Hybrid (biology)

For other uses, see Hybrid.

Hercules, a "liger", a lion/tiger hybrid

In biology a hybrid, also known as cross breed, is the result of mixing, through sexual reproduction, two animals or plants of different breeds, varieties, species or genera. Using genetic terminology, it may be defined as follows:

1. Hybrid generally refers to any offspring resulting from the breeding of two genetically distinct individuals, which usually will result in a high degree of heterozygosity, though hybrid and heterozygous are not, strictly speaking, synonymous;
2. genetic hybrid carries two different alleles of the same gene;
3. structural hybrid results from the fusion of gametes that have differing structure in at least one chromosome, as a result of structural abnormalities;
4. numerical hybrid results from the fusion of gametes having different haploid numbers of chromosomes;
5. permanent hybrid is a situation where only the heterozygous genotype occurs, because all homozygous combinations are lethal.

From a taxonomic perspective, hybrid refers to:

1. Offspring resulting from the interbreeding between two animal species or plant species. See also hybrid speciation.
2. Hybrids between different subspecies within a species (such as between the Bengal tiger and Siberian tiger) are known as intra-specific hybrids. Hybrids between different species within the same genus (such as between lions and tigers) are sometimes known as interspecific hybrids or crosses. Hybrids between different genera (such as between sheep and goats) are known as intergeneric hybrids. Extremely rare interfamilial hybrids have been known to occur (such as the guineafowl hybrids). No interordinal (between different orders) animal hybrids are known.
3. The third type of hybrid consists of crosses between populations, breeds or cultivars within a single species. This meaning is often used in plant and animal breeding, where hybrids are commonly produced and selected, because they have desirable characteristics not found or inconsistently present in the parent individuals or populations.

Terminology

The term hybrid is derived from Latin hybrida, meaning the "offspring of a tame sow and a wild boar", "child of a freeman and slave", etc. The term came into popular use in English in the 19th century, though examples of its use have been found from the early 17th century.

There is a popular convention of naming hybrids by forming portmanteau words. This became common in the 1920s, with the breeding of tiger-lion hybrids (liger and tigon). This was playfully (but unsystematically) extended to a number of other hybrids, or hypothetical hybrids, such as beefalo (1960s), humanzee (1980s), and cama (1998).

Types of hybrids

Depending on the parents, there are a number of different types of hybrids;

• Single cross hybrids  result from the cross between two true breeding organisms which produces an F1 generation called an F1 hybrid (F1 is short for Filial 1, meaning "first offspring"). The cross between two different homozygous lines produces an F1 hybrid that is heterozygous; having two alleles, one contributed by each parent and typically one is dominant and the other recessive. Typically, the F1 generation is also phenotypically homogeneous, producing offspring that are all similar to each other.
• Double cross hybrids result from the cross between two different F1 hybrids.
• Three-way cross hybrids result from the cross between an F1 hybrid and an inbred line.
• Triple cross hybrids result from the crossing of two different three-way cross hybrids.
• Population hybrids result from the crossing of plants or animals in a population with those of another population. These include crosses between organisms such as interspecific hybrids or crosses between different breeds.
• Stable hybrid is a horticultural term, usually referring to an annual plant that, if grown and bred in a small monoculture free of external pollen (e.g., an air-filtered greenhouse) will produce offspring that are "true to type" with respect to phenotype; i.e., a true breeding organism.
• Hybrid species result from hybrid populations evolving reproductive barriers against their parent species through hybrid speciation.

Interspecific hybrids

Interspecific hybrids are bred by mating two species, normally from within the same genus. The offspring display traits and characteristics of both parents. The offspring of an interspecific cross are often sterile; thus, hybrid sterility prevents the movement of genes from one species to the other, keeping both species distinct. Sterility is often attributed to the different number of chromosomes between the two species. For example, donkeys have 62 chromosomes, while horses have 64 chromosomes, and mules and hinnies have 63 chromosomes. Mules, hinnies, and other normally sterile interspecific hybrids cannot produce viable gametes, because differences in chromosome structure prevent appropriate pairing and segregation during meiosis, meiosis is disrupted, and viable sperm and eggs are not formed. However, fertility in female mules has been reported with a donkey as the father.

Most often other processes occurring in plants and animals maintain gametic isolation and species distinction. Species often have different mating or courtship patterns or behaviors, the breeding seasons may be distinct and even if mating does occur antigenic reactions to the sperm of other species prevent fertilization or embryo development. Hybridisation is much more common among organisms that spawn indiscriminately, like soft corals and among plants.

While it is possible to predict the genetic composition of a backcross on average, it is not possible to accurately predict the composition of a particular backcrossed individual, due to random segregation of chromosomes. In a species with two pairs of chromosomes, a twice backcrossed individual would be predicted to contain 12.5% of one species' genome (say, species A). However, it may, in fact, still be a 50% hybrid if the chromosomes from species A were lucky in two successive segregations, and meiotic crossovers happened near the telomeres. The chance of this is fairly high: $\left({\frac {1}{2}}\right)^{(2\times 2)}={\frac {1}{16}}$ (where the "two times two" comes about from two rounds of meiosis with two chromosomes); however, this probability declines markedly with chromosome number and so the actual composition of a hybrid will be increasingly closer to the predicted composition.

Hybrid species

Main article: Hybrid speciation

While not common, a few animal species have been recognized as the result of hybridization. The Lonicera fly is an example of a novel animal species that resulted from natural hybridization. The American red wolf appears to be a hybrid species between gray wolf and coyote, although its taxonomic status has been a subject of controversy. The European edible frog appears to be a species, but is actually a semi-permanent hybrid between pool frogs and marsh frogs. The edible frog population is dependent on the presence of at least one of the parent species to be maintained. Evidence found in cave paintings indicates that the European bison is a descendant of the "Higgs bison", a natural hybrid between aurochs and steppe bison.

Hybrid species of plants are much more common than animals. Many of the crop species are hybrids, and hybridization appears to be an important factor in speciation in some plant groups.

