Hybrid (biology)

In biology, hybrid has two meanings.^[1] The first meaning is the result of interbreeding between two animals or plants of different taxa. Hybrids between different species within the same genus are sometimes known as interspecific hybrids or crosses. Hybrids between different sub-species within a species are known as intra-specific hybrids. Hybrids between different genera are sometimes known as intergeneric hybrids. Extremely rare interfamilial hybrids have been known to occur (such as the guineafowl hybrids). The second type of "hybrid" are crosses between populations, breeds or cultivars within a single species. This second meaning is often used in plant and animal breeding. In plant and animal breeding, hybrids are commonly produced and selected because they have desirable characteristics not found or inconsistently present in the parent individuals or populations. This rearranging of the genetic material between populations or races is often called hybridization.

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Types of hybrids

There are a number of different types of hybrids; ^[2]

Single cross hybrids - result from the cross between two pure bred lines and produces an F1 (Filial 1, meaning "first offspring) generation called an F1 hybrid. The cross between two different homozygous lines produces a F1 hybrid that is heterozygous; having two alleles, one contributed by each parent and typically one is dominant and the other recessive. The F1 generation is also homogeneous, producing offspring that are all similar to each other.

Double cross hybrids - result from the cross between two different F1 hybrids.^[3]

Three-way cross hybrids - result from the cross between one parent that is a F1 hybrid and the other is from an inbred line.^[4]

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 another population. These include crosses between organisms such as interspecific hybrids or crosses between difference races.

Interspecific hybrids

An example of an intraspecific hybrid is a hybrid between a Bengal tiger and an Amur (Siberian) tiger. 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 very often are sterile; thus, hybrid sterility prevents the movement of genes from one species to the other, keeping both species distinct.^[5] Sterility is often attributed to the different number of chromosomes the two species have, 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 the extra chromosome cannot make a homologous pair at 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.^[6] Most often other mechanisms are used by plants and animals to keep gametic isolation and species distinction. Species often have different mating or courtship patterns or behavours, 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. The Lonicera fly is the first known animal species that resulted from natural hybridization. Until the discovery of the Lonicera fly, this process was known to occur in nature only 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, 1/2^(2×2)=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.

Hybrids are often named by the portmanteau method, combining the names of the two parent species. For example, a zeedonk is a cross between a zebra and a donkey. Since the traits of hybrid offspring often vary depending on which species was mother and which was father, it is traditional to use the father's species as the first half of the portmanteau. For example, a liger is a cross between a male lion and a female tiger, while a tigon is a cross between a male tiger and a female lion.

Examples of hybrid animals

• dog hybrids are crosses between different breeds and are often bred selectively.
• 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)
• 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.
  • Zubron, a hybrid between Wisent (European Bison) and domestic cow.
• Sheep-goat hybrids, such as the The Toast of Botswana.
• Ursid hybrids, such as the Grizzly-polar bear hybrid, occur between black bears, brown bears, Kodiak and polar bears.
• Felid hybrids
  • Savannah cats are the hybrid cross between an African serval cat and a Domestic cat
  • Ligers and Tigons (crosses between a Lion and a Tiger) and other Panthera hybrids such as the Lijagulep. Various other wild cat crosses are known involving the Lynx, Bobcat, Leopard, Serval, etc.
  • Bengal cat, a cross between the Asian Leopard cat and the domestic cat, one of many hybrids between the domestic cat and wild cat species. The domestic cat, African wild cat and European wildcat may be considered variant populations of the same species (Felis silvestris), making such crosses non-hybrids.
• Fertile Canid hybrids occur between coyotes, wolves, dingoes, jackals, foxes and domestic dogs.
• Hybrids between Black Rhinos & White Rhinos have been recognized.
• Hybrids between spotted owls and barred owls
• Cama, a cross between a Camel and a Llama, also an intergeneric hybrid.
• Wolphin, a fertile but very rare cross between a False Killer Whale and a Bottlenose Dolphin.
• A fertile cross between an albino King Snake and an albino Corn Snake.
• At Chester Zoo in the United Kingdom, a cross between African elephant (male) and Asian elephant (female). The male calf was named Motty. It died of gut infection after twelve days.
• Cagebird breeders sometimes breed hybrids between species of finch, such as Goldfinch x Canary. These birds are known as Mules.
• Gamebird hybrids, hybrids between gamebirds and domestic fowl, including Chickens, Guineafowl and Peafowl, interfamilial hybrids.
• Numerous Macaw hybrids are also known.
• Red Kite x Black Kite: 5 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.)
• 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 is genetic integrity. [2]
• Blood parrot cichlid, which is probably created by crossing a Gold Severum and a Midas Cichlid or Red Devil Cichlid

Hybrids should not be confused with chimaeras such as the chimera between sheep and goat known as the geep. Wider interspecific hybrids can be made via in vitro fertilization or somatic hybridization, however the resulting cells are not able to develop into a full organism. An example of interspecific hybrid cell lines is the humster (hamster x human) cells.

