AGB 211: Livestock and Poultry Breeding (2+1)

SPECIES HYBRIDISATION

Hybrids can occur where the species are closely related for the egg and sperm to result in a viable embryo. Where the two species are very closely related, the hybrids may even been partially or fully fertile. Some hybrids are bred for curiosity or public display, others are bred by researchers involved in genetic researcher and a few occur naturally. Chimeras are not the same as hybrids. Hybrids have intermediate features and each cell is a mix of chromosomes from the parental species. Chimeras are a mix of genetically different cells to form a mosaic animal.

Crossing the species boundary

• Speciation (one species evolving into two) is usually a slow process. It is generally accepted that different species usually cannot mate and reproduce - this is called "reproductive isolation". The exception was closely related species which can produce hybrids, although those hybrids have reduced fertility.
• Sometimes, one species can split into two through behavioural isolation. Some individuals develop behaviour patterns which limit their choice of mates e.g. they might be attracted to certain colours or might be active at different times of day. Though they are fully capable of interbreeding with the other group, their different behaviours keep them apart. If their habitat became permanently overcast, those behaviour barriers would break down and they would interbreed freely; their hybrids might become new species.
• Another way reproductive isolation occurs is when fragments of DNA accidentally jump from one chromosome to another in an individual i.e., chromosomal translocation. The mutant individuals cannot reproduce except with other mutant individuals - not much good unless the individual has mutant siblings to mate with! There are also "master genes" which govern general body plan (Hox genes) and those which switch other genes on and off. A small mutation to a master gene can mean a sudden big change to the individuals that inherit that mutation. Sometimes, those radical mutations can "undo" generations of evolution so that two unrelated species can mate with each other and produce fertile young (only seen in micro-organisms).
• In mammals, hybrid White-Tail/Mule Deer don't inherit either parent's escape strategy (White Deer dash. Mule Deer bound) and are easier prey than the pure-bred parents. Another example is seen in Galapagos Finches. 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.

Mechanisms for keeping species separate

• Physical separation: the species live in different geographic locations or occupy different ecological niches in the same location and so never have the chance to meet each other.
  Temporal isolation: the species that mate during different seasons or different time of day and cannot breed together.
• Behavioral isolation: members of different species may meet each other, but do not mate because neither performs the correct mating ritual. Imprinting by fostering the young of one species on a female of the other species can overcome this in some cases.
• Mechanical isolation: copulation may be impossible because of incompatible size and shape of the reproductive organs.
• Morphological isolation: copulation may be impossible because of the difference in body size or shape.
• Gametic isolation: the sperm and egg may not fuse and hence fertilization cannot occur; if it does occur then the embryo fails to get past the first few cell division.

Haldan's rule

• Haldane's Rule states that in animal species whose gender is determined by sex chromosomes, when in the first cross offspring of two different animal species, one of the sexes is absent, rare or sterile, that sex is the heterogametic sex. The "heterogametic sex" is the one with two different sex chromosomes (e.g. X and Y); usually the male. The "homogametic sex" has two copies of one type of sex chromosome (e.g. X and X) and is usually the female.
• Haldane's Rule for Hybrid Sterility states that a race of animals could diverge enough to be considered separate species, but could still mate to produce healthy hybrid offspring in a normal ratio of males and females. If any of the hybrid offspring were sterile, the sterile offspring would be the heterogametic offspring (males). If the heterogametic offspring was fertile, it produced the normal 50:50 ratio of X and Y sperm.
• Haldane's Rule for Hybrid Inviability states that if the divergence between the species became large enough to generate genic differences, but not to prevent mating, then parental gene products may fail to co-operate during development of the embryo, resulting in hybrid inviability (the hybrids are aborted, stillborn or don't survive to maturity). In this case, the male to female ratio of hybrid offspring is skewed with more homogametic offspring while the heterogametic offspring (males) are absent or rare.

By crossing two different species, sometimes we get good individuals. The mule is a good example of a commercially important species hybrid. Mare x Jackal ass = Mule, She ass x stallion – Hinny. Male Mules are always sterile as for as it yet known. A few cases of fertile mare mule have however reported, but they are very rare. Hinny is generally inferior to Mule as a worth animals. Hinny is also sterile. Horse having 32 pairs and Ass 31 pairs. Mules comes to possess 63 chromosomes in all. The mare mules have given birth to mule foal and horse foal when bred to Jack and stallion respectively. The inference is that the mare follicles occassionally produce an egg containing nothing but horse chromosomes, and all of the Ass chromosomes have been extruded in the polar body. The fertile mare mules essentially function as mare as far as the genetics of the egg is concerned. If all the horse chromosome where extruded in the polar body the Mules will function genetically as assess. But no case of this sort has been reported. Pure breeding of Mules as such also theoretically impossible.

