Hybrid

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This article is about a biological term. For other uses, see Hybrid (disambiguation).

In biology, hybrid has two meanings.

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 use of "hybrid" is 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.

Ernst Mayr wrote of Gregor Mendel:

"He was uncertain about the nature of the kinds of peas he crossed, and, like most plant breeders, he called heterozygotes "hybrids". When he tried to confirm the laws he had found by using "other hybrids" that were actually real species hybrids, he failed. The use of the same term "hybrid" for two entirely different biological phenomena thwarted his later efforts." [1]

Some dog hybrids (used in sense two, above), are becoming increasingly popular and are being bred selectively. In cats, term feral-domestic hybrid is often used to indicate a cross between the domestic cat and a wild cat species (erroneous because a "feral" animal is a domestic animal that has reverted to the wild state).

An example of an intra-specific hybrid is a hybrid between a Bengal tiger and an Amur (Siberian) tiger.

Plant hybrids, especially, are often stronger than either parent variety, a phenomenon which when present is known as hybrid vigour (heterosis) or heterozygote advantage. Plant breeders make use of a number of techniques to produce hybrids.

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.

[edit] Interspecific hybrids

It has been suggested that Hybrid animals be merged into this article or section. (Discuss)

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 may be sterile. 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 normally 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 both female mules and hinnies has been reported with a donkey as the father.[2]

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.

In order to calculate the chromosome count of a hybrid, determine the haploid number of chromosomes from both animals and add them together. The resulting number is the diploid number of the hybrid. Example: an male donkey which has 62 chromosomes mates with a zebra, which has 44 chromosomes (all counts diploid). Divide the 62 chromosome by half (31) and add that number to the number of haploid zebra chromosomes, 22. The resulting number is 53, which is the number of the hybrid animal, a zeedonk.

Hybrids are 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.

Hybrids examples

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. The resulting cells are however not able to develop into a full organism. An example of interspecific hybrid cell lines is the humster (hamster x human) cells.

[edit] Hybrid plants

Plant species hybridize more readily than animal species, and the resulting hybrids are generally fertile. Plant hybrids are often created in order to select characteristics from both parents in order to create an improved plant, by improving production of seeds or fruit, or to make a plant hardier or more disease resistant.

Unlike animal hybrids, many plant hybrids may reproduce, though there still exist sterile hybrids. Many plant species are the result of hybridization and polyplodism. Many plant species easily will cross pollinate and produce viable seeds, the distinction between each species is often maintained by geographically isolation or difference's 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 because the hybrids can not find mates that will except them. 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 chromosmally ballanced gametes.[3] 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.

Some plant hybrids include:

Some natural hybrids are:

[edit] 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. 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.[4] 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.

[edit] Mythological 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:

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

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

  • Nephilim - the offspring of a fallen angel and human woman.
  • Hippogriff - the offspring of a griffin and a horse, typically a male griffin and a mare.

[edit] Etymology

The word came from Ancient Greek `υβριδης, literally meaning "son of outrageous conduct", and used to mean "sheep-goat hybrid".

[edit] See also

[edit] References

  1. ^ Mayr, Ernst (1997). This is Biology. Belknap Press, p58. 
  2. ^ McBeath S, Tan PP, Bai Q, Speed RM. (1988).
  3. ^ http://www.colorado.edu/MCDB/MCDB2150Fall/notes00/L0033.html
  4. ^ "Hybrid bear shot dead in Canada", BBC News, 2006-05-13.

[edit] External links

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