Hybrid plants

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. Animals, being more mobile, have developed complex mating behaviors that maintain the species boundary and when hybrids do occur, natural selection tends to weed them out of the population since these hybrids generally can not find mates that will accept them or they are less adapted and fit for survival in their habitats. 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 hardy or improve its growth and/or appearance for use in horticulture. Much work is now being done with hybrids to produce more disease resistant plants for both agricultural and horticultural crops. In many groups of plants hybridization has been used to produce larger and more showy flowers and new flower colors.   Many plant genera and species have their origins in polyploidy. Autopolyploidy resulting from the sudden multiplication in the number of chromosomes in typical normal populations caused by unsuccessful separation of the chromosomes during meiosis. Tetraploids or plants with four sets of chromosomes 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 Oenthera gigas. Tetraploids can develop into a breeding population within the diploid population and when hybrids are formed with 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 hybrids. Usually the typical chromosome number is doubled in successful allopolyploid species, with four sets of chromosomes the genotypes can sort out to form a complete diploid set from the parent species, thus they can produce fertile offspring that can mate and reproduce with each other but can not back-cross with the parent species. Allopolyploidy in plants often gives them a condition called hybrid vigour, which results in plants that are larger and stronger growing than either of the two parent species. Allopolyploids are often more aggressive growing and can be invaders of new habitats. Sterility in a 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.^[7] While this is a negative in a crop such as wheat, when growing a crop which produces no seeds would be pointless, it is an attractive attribute in some fruits. Bananas and seedless watermelon, for instance, are intentionally bred to be triploid, so that they will produce no seeds. Many hybrids are created by humans, but natural hybrids occur as well. Plant hybrids, especially, are often stronger than either parent variety, a phenomenon which when present is known as hybrid vigour (heterosis) or heterozygote advantage.^[8] Plant breeders make use of a number of techniques to produce hybrids, including line breeding and the formation of complex hybrids.

Some plant hybrids include:

• Leyland Cypress, a hybrid between Monterey Cypress and Nootka Cypress.
• Limequat, lime and kumquat hybrid.
• Loganberry, a hybrid between raspberry and blackberry.
• London Plane, a hybrid between Oriental plane and American plane (American sycamore).
• Peppermint, a hybrid between spearmint and water mint.
• Tangelo, a hybrid of a Mandarin orange and a pomelo or a grapefruit which may have been developed in Asia about 3,500 years ago.
• Triticale, a wheat-rye hybrid.
• Wheat; most modern and ancient wheat breeds are themselves hybrids.

Some natural hybrids are:

• White Flag Iris, 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.

Some horticultural hybrids:

• Dianthus ×allwoodii, is a hybrid between Dianthus caryophyllus × Dianthus plumarius. This is an "interspecific hybrid" or hybrid between two species in the same genus.
• ×Heucherella tiarelloides, or Heuchera sanguinea × Tiarella cordifolia is an "intergeneric hybrid" a hybrid between two different genera.

Hybrids in nature

Hybridisation between two closely related species is actually a common occurrence in nature. 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 polar bear/grizzly bear hybrids.^[9] There have been reports of similar supposed hybrids, but this is the first to be confirmed by DNA analysis. In 1943, Clara Helgason described a male bear shot by hunters during her childhood. It was large and off-white with hair all over its paws. The presence of hair on the bottom of the feet suggests it was a natural hybrid of Kodiak and Polar bear.

In some species, hybridisation plays an important role in evolutionary biology. While most hybrids are disadvantaged as a result of genetic incompatibility, the fittest survive, regardless of species boundaries. They may have a beneficial combination of traits allowing them to exploit new habitats or to succeed in a marginal habitat where the two parent species are disadvantaged. This has been seen in experiments on sunflower species. Unlike mutation, which affects only one gene, hybridisation creates multiple variations across genes or gene combinations simultaneously. Successful hybrids could evolve into new species within 50-60 generations. This leads some scientists to speculate that life is a genetic continuum rather than a series of self-contained species.

Where there are two closely related species living in the same area, less than 1 in 1000 individuals are likely to be hybrids because animals rarely choose a mate from a different species (otherwise species boundaries would completely break down). In some closely related species there are recognized "hybrid zones".

Some species of Heliconius butterflies exhibit dramatic geographical polymorphism of their wing patterns, which act as aposematic signals advertising their unpalatability to potential predators. Where different-looking geographical races abut, inter-racial hybrids are common, healthy and fertile. Heliconius hybrids can breed with other hybrid individuals and with individuals of either parental race. These hybrid backcrosses are disadvantaged by natural selection because they lack the parental form's warning coloration, and are therefore not avoided by predators.