• European cattle and American Bison when crossed produce sterile Males and Fertile females. By Back crossing the females to Bison and Cattle attempts are being made to form a new breed of cattle called cattallo.
• Male Jackals only mate with domestic bitches if the Jackal pups are raised by a domestic bitch (to become imprinted on dogs). There is a psychological barrier, but the offspring are fertile (pre-zygotic barrier, but no post-zygotic barrier).
• Lions and Tigers must overcome behavioural (courtship) barriers, but produce fertile female offspring and sterile male offspring (pre-zygotic and post-zygotic barriers). Lions and leopards have some physical barriers (size), but these are overcome if the lioness lies on her side to let the leopard mount her; the male Leopons are sterile, though female offspring are fertile (pre-zygotic and post-zygotic barriers). In these cases, pre-zygotic barriers are overcome by rearing the two species together (in whales and dolphins this occurs naturally).

Some cases seem to need additional rules! In Beefalo, Domestic cows may have an immune response against Bison/Cow hybrid calves - this is a physiological barrier, but does not prevent conception. Bison cows don't have this immune response against hybrid calves and hybrid Beefalo males can be fertile. In some hybrids of domestic cats with small wildcats, a proportion of hybrid males are claimed to be partially fertile (incomplete post-zygotic barrier?) and though the hybrid females are fertile they may not successfully raise their young - a psychological barrier, but one which does not prevent mating/conception.

By crossing the two different species, sometimes good, visible individuals are produced. The mule is a good example of species hybridisation.

Several other species hybrids have been produced. Some of them are

S.No.	Hybrids	Sire	Dam	Remarks
1	Hinny	Stallion	Jennet	It is inferior to mule as a work animal and is also sterile
2	Zebroid	Zebra	Horse	Popular in tropics – docile – better disease and heat resistance
3	Cattalo	Cattle	Bison	Bison is known as American buffalo. Males are sterile and females are fertile. domestic bull/Bison cow crossings have a lower infant mortality rate (cow immune systems can reject hybrid calves)
4	Beefalo	American Bison	Domestic Cattle	Beefalo have been back-crossed to Bison and to domestic cattle; some of these resemble pied Bison with smooth coats and a maned hump. The aim is to produce high protein, low fat and low cholesterol beef on animals which have "less hump and more rump". Although Bison bull/domestic cow crossings are more usual,
5	Pien niu	Cattle	Yak	Found in Tibet.
6	Goep	Goat	Sheep	Sheep and goat are not so closely related. When crosses are made between them fertilization sometimes takes place. However the embroys die before parturition and are resorbed or aborted.
7	Zubron	Domestic cattle	Wisent (European Bison, Bison bonasus).	Zubron was considered as a possible replacement for domestic cattle as they were durable and resistant to many cattle diseases. They also thrived on poor pasture, in harsh weather and with minimal husbandry. First generation Zubron males are infertile and cannot be used for breeding, but the females are fertile and may be bred back (back-crossed) to either Wisent or to domestic bulls. Males from these back-crosses are fertile.
8	Yakalo	Bison (American "Buffalo")	Domestic Tibetan Yak	In Nepal, Yak/Cow hybrids are bred using Yak bulls on domestic cows or, less often, domestic bulls on Yak cows. The Yak-Cow females are fertile, the males are sterile and the meat is considered superior to beef. In Nepalese, the hybrid is called a Khainag or Dzo (male)/Dzomo (female). A Dzomo crossed with either a domestic bull or yak bull results in an Ortoom (three-quarter-bred) and an Ortoom crossed with a domestic bull or yak bull results in a Usanguzee (one eighth bred).
9	Geep	Goat embryo	Sheep embryo	Although often cited as a hybrid, the famous "Geep" is not a true goat/sheep hybrid, but was a laboratory experiment which fused a sheep embryo with a goat embryo (a type of animal called a chimera). The geep is a mosaic of mismatched goat and sheep parts; the parts which grew from the sheep embryo are woolly while those which grew from the goat embryo are hairy. Each set of cells kept their own species identity instead of being intermediate in type. It could be fertile, but will produce either goats or sheep depending on whether its reproductive organs grew from the goat embryo or from the sheep embryo.
10	Cama	Camel	Llama	Llama is a hybrid
11	Iron Age Pigs	American wild hogs	Tamworth pigs	Resemble early domestic pigs