Examples of hybrid animals and animal populations derived from hybrids

Mammals

A "zonkey", a zebra/donkey hybrid

A "jaglion", a jaguar/lion hybrid

• Equid hybrids
  • Mule, a cross of female horse and a male donkey
  • Hinny, a cross between a female donkey and a male horse (Mule and hinny are examples of reciprocal hybrids)
  • Zebroids
    • Zeedonk or Zonkey, a zebra/donkey cross
    • Zorse, a zebra/horse cross
    • Zony or Zetland, a zebra/pony cross ("zony" is a generic term; "zetland" is specifically a hybrid of the Shetland pony breed with a zebra)
  • hybrid ass, a cross between a donkey and an onager or Asian wild ass
• Bovid hybrids
  • Dzo, zo or yakow; a cross between a domestic cow/bull and a yak
  • Beefalo, a cross of an American bison and a domestic cow. This is a fertile breed; this along with genetic evidence has caused them to be recently reclassified into the same genus, Bos.
  • Żubroń, a hybrid between wisent (European bison) and domestic cow
  • "Higgs bison", a naturally occurring hybrid between aurochs and steppe bison that appeared during the Pleistocene; likely gave rise to the wisent.
• Sheep-goat hybrid is the cross between a sheep and a goat, which belong to different genera.
• Ursid hybrids, such as the grizzly-polar bear hybrid, occur between black bears, brown bears, and polar bears.
• Felid hybrids
  • Savannah cat is a fertile breed developed originally from a cross between the serval (Leptailurus serval) and domestic cats (Felis catus).
  • A hybrid between a Bengal tiger and a Siberian tiger is an example of an intra-specific hybrid. It also includes the Indochinese tiger, Sumatran tiger.
  • Pumapards are the hybrid crosses between a puma and a leopard.
  • Ligers and tigons (crosses between a lion and a tiger - the difference in name due to which parent is of which species - ligers have a lion father and a tiger mother) and other Panthera hybrids such as the lijagulep. Various other wild cat crosses are known involving the lynx, bobcat, leopard, serval, etc.
    • Liligers are the hybrid cross between a male lion and a ligress.
  • Bengals are a fertile breed developed originally from a cross between the Asian leopard cat [Prionailurus bengalensis] and the domestic cat [Felis catus].
• Fertile canid hybrids occur between coyotes, wolves, dingoes, jackals and domestic dogs.
• Hybrids between black and white rhinoceroses have been recognized.
• Hybrid camel, a cross between a bactrian camel and a dromedary camel
• Cama, a cross between a camel and a llama, also an intergeneric hybrid.
• Wholphin, a fertile but very rare cross between a false killer whale and a bottlenose dolphin.
• At Chester Zoo in the United Kingdom, a cross between an African elephant (male) and an Asian elephant (female). The male calf was named Motty. He died of intestinal infection after twelve days.
• Bornean and Sumatran orangutan hybrids have occurred in captivity.

Birds

See also: Bird hybrid

A mule, a domestic canary/goldfinch hybrid

• Hybrids between spotted owls and barred owls
• Cagebird breeders sometimes breed hybrids between species of finch, such as goldfinch × canary. These birds are known as mules.
• The perlin is a peregrine falcon – merlin hybrid.
• Gamebird hybrids, hybrids between gamebirds and domestic fowl, including chickens, guineafowl and peafowl, interfamilial hybrids.
• Numerous macaw hybrids and lovebird hybrids are also known in aviculture.
• Red kite × black kite: five bred unintentionally at a falconry center in England. (It is reported that the black kite (the male) refused female black kites but mated with two female red kites.)
• The mulard duck, hybrid of the domestic pekin duck and domesticated muscovy ducks.
• In Australia, New Zealand and other areas where the Pacific black duck occurs, it is hybridised by the much more aggressive introduced mallard. This is a concern to wildlife authorities throughout the affected area, as it is seen as Genetic pollution of the black duck gene pool.
• Hybridisation in gulls is a reasonably frequent occurrence in the wild.

Reptiles

• Hybrid iguana, a single‐cross hybrid resulting from natural interbreeding between male marine iguanas and female land iguanas since the late 2000s.
• Crestoua, a cross between a Rhacodactylus Ciliatus (crested gecko) and a Rhacodactylus Chahoua.
• Colubrid snakes of the tribe Lampropeltini have been shown to produce fertile hybrid offspring.
• Hybridization between the endemic Cuban crocodile (Crocodilus rhombifer) and the widely distributed American crocodile (Crocodilus acutus) is causing conservation problems for the former species as a threat to its genetic integrity.
• Saltwater crocodiles (Crocodylus porosus) have mated with Siamese crocodiles (Crocodylus siamensis) in captivity producing offspring which in many cases have grown over 20 feet (6.1 metres) in length. It is likely that wild hybridization occurred historically in parts of southeast Asia.
• Many species of boas and pythons are known to produce hybrids, such as carball (a cross between a ball python and a carpet python) or a bloodball (a cross between a blood python and a ball python); however most of these only occur in captivity. Contrary to popular belief, boa–python hybrids are not possible due to their differing reproductive functions. Boas only produce hybrids with other species of boas, and pythons only produce hybrids with other species of pythons.

Amphibians

• Japanese giant salamanders and Chinese giant salamanders have created hybrids that threaten the survival of Japanese giant salamanders due to the competition for similar resources in Japan.

Fish

• Blood parrot cichlid, which is probably created by crossing a red head cichlid and a Midas cichlid or red devil cichlid
• A group of about 50 hybrids between Australian blacktip shark and the larger common blacktip shark was found by Australia's East Coast in 2012. This is the only known case of hybridization in sharks.
• Silver bream and common bream commonly produce sterile hybrids.
• Tiger muskie is a sterile hybrid between northern pike and muskellunge.

Insects

• Killer bees were created during an attempt to breed a strain of bees that would produce more honey and be better adapted to tropical conditions. This was done by crossing a European honey bee and an African bee.
• The Colias eurytheme and C. philodice butterflies have retained enough genetic compatibility to produce viable hybrid offspring.

