Thursday, December 29, 2005

O'Dea J. D. (2006). Did Conflict Between Chromosomes Drive the Evolution of Sex? Calodema, 8: 33-34.

Did Conflict Between Chromosomes Drive the Evolution of Sex?

J.D. O'Dea
Visiting Fellow, School of Botany and Zoology, Australian National University

One of the most prominent of the many theories on why sex evolved is that it provides constant genetic recombination, which will allow more rapid evolution of new types of parasite resistance (Hamilton et al., 1990). Hamilton was thinking about multicellular parasites such as worms. But genetic elements such as chromosomes can also be parasitic and conflict within the genome is currently a hot topic in biology (Burt and Trivers, 2006). Putting some of these thoughts together, I propose that sex might have evolved to rid cells of worthless ("parasitic") genetic material.

Imagine a primitive cell with some good chromosomes and some ineffective chromosomes (that is, genetic material that is not advantageous to the cell but is merely "along for the ride"). How are the other chromosomes to solve this "free rider" problem? Suppose the effective chromosomes act together to override the ineffective chromosomes and force a process of recombination (that is, sex). If a new generation of cells end up with nothing but effective chromosomes, they will tend to proliferate more effectively than the older generation. Cells which contain mostly good chromosomes with one or two parasitic genetic elements or chromosomes and end up, after recombination, with a new small set of undesirable chromosomes are no worse off and will proliferate as quickly as before. But new generation cells that are the result of recombination events that result in a concentration of undesirable genetic elements or chromosomes may proliferate poorly or even die. The net effect: a "cleansing" of undesirable genetic elements, resulting from what is effectively a "warfare" between chromosomes.

This is not a "group selection" proposal. It merely requires "good" chromosomes to act in concert to override a smaller number of parasitic chromosomes to force recombination (sex), which could result in the effective expunging of the parasitic, non-contributing genetic material.

That is, sex may have evolved as a mechanism resulting from intragenomal conflict.

This suggestion in some ways resembles that of Kondrashov (1988), except that it places the emphasis not on competition between individuals but on competition between chromosomes or genetic material: that is, on intragenomic conflict. Also, the present suggestion might apply to the initial evolution of sex: other functions (such as that suggested by Hamilton in relation to resistance to parasites) might have helped maintain it once it had evolved in the first place.

A calculation:

It is possible to calculate how advantageous it would be to a functional chromosome to have the cell in which it resides (along with a parasitic non-functional chromosome) recombine with another similar cell. If one assumes (for the sake of calculation) that both cells contain three functional chromosomes and one parasitic chromosome, the following are the results to be expected from sexual recombination of the chromosomes - with two new cells resulting from the random mutual exchange of half of their chromosomes (that is two from each cell):

For a functional chromosome, following recombination, there are 6/18 chances of an improvement in its situation (that is ending up in a cell with fewer parasitic chromosomes). There are 9/18 chances of no change (ending up in a cell with the same number of parasitic chromosomes). And there are 3/18 chances of ending up in a new cell with more parasitic chromosomes. In short, recombination provides a greater chance of ending up in a healthier cell than in a less healthy cell - that is, of ending up in a cell with less parasitic, useless genetic material (parasitic chromosomes).

In contrast, for a parasitic chromosome, recombination will never improve its situation. There is a 9/18 chance of no change and a 9/18 chance of actually ending up in a new cell with a higher level of parasitic material. Such a cell is likely to be less viable and may even die, destroying the original parasitic chromosome with it.

In conclusion, it will have been to the advantage of normal, functioning chromosomes to code for and promote sexual recombination with other cells. Therefore, sex will have evolved.

Burt, A. and Trivers, R. (2006) Genes in Conflict: The Biology of Selfish Genetic Elements. The Belknap Press of Harvard University Press, Cambridge, Massachusetts and London, England.

Hamilton, W. D. et al. (1990) "Sexual reproduction as an adaptation to resist parasites". Proceedings of the National Academy of Sciences (PNAS) 87:3566-3573.

Kondrashov, A. S. (1988) "Deleterious mutations and the evolution of sexual reproduction". Nature 336:435-440.

An updated, clearer version of this paper can be found at:


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