Hybrid (biology)

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Offspring of cross-species reproduction

A mule is a sterile hybrid of a male donkey and a female horse. Mules are smaller than horses but stronger than donkeys, making them useful as pack animals

.

In biology, a hybrid is the offspring resulting from combining the qualities of two organisms of different breeds, varieties, species or genera through sexual reproduction. Generally, it means that each cell has genetic material from two different organisms, whereas an individual where some cells are derived from a different organism is called a chimera.^[1] Hybrids are not always intermediates between their parents (such as in blending inheritance), but can show hybrid vigor, sometimes growing larger or taller than either parent. The concept of a hybrid is interpreted differently in animal and plant breeding, where there is interest in the individual parentage. In genetics, attention is focused on the numbers of chromosomes. In taxonomy, a key question is how closely related the parent species are.

Species are

fertilization and others after it. Similar barriers exist in plants, with differences in flowering times, pollen vectors, inhibition of pollen tube growth, somatoplastic sterility, cytoplasmic-genic male sterility and the structure of the chromosomes. A few animal species and many plant species, however, are the result of hybrid speciation, including important crop plants such as wheat

, where the number of chromosomes has been doubled.

breeds (usually called cultivars in reference to plants); crossbreeds between them (without any wild stock

) are sometimes also imprecisely referred to as "hybrids".

Neanderthals

and anatomically modern humans are thought to have interbred as recently as 40,000 years ago.

Mythological hybrids appear in human culture in forms as diverse as the Minotaur, blends of animals, humans and mythical beasts such as centaurs and sphinxes, and the Nephilim of the Biblical apocrypha described as the wicked sons of fallen angels

and attractive women.

Etymology

Liger

, a lion/tiger hybrid bred in captivity

The term hybrid is derived from Latin hybrida, used for crosses such as of a tame sow and a wild boar. 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.

portmanteau words, starting in the 1920s with the breeding of tiger–lion hybrids (liger and tigon).^[3]

As seen by different disciplines

Animal, and plant breeding

From the point of view of animal, and plant breeders, there are several kinds of hybrid formed from crosses within a species, such as between different

breeds.^[4]

Single cross hybrids result from the cross between two

recessive. Typically, the F1 generation is also phenotypically homogeneous, producing offspring that are all similar to each other.^[5]

Double cross hybrids result from the cross between two different F1 hybrids (i.e., there are four unrelated grandparents).[6] 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.^[7] Top cross (or "topcross") hybrids result from the crossing of a top quality or pure-bred male and a lower quality female, intended to improve the quality of the offspring, on average.^[8]

Population hybrids result from the crossing of plants or animals in one population with those of another population. These include interspecific hybrids or crosses between different breeds.^[9] In biology, the result of crossing of two populations is called a synthetic population.^[10]

In horticulture, the term stable hybrid is used to describe an annual plant that, if grown and bred in a small monoculture free of external pollen (e.g., an air-filtered greenhouse) produces offspring that are "true to type" with respect to phenotype; i.e., a true-breeding organism.^[11]

Biogeography

Further information: Hybrid zone

Hybridization can occur in the hybrid zones where the geographical ranges of species, subspecies, or distinct genetic lineages overlap. For example, the butterfly Limenitis arthemis has two major subspecies in North America, L. a. arthemis (the white admiral) and L. a. astyanax (the red-spotted purple). The white admiral has a bright, white band on its wings, while the red-spotted purple has cooler blue-green shades. Hybridization occurs between a narrow area across New England, southern Ontario, and the Great Lakes, the "suture region". It is at these regions that the subspecies were formed.^[12] Other hybrid zones have formed between described species of plants and animals.

Genetics

Oenothera lamarckiana is a permanent natural hybrid, studied intensively by the geneticist Hugo de Vries

. Illustration by De Vries, 1913.

From the point of view of genetics, several different kinds of hybrid can be distinguished.[13]^[14] A genetic hybrid carries two different

Oenothera lamarckiana,^[15] because all homozygous combinations are lethal.^[13][14] In the early history of genetics, Hugo de Vries supposed these were caused by mutation.^[16][17]

Genetic complementation

Genetic complementation is a hybridization test widely used in genetics to determine whether two separately isolated mutants that have the same (or similar) phenotype are defective in the same gene or in different genes (see Complementation (genetics) article).^[18] If a hybrid organism containing the genomes of two different mutant parental organisms displays a wild type phenotype, it is ordinarily considered that the two parental mutant organisms are defective in different genes. If the hybrid organism displays a distinctly mutant phenotype, the two mutant parental organisms are considered to be defective in the same gene. However, in some cases the hybrid organism may display a phenotype that is only weakly (or partially) wild-type, and this may reflect intragenic (interallelic) complementation.