Hybrid plants

A sterile hybrid between Trillium cernuum and T. grandiflorum

An ornamental lily hybrid known as Lilium 'Citronella'

Many hybrid plants are created by humans, but natural hybrids also occur. Plant species hybridize more readily than animal species, and the resulting hybrids are more often fertile hybrids and may reproduce, though there still exist sterile hybrids and selective hybrid elimination where the offspring are less able to survive and are thus eliminated before they can reproduce. A number of plant species are the result of hybridization and polyploidy with many plant species easily cross-pollinating and producing viable seeds, the distinction between each species is often maintained by geographical isolation or differences in the flowering period. Since plants hybridize frequently without much work, they are often created by humans in order to produce improved plants. These improvements can include the production of more or improved seeds, fruits or other plant parts for consumption, or to make a plant more winter- or heat-resistant, or to improve its growth and or appearance for use in horticulture. Much work is now being done with hybrids to produce disease-resistant plants for both agricultural and horticultural crops. In many plant species, hybridization is used to produce larger and more showy flowers and new flower colors. Hybridization may be restricted to the desired parent species through the use of pollination bags.

Many plant genera and species have their origins in polyploidy. Autopolyploidy results from the sudden multiplication in the number of chromosomes in typical normal populations caused by unsuccessful separation of the chromosomes during meiosis. Tetraploids (plants with four sets of chromosomes rather than two) are common in a number of different groups of plants and over time these plants can differentiate into distinct species from the normal diploid line. In Oenothera lamarchiana the diploid species has 14 chromosomes, this species has spontaneously given rise to plants with 28 chromosomes that have been given the name Oenothera gigas. When hybrids are formed between the tetraploids and the diploid population, the resulting offspring tend to be sterile triploids, thus effectively stopping the intermixing of genes between the two groups of plants (unless the diploids, in rare cases, produce unreduced gametes).

Another form of polyploidy called allopolyploidy occurs when two different species mate and produce polyploid hybrids. Usually the typical chromosome number is doubled, and the four sets of chromosomes can pair up during meiosis, thus the polyploids can produce offspring. Usually, these offspring can mate and reproduce with each other but cannot back-cross with the parent species. Allopolyploids may be able to adapt to new habitats that neither of their parent species inhabited.

