MadSci Network: Genetics

Re: What effect would such a low chromosome number have on sexual reproduction?

Date: Mon Oct 29 06:56:06 2001
Posted By: Rob Cruickshank, Post-doc/Fellow
Area of science: Genetics
ID: 1003156373.Ge

Hi Andrew,

There are two aspects to your question; haplodiploidy and low chromosome number. Putting aside the issue of haplodiploidy for a moment, on the face of it chromosome number appears to have very little effect on reproduction. The classic examples are Indian and Chinese muntjac. The Indian muntjac has 6 pairs of chromosomes and the Chinese muntjac has 46 pairs. The total length of the chromosomes is about the same, so the only major difference between the genomes of these closely related species is the number of chromosomes they are partitioned into. This appears to have very little effect on their reproductive biology.

However, there are some possible effects of low chromosome number. One effect would be a decrease in the variability of the gametes (sperm and eggs). As I'm sure you know, gametes are produced by a process called meiosis. This process generates genetic variety amongst the gametes by two processes: [1] recombination between homologous chromosomes (also called crossing-over) and [2] random assortment of chromosome pairs. If there were only a single pair of chromosomes the second of these mechanisms would not operate. This would decrease the variability of the gametes.

A decrease in gamete variability will be correlated with a decrease in offspring variability. This will have consequences for natural selection since selection requires variability on which to act. A decrease in offspring variability would decrease the rate at which a species will become adapted to its environment. In rapidly changing environments this may lead to the extinction of the species (i.e. there may be species selection against low chromosome number). However, this effect is likely to be small, since variability will continue to be generated by mutation and recombination.

Another effect of having a single pair of chromosomes is that non- disjunction during meiosis would lead to triploid rather than aneuploid offspring. I'll leave you to figure out the possible consequences of this.

Returning to the question of haplodiploidy, there is a large literature on this and I won't elaborate any further here. For an introduction to this literature I suggest you start with The Narrow Roads of Gene Land, Volume 1 by the late Bill Hamilton.

Here are links to a few web sites that may be also be of interest...

Chromosomal evolution
Evolution of chromsome number in plants
Changes in chromosome number during evolution


Rob Cruickshank

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