Taxonomy

See also: Hybrid name

From the point of view of taxonomy, hybrids differ according to their parentage. Hybrids between different

between chickens and guineafowl or pheasants, are reliably described but extremely rare.^[23] Interordinal hybrids (between different orders) are few, but have been engineered between the sea urchin Strongylocentrotus purpuratus (female) and the sand dollar Dendraster excentricus (male).^[24]

Biology

Expression of parental traits

Hybrid between Lady Amherst's pheasant (Chrysolophus amherstiae) and another species, probably golden pheasant

(Chrysolophus pictus)

When two distinct types of organisms breed with each other, the resulting hybrids typically have intermediate traits (e.g., one plant parent has red flowers, the other has white, and the hybrid, pink flowers).[25] 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).^[25]

Mechanisms of reproductive isolation

Main article: Reproductive isolation

Interspecific hybrids are bred by mating individuals from two species, normally from within the same genus. The offspring display traits and characteristics of both parents, but are often

chromosomes, horses have 64 chromosomes, and mules or 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.^[27]

A variety of mechanisms limit the success of hybridization, including the large genetic difference between most species. Barriers include morphological differences, differing times of fertility, mating behaviors and cues, and physiological rejection of sperm cells or the developing embryo. Some act before fertilization; others after it.[28]^[29][30][31]

In plants, some 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.^[32]

Speciation

wild emmer wheat, which is a hybrid of two diploid wild grasses, Triticum urartu and a wild goatgrass such as Aegilops searsii or Ae. speltoides.^[33]

Main articles: Hybrid speciation and Hybrid zone

A few animal species are the result of hybridization. The

gray wolf and the coyote,^[34] although its taxonomic status has been a subject of controversy.^[35][36][37] The European edible frog is a semi-permanent hybrid between pool frogs and marsh frogs; its population requires the continued presence of at least one of the parent species.^[38] Cave paintings indicate that the European bison is a natural hybrid of the aurochs and the steppe bison.^[39][40]

diploid), so hybridization events likely involved the doubling of chromosome sets, causing immediate genetic isolation.^[41]

Hybridization may be important in speciation in some plant groups. However, homoploid hybrid speciation (not increasing the number of sets of chromosomes) may be rare: by 1997, only eight natural examples had been fully described. Experimental studies suggest that hybridization offers a rapid route to speciation, a prediction confirmed by the fact that early generation hybrids and ancient hybrid species have matching genomes, meaning that once hybridization has occurred, the new hybrid genome can remain stable.^[42]

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. Recently DNA analysis of a bear shot by a hunter in the Northwest Territories confirmed the existence of naturally occurring and fertile grizzly–polar bear hybrids.^[43]

Hybrid vigour

Hybrid vigour: Salvia jurisicii x nutans hybrids (top centre, with flowers) are taller than their parents Salvia jurisicii (centre tray) or Salvia nutans

(top left).

Main article: Heterosis

Hybridization between reproductively isolated species often results in hybrid offspring with lower fitness than either parental. However, hybrids are not, as might be expected, always intermediate between their parents (as if there were blending inheritance), but are sometimes stronger or perform better than either parental lineage or variety, a phenomenon called heterosis, hybrid vigour, or heterozygote advantage. This is most common with plant hybrids.^[44] A transgressive phenotype is a phenotype that displays more extreme characteristics than either of the parent lines.^[45] 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.^[46]

In a hybrid, any trait that falls outside the range of parental variation (and is thus not simply intermediate between its parents) is considered heterotic. Positive heterosis produces more robust hybrids, they might be stronger or bigger; while the term negative heterosis refers to weaker or smaller hybrids.