Species	Common name	Family	Hybridization	Confirmed or putative hybridization?	Putative parental/introgressive species	Polyploid or homoploid?	Polyploid chromosome count	References	Notes
Abelmoschus esculentus (L.) Moench	Okra	Malvaceae	Allopolyploid origin	Putative	Uncertain	Polyploid (tetraploid)	usually 2n=4x=130	Joshi and Hardas, 1956; Schafleitner et al., 2013	Variable ploidy
Actinidia deliciosa (A. Chev.) C.F.Liang & A.R.Ferguson	Kiwifruit	Actinidiaceae	Allopolyploid origin	Putative	Actinidia chinensis Planch. and Unknown	Polyploid (hexaploid)	2n=6x=174	Atkinson et al., 1997
Agave fourcroydes Lem.	Henequen	Asparagaceae	Allopolyploid origin	Confirmed	Uncertain	Polyploid (usu. pentaploid, triploid)	2n=5x(3x)=150(90)	Robert et al., 2008; Hughes et al., 2007	Variable ploidy, polyploid event not recent
Agave sisalana Perrine	Sisal	Asparagaceae	Allopolyploid origin	Confirmed	Uncertain	Polyploid (usu. pentaploid, hexaploid)	2n=5x(6x)=150(180)	Robert et al., 2008	Variable ploidy, polyploid event not recent
Allium ampeloprasum L.	Great headed garlic	Amaryllidaceae	Intraspecific hybrid origin	Putative	Allium ampeloprasum L.	Homoploid	–	Guenaoui et al., 2013
Allium cepa L.	Common onion	Amaryllidaceae	Interspecific hybrid origin	Putative	Uncertain: Allium vavilovii Popov & Vved., A. galanthum Kar. & Kir. or A. fistulosum L.	Homoploid	–	Gurushidze et al., 2007
Allium cornutum Clementi	Triploid onion	Amaryllidaceae	Triparental allopolyploid origin	Confirmed	Allium cepa L., A. roylei Stearn, unknown	Polyploid (triploid)	2n=3x=24	Fredotovic et al., 2014
Ananas comosus (L.) Merr is	Pineapple	Bromeliaceae	Interspecific introgression	Putative	Ananas ananassoides (Baker) L.B. Smith	Homoploid	–	Duval et al., 2003
Annona x atemoya	Atemoya	Annonaceae	Interspecific hybrid origin	Confirmed	Annona cherimola Mill. and A. squamosa L.	?	–	Perfectti et al., 2004; Jalikop, 2010
Arachis hypogaea L.	Peanut	Fabaceae	Allopolyploid origin	Confirmed	Arachis duranensis Krapov. & W.C. Greg. and A. ipaënsis Krapov. & W.C. Greg.	Polypoid (tetraploid)	2n=4x=40	Kochert et al., 1996; Bertioli et al., 2011
Armoracia rusticana P.Gaertn., B.Mey. & Scherb.	Horseradish	Brassicaceae	Interspecific hybrid origin	Putative	Uncertain	?	–	Courter and Rhodes, 1969
Artocarpus altilis (Parkinson ex F.A.Zorn) Fosberg	Breadfruit	Moraceae	Interspecific introgression	Putative	Artocarpus mariannensis Trécul	?	–	Zerega et al., 2005; Jones et al., 2013
Avena sativa L.	Oat	Poaceae	Allopolyploid origin	Confirmed	Uncertain	Polyploid (hexaploid)	2n=6x=42	Linares et al., 1998; Oliver et al., 2013
Brassica carinata A.Braun	Ethiopian mustard	Brassicaceae	Allopolyploid origin	Confirmed	Brassica oleracea L. and B. nigra (L.) K.Koch	Polyploid (tetraploid)	2n=4x=19	Arias et al., 2014
Brassica juncea (L.) Czern.	Indian mustard	Brassicaceae	Allopolyploid origin	Confirmed	Brassica nigra (L.) K.Koch and B. rapa L.	Polyploid (tetraploid)	2n=4x=18	Arias et al., 2014
Brassica napus L.	Rapeseed, rutabega	Brassicaceae	Allopolyploid origin	Confirmed	Brassica rapa L. and B. oleracea L.	Polyploid (tetraploid)	2n=4x=19	Arias et al., 2014
Cajanus cajan (L.) Millsp.	Pigeon pea	Fabaceae	Intraspecific introgression, interspecific introgression	Putative	Wild Cajanus cajan and other species	Homoploid	–	Kassa et al., 2012
Cannabis sativa L.	Hemp	Cannabaceae	Intraspecific introgression	Putative	Cannabis sativa L. 'Indica' and 'Sativa' types	Homoploid	–	de Meijer and van Soest, 1992
Carica pentagona Heilborn	Babaco	Caricaceae	Interspecific hybrid origin	Confirmed	Uncertain (Carica stipulata V.M.Badillo, Vasconcellea pubescens A.DC., Vasconcellea weberbaueri (Harms) V.M. Badillo)	?	–	Van Droogenbroeck et al., 2002; Van Droogenbroeck et al., 2006
Carya illinoinensis (Wangenh.) K.Koch	Pecan	Juglandaceae	Interspecific hybrid origin	Putative	Uncertain	?	–	Grauke et al., 2011
Castanea dentata (Marshall) Borkh	Chestnut	Fagaceae	Interspecific introgression	Confirmed	Castanea pumila (L.) Mill.	Homoploid	–	Li and Dane, 2013	Also ongoing efforts to introgress blight resistance from Castanea mollissima Blume (see Jacobs et al., 2013)
Castanea sativa Mill.	Chestnut	Fagaceae	Intraspecific introgression	Confirmed	Castanea sativa Eurosiberian and Mediterranean populations	Homoploid	–	Villani et al., 1999; Mattioni et al., 2013
Chenopodium quinoa Willd.	Quinoa	Chenopodiaceae	Allopolyploid origin	Putative	Uncertain	Polyploid (tetraploid)	–	Heiser, 1974; Ward, 2000; Maughan et al., 2004
Cicer arietinum L.	Chickpea (pea-shaped)	Fabaceae	Intraspecific hybrid origin	Putative	Cicer arietinum L. Desi and Kabuli Germplasm	?	–	Upadhyaya et al., 2008; Keneni et al., 2011
Cichorium intybus L.	Radicchio	Asteraceae	Interspecific introgression	Confirmed	Wild Cichorium intybus L.	Homoploid	–	Kiaer et al., 2009
Citrus aurantiifolia (Christm.) Swingle	Key lime	Rutaceae	Interspecific hybrid origin	Confirmed	Citrus medica L. and C. subg. Papeda	?	–	Ollitrault and Navarro, 2012; Penjor et al., 2014; Nicolosi et al., 2000; Moore, 2001
Citrus aurantium L.	Sour oranges	Rutaceae	Interspecific hybrid origin	Confirmed	Citrus maxima (Burm.) and C. reticulata Blanco	?	–	Wu et al., 2014; Moore, 2001
Citrus clementina hort.	Clementine	Rutaceae	Interspecific hybrid origin	Confirmed	Citrus sinensis (L.) Osbeck and C. reticulata Blanco	?	–	Wu et al., 2014
Citrus limon (L.) Osbeck	Lemon, lime	Rutaceae	Interspecific hybrid origin	Confirmed	Citrus medica L., C. aurantiifolia (Christm.) Swingle, and uncertain	?	–	Nicolosi et al., 2000; Moore, 2001
Citrus paradisi Macfad.	Grapefruit	Rutaceae	Interspecific hybrid origin	Confirmed	Citrus sinensis (L.) Osbeck and C. maxima (Burm.)	?	–	Wu et al., 2014; Moore, 2001
Citrus reticulata Blanco	Mandarin	Rutaceae	Interspecific introgression	Confirmed	Citrus maxima (Burm.)	?	–	Wu et al., 2014
Citrus sinensis (L.) Osbeck	Sweet orange (blood, common)	Rutaceae	Interspecific hybrid origin	Confirmed	Uncertain	?	–	Wu et al., 2014; Moore, 2001
Cocos nucifera L.	Coconut	Arecaceae	Intraspecific introgression	Confirmed	Cocos nucifera L. Indo-Atlantic and Pacific lineages	Homoploid	–	Gunn et al., 2011
