MadSci Network: Genetics
Query:

Re: Chromosome number compatibility (eg crossbreeding viability)

Date: Thu May 31 21:38:36 2012
Posted By: Alex Brands, Post-doc/Fellow, Biological ciences, Lehigh University
Area of science: Genetics
ID: 1337258937.Ge
Message:

Hi BAM,

Whether or not two species can crossbreed depends on many factors. As it turns out, chromosome number is not the most critical. In theory, it should not really matter how many chromosomes the two parents have, if you are simply looking at the ability to produce offspring. However, if the two parents do have different numbers of chromosomes, any resulting offspring will most likely be sterile.

The classic example of this is the mule, which is the product of a female horse (64 chromosomes) mating with a male donkey (62 chromosomes). During meiosis, the horse produces gametes with 32 chromosomes, and the donkey produces gametes with 31 chromosomes. When the gametes combine, the result is a mule with 63 chromosomes. While horses and donkeys are clearly different, they are, from a genetic perspective, quite similar. That is, they carry basically the same set of genes, (but with some subtle variations in those genes that accounts for the physical differences between them). The more important difference is that those genes are organized onto different numbers of chromosomes. For the developing mule, this is not a problem. The 32 chromosomes from the mother combined with the 31 chromosomes from the father each represent a complete haploid set of genetic information, and combine to form a complete diploid set of genetic information. For the cells of the mule to get along with their lives, that is enough, it does not matter what the arrangement of chromosomes is. The problem is with meiosis, when that genetic information gets reshuffled and split up to form gametes. Without going into a detailed lesson on meiosis, I’ll just say that having one set of information arranged into 31 chromosomes and another set arranged into 32 chromosomes creates problems when it comes to making gametes. Briefly, the problem comes when one chromosome winds up partially pairing with two or more other chromosomes.

There are many examples of hybrid animals resulting from parents with different chromosome numbers. So why can’t any two animals hybridize? There are many barriers to hybridization, and these are broadly classified as prezygotic and postzygotic.

Prezygotic barriers prevent the zygote from ever forming. This could be because the two species occupy different habitats, so they never meet. It could be because they mate at different times of year. It could be because they do not recognize each other as potential mates, due to different mating rituals. Those barriers can be overcome by intervention on the part of humans, perhaps by introducing the two species in a zoo, or by artificial insemination. Another prezygotic barrier results from a lack of biochemical recognition between egg and sperm. In this case, even of the two species meet and mate, there is no fusion between egg and sperm, so no zygote forms, even if the gametes are mixed together in a test tube. This can sometimes be overcome by stripping the egg of its protective coating, or injecting the sperm directly into the egg.

Postzygotic barriers refer to problems that occur after formation of the zygote, and are the result of some kind of genetic incompatibility that affects development of the embryo. In some cases, the embryo does not develop, in other cases, offspring may be produced, but they are short lived or otherwise compromised. The genetic interactions in these cases are probably enormously complicated and vary from case to case. I don’t know of any example where the basis of the genetic incompatibility has been studied to any extent.

Dr. Alex Brands


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