Chrysolophus pictus).^[48] Spurs are absent in hybrids of the former type, although present in both parents.^[49]

Human influence

Anthropogenic hybridization

Hybridization is greatly influenced by human impact on the environment,

biological control, and unintentionally, as with accidental escapes of individuals. Introductions can drastically affect populations, including through hybridization.^[14][52]

Management

Examples of hybrid flowers from hybrid swarms of Aquilegia pubescens and Aquilegia formosa

There is a kind of continuum with three semi-distinct categories dealing with anthropogenic hybridization: hybridization without introgression, hybridization with widespread introgression (backcrossing with one of the parent species), and

Global climate change is creating other changes such as difference in population distributions which are indirect causes for an increase in anthropogenic hybridization.^[50]

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 a population becomes a complete mixture, the goal becomes to conserve those hybrids to avoid their loss. Conservationists treat each case on its merits, depending on detecting hybrids within the population. It is nearly impossible to formulate a uniform hybridization policy, because hybridization can occur beneficially when it occurs "naturally", and when hybrid swarms are the only remaining evidence of prior species, they need to be conserved as well.[50]

Genetic mixing and extinction

Main article: Genetic pollution

Further information:

Genetic mixing and Introgression

Regionally developed

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.^[54][55]

Effect on biodiversity and food security

high-yielding varieties, along with increased reliance on inputs of fertilizers, pesticides, and irrigation.^[56]

In agriculture and

Mammals

Familiar examples of

The first known instance of hybrid speciation in marine mammals was discovered in 2014. The clymene dolphin (Stenella clymene) is a hybrid of two Atlantic species, the spinner and striped dolphins.^[64] In 2019, scientists confirmed that a skull found 30 years earlier was a hybrid between the beluga whale and narwhal, dubbed the narluga.^[65]

Birds

Cagebird breeders sometimes breed bird hybrids known as

Among amphibians, Japanese giant salamanders and Chinese giant salamanders have created hybrids that threaten the survival of Japanese giant salamanders because of competition for similar resources in Japan.^[67]

Fish

Among fish, a group of about fifty natural hybrids between Australian blacktip shark and the larger common blacktip shark was found by Australia's eastern coast in 2012.^[68]

Russian sturgeon and American paddlefish were hybridized in captivity when sperm from the paddlefish and eggs from the sturgeon were combined, unexpectedly resulting in viable offspring. This hybrid is called a sturddlefish.^[69][70]

Cephalochordates

The two genera Asymmetron and Branchiostoma are able to produce viable hybrid offspring, even if none have lived into adulthood so far, despite the parents' common ancestor living tens of millions of years ago.^[71][72]

Insects

Among insects, so-called

The Colias eurytheme and C. philodice butterflies have retained enough genetic compatibility to produce viable hybrid offspring.^[74] Hybrid speciation may have produced the diverse Heliconius butterflies,^[75] but that is disputed.^[76]

The two closely related harvester ant species Pogonomyrmex barbatus and Pogonomyrmex rugosus have evolved to depend on hybridization. When a queen fertilizes her eggs with sperm from males of her own species, the offspring is always new queens. And when she fertilizes the eggs with sperm from males of the other species, the offspring is always sterile worker ants (and because ants are haplodiploid, unfertilized eggs become males). Without mating with males of the other species, the queens are unable to produce workers, and will fail to establish a colony of their own.^[77]

• 

  A "zonkey", a zebra/donkey hybrid

• 

  A "

  jaglion

  ", a jaguar/lion hybrid

• 

  A

  domestic canary/goldfinch hybrid

In plants

The London plane, Platanus × acerifolia

, is a natural hybrid, popular for street planting.

Plant species hybridize more readily than animal species, and the resulting hybrids are fertile more often. Many plant species are the result of hybridization, combined with polyploidy, which duplicates the chromosomes. Chromosome duplication allows orderly meiosis and so viable seed can be produced.^[78]

Plant hybrids are generally given names that include an "×" (not in italics), such as Platanus × acerifolia for the London plane, a natural hybrid of P. orientalis (oriental plane) and P. occidentalis (American sycamore).^[79][80] The parent's names may be kept in their entirety, as seen in Prunus persica × Prunus americana, with the female parent's name given first, or if not known, the parent's names given alphabetically.^[81]

Plant species that are genetically compatible may not hybridize in nature for various reasons, including geographical isolation, differences in flowering period, or differences in pollinators. Species that are brought together by humans in gardens may hybridize naturally, or hybridization can be facilitated by human efforts, such as altered flowering period or artificial pollination. Hybrids are sometimes created by humans to produce improved plants that have some of the characteristics of each of the parent species. Much work is now being done with hybrids between crops and their wild relatives to improve disease resistance or climate resilience for both agricultural and horticultural crops.^[82]

Some

• 

  A sterile hybrid between Trillium cernuum and T. grandiflorum^{[citation needed]}

• 

  An ornamental lily hybrid known as Lilium 'Citronella'^[90]

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, which leaves them unable to produce chromosomally balanced