Coffea arabica L.	Coffee	Rubiaceae	Allopolyploid origin	Confirmed	Coffea eugenioides S.Moore and C. canephora Pierre ex A.Froehner	Polyploid (tetraploid)	2n=4x=44	Lashermes et al., 1999
Corylus avellana L.	Hazelnut	Betulaceae	Intraspecific introgression	Confirmed	Wild Corylus avellana L. in Southern Europe	Homoploid	–	Campa et al., 2011; Boccacci et al., 2013
Cucurbita pepo L.	Winter squash, pumpkin	Cucurbitaceae	Intraspecific introgression	Putative	Cucurbita pepo var. texana (Scheele) D.S.Decker	Homploid	–	Kirkpatrick and Wilson, 1988
Daucus carota subsp. sativus (Hoffm.) Arcang.	Carrot	Apiaceae	Intraspecific introgression	Confirmed	Daucus carota L. subsp. carota	Homoploid	–	Iorizzo et al., 2013; Simon, 2000
Dioscorea L. spp.	Yam	Dioscoreaceae	Interspecific hybrid origin, introgression	Putative	Uncertain	Variable	–	Terauchi et al., 1992; Dansi et al., 1999; Bhattacharjee et al., 2011; Mignouna et al., 2002	Multiple species of putative hybrid (perhaps allopolyploid) origin including Dioscorea cayennensis subsp. rotundata (Poir.) J.Miège. and D. cayennensis Lam.
Diospyros kaki L.f.	Persimmon	Ebenaceae	Allopolyploid origin	Putative	Uncertain	Polyploid (hexaploid)	Yonemori et al., 2008
Ficus carica L.	Fig	Moraceae	Interspecific introgression	Putative	Uncertain	?	Aradhya et al., 2010
Fragaria ananassa (Duchesne ex Weston) Duchesne ex Rozier	Strawberries	Rosaceae	Interspecific hybrid origin	Confirmed	Fragaria virginiana Mill. (octoploid), F. chiloensis (L.) Mill. (octoploid)	Homoploid relative to parentals (octoploid)	2n=8x=56 Evans, 1977; Hirakawa et al., 2014	Uncertain which species formed the octoploid progenitors
Garcinia mangostana L.	Mangosteen	Clusiaceae	Allopolyploid origin	Putative	Garcinia celebica L. and G. malaccensis Hook.f. ex T.Anderson	Polyploid (tetraploid)	? Richards, 1990	Recent work shows this may not be of hybrid origin (Nazre, 2014)
Gossypium hirsutum L.	Upland cotton	Malvaceae	hybrid origin	Confirmed	Uncertain, referred to as 'A' and 'D'	Polyploid (formed <1MYA)	2n =4x=52 Wendel and Cronn 2003	Polyploidization likely led to agronomically significant traits (Applequist et al., 2001)
Hibiscus sabdariffa L.	Roselle	Malvaceae	Allopolyploid origin	Putative	Uncertain	Polyploid (tetraploid)	2n=4x=72 Menzel and Wilson, 1966; Satya et al., 2013
Hordeum vulgare L.	Barley	Poaceae	Introgression	Confirmed	Hordeum spontaneum K.Koch	Homoploid	Badr et al., 2000; Dai et al., 2012
Humulus lupulus L.	Hops	Cannabaceae	Intraspecific introgression	Confirmed	Humulus lupulus L. North American and European Germplasm	Homoploid	Reeves and Richards, 2011; Stajner et al., 2008; Seefelder et al., 2000
Ipomoea batatas (L.) Lam.	Sweet potato	Convolvulaceae	Intraspecific introgression; Interspecific introgression?	Putative	Ipomoea batatas (L.) Lam. Central American and South American Germplasm	Homoploid relative to parentals	Roullier et al., 2013
Juglans regia L.	Walnut	Juglandaceae	Interspecific hybridization	Confirmed	Juglans sigillata Dode	Homoploid	Gunn et al., 2010
Lactuca sativa L.	Lettuce	Asteraceae	Intraspecific hybrid origin	Putative	Lactuca serriola L. and other L. spp.	Homoploid	de Vries, 1997
Lagenaria siceraria (Molina) Standl.	Bottle Gourd	Cucurbitaceae	Intraspecific introgression	Confirmed	Lagenaria siceraria (Molina) Standl. African/American and Asian Germplasm	Homoploid	Clarke et al., 2006
Lens culinaris Medik. ssp. culinaris	Lentil	Fabaceae	Intraspecific introgression	Putative	Wild lentil, Lens culinaris subsp. orientalis (Boiss.) Ponert	Homoploid	Erskine et al., 2011
Macadamia integrifolia Maiden & Betche	Macadamia	Proteaceae	Interspecific hybrid origin, interspecific introgression	Confirmed	Macadamia tetraphylla L.A.S.Johnson, and other M. spp.	Homoploid	Hardner et al., 2009; Steiger et al., 2003; Aradhya et al., 1998
Malus domestica Borkh.	Apple	Rosaceae	Interspecific hybrid origin	Confirmed	Malus sieversii (Ledeb.) M.Roem., M. sylvestris (L.) Mill., and possibly others	Homoploid	Cornille et al., 2012
Mentha piperita L.	Peppermint	Lamiaceae	Allopolyploid origin	Confirmed	Mentha aquatica L. and M. spicata L.	Polyploid (12-ploid)	2n=12x=66 or 72 Harley and Brighton, 1977; Gobert et al., 2002
Musa paradisiaca L.	Banana	Musaceae	Allopolyploid origin	Confirmed	Musa acuminata Colla, M. balbisiana Colla	Polyploid (usually triploid)	2n=3x=33 Simmonds and Shepherd, 1955; Heslop-Harrison and Schwarzacher, 2007; De Langhe et al., 2010
Nicotiana tabacum L.	Tobacco	Solanaceae	Allopolyploid origin	Confirmed	Uncertain (Nicotiana sylvestris Speg. & S. Comes and N. tomentosiformis Goodsp.)	Polyploid (tetraploid)	2n=4x=48 Kenton et al., 1993; Murad et al., 2002
Olea europaea L.	Olive	Oleaceae	Intraspecific introgression	Putative	Wild Olea europaea L., Eastern and Western Germplasm	Homoploid	Kaniewski et al., 2012; Besnard et al., 2013; Breton et al., 2006; Rubio de Casas et al., 2006; Besnard et al., 2007; Besnard et al., 2000
Opuntia L. spp.	Opuntia	Cactaceae	Interspecific hybrid origin, Allopolyploid origin	Putative	Including Opuntia ficus-indica (L.) Mill.	Polyploid, homoploid	Hughes et al., 2007; Griffith, 2004
Oryza sativa L.	Rice	Poaceae	Intraspecific introgression, interspecific introgression	Putative	Oryza sativa L. 'Japonica' and 'Indica' Germplasm, Oryza rufipogon Griff.	Homoploid	Caicedo et al., 2007; Gao and Innan, 2008
Oxalis tuberosa Molina	Oca	Oxalidaceae	Allopolyploid origin	Putative	Uncertain	Polyploid (octaploid)	2n=8x=64 Emswhiller and Doyle, 2002; Emshwiller 2002; Emswhiller et al., 2009
Pennisetum glaucum (L.) R.Br.	Pearl millet	Poaceae	Intraspecific introgression	Confirmed	Wild Pennisetum glaucum (L.) R.Br.	Homoploid	Oumar et al., 2008
Persea americana Mill.	Avocado (Hass and other cultivars)	Lauraceae	Intraspecific introgression	Confirmed	Persea americana Mill. 'Guatamalensis', 'Drymifolia', and' Americana'	Homoploid	Chen et al., 2008; Davis et al., 1998; Ashworth and Clegg, 2003; Douhan et al., 2011
Phoenix dactylifera L.	Date palm	Arecaceae	Interspecific hybrid origin	Putative	Uncertain	Homoploid	El Hadrami et al., 2011; Bennaceur et al., 1991