A similar case in mammals is hybrid White-Tail/Mule Deer. The hybrids don't inherit either parent's escape strategy. White-tail Deer dash while Mule Deer bound. The hybrids are easier prey than the parent species.

In birds, healthy Galapagos Finch hybrids are relatively common, but their beaks are intermediate in shape and less efficient feeding tools than the specialised beaks of the parental species so they lose out in the competition for food. Following a major storm in 1983, the local habitat changed so that new types of plants began to flourish, and in this changed habitat, the hybrids had an advantage over the birds with specialised beaks - demonstrating the role of hybridization in exploiting new ecological niches. If the change in environmental conditions is permanent or is radical enough that the parental species cannot survive, the hybrids become the dominant form. Otherwise, the parental species will re-establish themselves when the environmental change is reversed, and hybrids will remain in the minority.

Natural hybrids may occur when a species is introduced into a new habitat. In Britain, there is hybridisation of native European Red Deer and introduced Chinese Sika Deer. Conservationists want to protect the Red Deer, but evolution favors the Sika Deer genes. There is a similar situation with White-headed Ducks and Ruddy Ducks.

Hybrid expression

When two species or races breed with each other the resulting hybrids typically represent two different types; one is intermediate individuals that show characteristics between the two parents, and other individuals show combined characteristics of the two parents. Hybrids do not have any characteristics that are not already represented in the parents, thought they can show mixed combinations that do not exist within the parents. An intermediate hybrid from the cross between a plant with red flowers and one with white flowers, has pink or rose colored flowers.

Heterotic hydrides produce offspring that are different than the parents, that is they are not intermediate or combined in appearance. Positive heterosis produces more robust hybrids, they might be stronger, bigger or show greater variation; while negative heterosis tends to produces weaker or smaller hybrids that might show less variation than the parents.^[10] Heterosis is most common in plant hybrids but also occurs in other organisms too.

Since animal and plant populations over time produce individuals that conform to a standard range of expressions that are uniformly produced threw out the entire population, hybrid populations can often be surmised when individuals show a narrow conformity or extreme variation. When populations cross with each other, often the first generation produced are very uniform hybrids; and as those individuals breed with each other or back cross and breed with the original population, then next generation produced is highly variable. Field researches use these differences to ascertain whether a population is of hybrid origin or a population of a stable species.

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.^[11] 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 effecting the population to such a degree than none of the originally genetically distinct population remains.^[12][13].

Effect on biodiversity and food security

Main articles: biodiversity and food security

In agriculture and animal husbandry, the green revolutions 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.^[14] Since the indigenous breeds are often better adapted to local extremes in climate and have immunity to local pathogens this represents a significant genetic erosion of the gene pool for future breeding. Newer, genetically engineered (GE) varieties are be a problem for local biodiversity. Some of these plants contain designer genes that would be unlikely to evolve in nature, even with conventional hybridization.^[15][16] These may pass into the wild population with unpredictable consequences and may be detrimental for the success of future breeding programs.

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 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.^[17]

Mythical and legendary hybrids

In ancient folktales many fictional hybrids have become part of the mythological narrative. Many mythological creatures are simple composites of known animals:

• Basilisk and Cockatrice - both a combination of a cockerel and lizard or snake.
• Bonnacon - a mixture between a horse and a bull.
• Chimera - a fire breathing mixture between a goat, a snake, and a lion.
• Griffin - beast with the body of a lion and the head and wings of an eagle.
• Manticore - the face of a man, the body of a lion and the tail of a scorpion.
• Mermaid and Merman - half fish, half human.
• Satyr - the torso of a man, the legs and feet of a goat.

Some mythological hybrids were said to be the result of two species mixing.

• Centaur - the offspring of Centaurus and the mares of Thessaly. Has the body of a horse with its neck and head replaced by the torso and head of a man.
• Harpy - the torso of a woman with the wings and feet of a bird.
• Hippogriff - the offspring of a griffin and a horse, typically a male griffin and a mare.
• Minotaur - the offspring of Pasiphaë and a white bull. Has the body of a man and the head of a bull.
• Nephilim - the offspring of a fallen angel and human woman.

Etymology

The word has a Latin root: hybrida (or ibrida) which in turn is a loan-word related to Greek hubris (ὕβρις, meaning overbearing pride, outrage), and was meant to be "the offspring of a tame sow and wild boar", probably in interpreting such an offspring as a result of an outrageous "interracial miscegenation". The term entered into popular use in English in the 19th century, though examples of its use have been found from the early 17th century.^[18]

See also

References