Piper methysticum G.Forst.	Kava	Piperaceae	Allopolyploid origin	Putative	Piper wichmannii C. DC. and P. gibbiflorum C.DC.	Polyploid (decaploid)	2n=10x=130 Singh, 2004; Lebot et al., 1991
Pistacia vera L.	Pistachio	Anacardiaceae	Interspecific introgression	Putative	Pistacia atlantica Desf., P. chinensis subsp. integerrima (J. L. Stewart ex Brandis) Rech. f.	Homoploid	Kafkas et al., 2001
Pisum abyssinicum A.Braun	Pea	Fabaceae	Interspecific hybrid origin	Confirmed	Uncertain (Pisum fulvum Sibth. & Sm. and other P. spp.)	Homoploid	Vershinin et al., 2003
Pisum sativum L.	Pea	Fabaceae	Interspecific hybrid origin	Confirmed	Uncertain (Pisum sativum subsp. elatius (M.Bieb.) Asch. & Graebn. and other P. spp.)	Homoploid	Vershinin et al., 2003
Plinia cauliflora (Mart.) Kausel	Jaboticaba	Myrtaceae	Intraspecific hybridization, interspecific hybridization	Putative	Plinia 'Jaboticaba' and 'Cauliflora' Germplasm; P. peruviana (Poir.) Govaerts	Homoploid	Balerdi et al., 2006
Prunus cerasus L.	Cherry	Rosaceae	Allopolyploid origin	Confirmed	Prunus avium (L.) L. and P. fruticosa Pall.	Polyploid (tetraploid)	2n=4x=32 Tavaud et al., 2004; Olden and Nybom, 1968
Prunus domestica L.	Plum	Rosaceae	Allopolyploid origin	Confirmed	Uncertain (P. cerasifera Ehrh. and P. spinosa L.)	Polyploid (hexaploid)	2n=6x=48 Zohary, 1992; Hartmann and Neumuller, 2009	Japanese plum is also of hybrid origin (see Hartmann and Neumuller 2009). Also hybridizes with other cultivated Prunus spp.
Prunus dulcis (Mill.) D.A.Webb	Almond	Rosaceae	Interspecific introgression	Confirmed	Prunus orientalis (Mill.) Koehne and other P. spp.	Homoploid	Delplancke et al., 2012; Delplancke et al., 2013
Pyrus L. species	Pear	Rosaceae	Interspecific hybrid origin	Confirmed	Many species	Homoploid	Silva et al., 2014	Also introgression with semidomesticated populations (see Iketani et al. 2009)
Raphanus raphanistrum subsp. sativus (L.) Domin	Radish	Brassicaceae	Intraspecific introgression	Confirmed	Raphanus raphanistrum L. subsp. raphanistrum	Homoploid	Ridley et al., 2008
Rheum L. cultivated species	Rhubarb	Polygonaceae	Interspecific hybrid origin	Putative	Unclear	Homoploid relative to parentals (tetraploid)	Foust and Marshall, 1991; Kuhl and Deboer, 2008	Hybrids include: Rheum rhaponticum L., R. rhabarbarum L., R. palmatum L.
Rubus L. spp.	Red raspberry, blackberry, tayberry, boysenberry, etc.	Rosaceae	Allopolyploid origin, interspecific hybridization	Confirmed	Many	Polyploid	Alice et al., 2014; Alice and Campbell, 1999
Saccharum spp.	Sugarcane	Poaceae	Allopolyploid origin	Confirmed	Saccharum officinarum L. and S. spontaneum L.	Polyploid	Variable, 2n=10-13x=100-130 Grivet et al., 1995; D'Hont et al., 1996
Secale cereale L.	Rye	Poaceae	Interspecific hybrid origin	Confirmed	Uncertain (Secale montanum Guss., S. vavilovii Grossh.)	Homoploid	Bartos et al., 2008; Korzun et al., 2001; Hillman, 1978; Tang et al., 2011; Salamini et al., 2002
Sechium edule (Jacq.) Sw.	Chayote	Cucurbitaceae	Interspecific introgression	Putative	Sechium compositum (Donn. Sm.) C. Jeffrey	Homoploid	Newstrom, 1991
Setaria italica (L.) P.Beauv.	Foxtail millet	Poaceae	Interspecific introgression	Confirmed	Setaria viridis (L.) P.Beauv.	Homoploid	Till-Bottraud et al., 1992
Solanum L. spp. Section Petota	Potato	Solanaceae	Interspecific hybrid origin, allopolyploid origin, interspecific introgression	Confirmed	Including Solanum tuberosum L., S. ajanhuiri Juz. & Bukasov, S. curtilobum Juz. & Bukasov, S. juzepczukii Bukasov	Homoploid and Polyploid	Rodriguez et al., 2010
Solanum lycopersicum L.	Tomato	Solanaceae	Intraspecific introgression, interspecific introgression	Confirmed	Solanum lycopersicum var. cerasiforme (Dunal) D.M. Spooner, G.J. Anderson & R.K. Jansen and S. pimpinellifolium L.	Homoploid	Blanca et al., 2012; Causse et al., 2013; Rick, 1950
Solanum melongena L.	Eggplant	Solanaceae	Interspecific hybrid origin, interspecific introgression, intraspecific introgression	Confirmed	Solanum undatum Lam. and others; wild S. melongena L. (=S. insanum L.)	Homoploid	Knapp et al., 2013; Meyer et al., 2012	Hybrid origin is not confirmed, but introgression is well documented
Solanum muricatum Aiton	Pepino dulce	Solanaceae	Interspecific hybrid origin, interspecific introgression	Likely	Solanum species in Series Caripensia	Homoploid	Blanca et al., 2007	Polyphyletic origin and extensive, ongoing introgression with wild species
Solanum tuberosum	Potato	Solanaceae	Intraspecific Hybrid	Confirmed	Diploid potato lines crossed with S. chacoense containing Sli gene	Homoploid	Lindhout et al., 2011
Spondias purpurea L.	Jocote	Anacardiaceae	Interspecific introgression	Putative	Spondias mombin L.	Homoploid	Miller and Schaal, 2005
Theobroma cacao L.	Cacao (Trinitario-type)	Malvaceae	Intraspecific hybrid origin	Confirmed	Theobroma cacao L. 'Forastero' and 'Criollo' Germplasm	Homoploid	Yang et al., 2013
Triticum aestivum L.	Bread wheat, spelt	Poaceae	Allopolyploid origin	Confirmed	Triticum turgidum L. (tetraploid) with Aegilops tauschii Coss.	Polyploid (hexaploid)	2n=6x=42 Matsuoka, 2011; Dvorak, 2012
Triticum turgidum L.	Emmer wheat, durum wheat	Poaceae	Allopolyploid origin	Confirmed	Triticum urartu Thumanjan ex Gandilyan and Aegilops speltoides Tausch	Polyploid (tetraploid)	2n=4x=28 Dvorak et al., 2012; Matsuoka, 2011; Yamane and Kawahara, 2005
Vaccinium corymbosum L.	Highbush blueberry	Ericaceae	Interspecific hybrid origin, interspecific introgression	Putative	Vaccinium tenellum Aiton, V. darrowii Camp, (V. virgatum Aiton, V. angustifolium Aiton)	Uncertain	Vander Kloet, 1980; Bruederle et al., 1994; Lyrene et al., 2003; Boches et al., 2006	Possible hybrid origin during the Plesitocene
Vanilla tahitensis J.W. Moore	Tahitian vanilla	Orchidaceae	Allopolyploid origin	Confirmed	Vanilla planifolia Jacks. ex Andrews and V. odorata C.Presl	Polyploid	Variable, 2n=2x(4x)=32(64) Lubinsky et al., 2008
Vitis rotundifolia Michx.	Grape	Vitaceae	Interspecific introgression	Confirmed	ManyVitis spp.	Homoploid	2n=2x=38 Reisch et al., 2012; This et al., 2006
Zea mays L.	Maize	Poaeceae	Intraspecific introgression	Confirmed	Wild Zea mays L. (teosinte, =subsp. parviglumis Iltis & Doebley)	Homoploid	Van Heerwaarden et al., 2011; Hufford et al., 2013

Sterility in a non-polyploid hybrid is often a result of chromosome number; if parents are of differing chromosome pair number, the offspring will have an odd number of chromosomes, leaving them unable to produce chromosomally balanced gametes. While this is undesirable in a crop such as wheat, where growing a crop which produces no seeds would be pointless, it is an attractive attribute in some fruits. Triploid bananas and watermelons are intentionally bred because they produce no seeds (and are parthenocarpic).

Heterosis

Main article: heterosis

Hybrids are sometimes stronger than either parent variety, a phenomenon most common with plant hybrids, which when present is known as hybrid vigor (heterosis) or heterozygote advantage. A transgressive phenotype is a phenotype displaying more extreme characteristics than either of the parent lines. Plant breeders use several techniques to produce hybrids, including line breeding and the formation of complex hybrids. An economically important example is hybrid maize (corn), which provides a considerable seed yield advantage over open pollinated varieties. Hybrid seed dominates the commercial maize seed market in the United States, Canada and many other major maize-producing countries.

Examples of plant hybrids

The multiplication symbol × (not italicised) indicates a hybrid in the Latin binomial nomenclature. Placed before the binomial it indicates a hybrid between species from different genera (intergeneric hybrid):-

• × Fatshedera lizei, a hybrid between Hedera helix and Fatsia japonica
• × Heucherella, a hybrid genus between Heuchera and Tiarella
• × Philageria veitchii is a hybrid between Lapageria rosea and Philesia magellanica; it is more similar in appearance to the former
• Leyland cypress, [× Cupressocyparis leylandii] hybrid between Monterey cypress and Nootka cypress
• Triticale, [× Triticosecale] a wheat–rye hybrid
• × Urceocharis, a hybrid between Eucharis and Urceolina

Interspecific plant hybrids include:

• Dianthus × allwoodii (Dianthus caryophyllus × Dianthus plumarius)
• Limequat Citrus × floridana, key lime Citrus aurantiifolia and kumquat Citrus japonica hybrid
• Loganberry Rubus × loganobaccus, a hybrid between raspberry Rubus idaeus and blackberry Rubus ursinus
• London plane (Platanus orientalis × Platanus occidentalis), thus forming Platanus × acerifolia
• Magnolia × alba (Magnolia champaca × Magnolia montana)
• Peppermint, a hybrid between spearmint and water mint
• Quercus × warei (Quercus robur × Quercus bicolor) 'Nadler' (marketed in the United States under the trade name Kindred Spirit hybrid oak)
• Tangelo, a hybrid of a Mandarin orange and a pomelo which may have been developed in Asia about 3,500 years ago
• Wheat; most modern and ancient wheat breeds are themselves hybrids. Bread wheat is a hexaploid hybrid of three wild grasses; durum (pasta) wheat is a tetraploid hybrid of two wild grasses
• Grapefruit, hybrid between a pomelo and the Jamaican sweet orange

Some natural hybrids:

• Iris albicans, a sterile hybrid which spreads by rhizome division
• Evening primrose, a flower which was the subject of famous experiments by Hugo de Vries on polyploidy and diploidy

Hybrids in nature

Hybridization between two closely related species is actually a common occurrence in nature but is also being greatly influenced by anthropogenic changes as well. Hybridization is a naturally occurring genetic process where individuals from two genetically distinct populations mate. It can occur both intraspecifically, between different distinct populations within the same species, and interspecifically, between two different species. Hybrids can be either sterile/not viable or viable/fertile. This affects the impact which the hybrid will have on its and other populations that it interacts with. Many hybrid zones are known where the ranges of two species meet, and hybrids are continually produced in great numbers. These hybrid zones are useful as biological model systems for studying the mechanisms of speciation (Hybrid speciation). Recently DNA analysis of a bear shot by a hunter in the North West Territories confirmed the existence of naturally-occurring and fertile grizzly–polar bear hybrids.

Anthropogenic hybridization

Changes to the environment caused by humans, such as fragmentation and Introduced species, are becoming more widespread. This increases the challenges in managing certain populations that are experiencing introgression, and is a focus of conservation genetics.

Introduced species and habitat fragmentation

Humans have introduced species worldwide to environments for a long time, both intentionally such as establishing a population to be used as a biological control, and unintentionally such as accidental escapes of individuals out of agriculture. This causes drastic global effects on various populations, including through hybridization.

When habitats become broken apart, one of two things can occur, genetically speaking. The first is that populations that were once connected can be cut off from one another, preventing their genes from interacting. Occasionally, this will result in a population of one species breeding with a population of another species as a means of surviving such as the case with the red wolves. Their population numbers being so small, they needed another means of survival. Habitat fragmentation also led to the influx of generalist species into areas where they would not have been, leading to competition and in some cases interbreeding/incorporation of a population into another. In this way, habitat fragmentation is essentially an indirect method of introducing species to an area.

The hybridization continuum

There is a kind of continuum with three semi-distinct categories dealing with anthropogenic hybridization: hybridization without Introgression, hybridization with widespread introgression, and essentially a Hybrid swarm. Depending on where a population falls along this continuum, the management plans for that population will change. Hybridization is currently an area of great discussion within Wildlife management and habitat management. Global climate change is creating other changes such as difference in population distributions which are indirect causes for an increase in anthropogenic hybridization.

Consequences

Hybridization can be a less discussed way toward extinction than within detection of where a population lies along the hybrid continuum. The dispute of hybridization is how to manage the resulting hybrids. When a population experiences hybridization with substantial introgression, there still exists parent types of each set of individuals. When a complete hybrid swarm is created, all the individuals are hybrids.

Management of hybrids

Conservationists disagree on when is the proper time to give up on a population that is becoming a hybrid swarm or to try and save the still existing pure individuals. Once it becomes a complete mixture, we should look to conserve those hybrids to avoid their loss. Most leave it as a case-by-case basis, depending on detecting of hybrids within the group. It is nearly impossible to regulate hybridization via policy because hybridization can occur beneficially when it occurs "naturally" and there is the matter of protecting those previously mentioned hybrid swarms because if they are the only remaining evidence of prior species, they need to be conserved as well.

Expression of parental traits in hybrids

When two distinct types of organisms breed with each other, the resulting hybrids typically have intermediate traits (e.g., one parent has red flowers, the other has white, and the hybrid, pink flowers). Commonly, hybrids also combine traits seen only separately in one parent or the other (e.g., a bird hybrid might combine the yellow head of one parent with the orange belly of the other).

In a hybrid, any trait that falls outside the range of parental variation is termed heterotic. Heterotic hybrids do have new traits, that is, they are not intermediate. Positive heterosis produces more robust hybrids, they might be stronger or bigger; while the term negative heterosis refers to weaker or smaller hybrids. Heterosis is common in both animal and plant hybrids. For example, hybrids between a lion and a tigress ("ligers") are much larger than either of the two progenitors, while a tigon (lioness × tiger) is smaller. Also the hybrids between the common pheasant (Phasianus colchicus) and domestic fowl (Gallus gallus) are larger than either of their parents, as are those produced between the common pheasant and hen golden pheasant (Chrysolophus pictus). Spurs are absent in hybrids of the former type, although present in both parents.

Genetic mixing and extinction

Main article: Genetic pollution

Regionally developed ecotypes can be threatened with extinction when new alleles or genes are introduced that alter that ecotype. This is sometimes called genetic mixing. Hybridization and introgression of new genetic material can lead to the replacement of local genotypes if the hybrids are more fit and have breeding advantages over the indigenous ecotype or species. These hybridization events can result from the introduction of non-native genotypes by humans or through habitat modification, bringing previously isolated species into contact. Genetic mixing can be especially detrimental for rare species in isolated habitats, ultimately affecting the population to such a degree that none of the originally genetically distinct population remains.

Effect on biodiversity and food security

Main articles: biodiversity and food security

In agriculture and animal husbandry, the Green Revolution's use of conventional hybridization increased yields by breeding "high-yielding varieties". The replacement of locally indigenous breeds, compounded with unintentional cross-pollination and crossbreeding (genetic mixing), has reduced the gene pools of various wild and indigenous breeds resulting in the loss of genetic diversity. Since the indigenous breeds are often well-adapted to local extremes in climate and have immunity to local pathogens, this can be a significant genetic erosion of the gene pool for future breeding. Therefore, commercial plant geneticists strive to breed "widely adapted" cultivars to counteract this tendency.

Limiting factors

A number of conditions exist that limit the success of hybridization; the most obvious is great genetic diversity between most species. But in animals and plants that are more closely related, hybridization barriers can include morphological differences, differing times of fertility, mating behaviors and cues, physiological rejection of sperm cells or the developing embryo.

In plants, barriers to hybridization include blooming period differences, different pollinator vectors, inhibition of pollen tube growth, somatoplastic sterility, cytoplasmic-genic male sterility and structural differences of the chromosomes.

Mythical, legendary and religious hybrids

Main article: Mythological hybrid

Ancient folktales often contain mythological creatures, sometimes described as hybrids (e.g., hippogriff as the offspring of a griffin and a horse, and Minotaur, the offspring of Pasiphaë and a white bull). More often they are kind of chimera, i.e., a composite of the physical attributes of two or more kinds of animals, mythical beasts, and often humans, with no suggestion that they are the result of interbreeding, e.g., harpies, mermaids, and centaurs.

In the Bible, the Old Testament contains several passages which talk about a first generation of hybrid giants who were known as the Nephilim. The Book of Genesis (6:4) states that "the sons of God went to the daughters of humans and had children by them". As a result, the offspring were born as hybrid giants who became mighty heroes of old and legendary famous figures of ancient times. In addition, the Book of Numbers (13:33) says that the descendants of Anak came from the Nephilim, whose bodies looked exactly like men, but with an enormous height. According to the apocryphal Book of Enoch the Nephilim were wicked sons of fallen angels who had lusted with attractive women.

See also

• Artificial selection
• Bird hybrids
• Cabbit
• Canid hybrid
• Chimera (genetics)
• Chloroplast capture (botany)
• Felid hybrids
• F1 hybrids
• Genetic admixture
• Genetic erosion
• Grex (horticulture)
• Heterosis (hybrid vigor)
• Hybrid lovebird
• Hybrid (mythology)
• Hybrid name (botany)
• Hybrid speciation
• Hybrid swarm
• Hybrid zone
• Hybrot
• Inbreeding
• Interspecific pregnancy
• Intraspecific breeding
• Macropod hybrids
• Purebred
• Selective breeding
• Sheep-goat hybrids
• Species barrier
• Synergy

External links

• Artificial Hybridisation – Artificial Hybridisation in orchids
• Domestic Fowl Hybrids
• Hybrid Mammals
• Hybridisation in animals Evolution Revolution: Two Species Become One, Study Says (nationalgeographic.com)
• Hybrids of wildcats with domestic cats
• Scientists Create Butterfly Hybrid – Creation of new species through hybridization was thought to be common only in plants, and rare in animals.
• What is a human admixed embryo?
• Video of Mirror carp & Gold fish spawning at a fishing venue in France