Selfish Genes and Sex Death

A talk given by Tom Price in November 2018

            Genes co-operate to create a range of cells and systems, as well as helping animals to co-operate in hives or packs. But sometimes genes cheat the system. An example of one of these genes is paternal sex drive in Nasonia, a genus of parasitic wasps. Males have a single copy of chromosomes and females have two, meaning unfertilised eggs become males. The paternal SR (sex ratio) gene is an extra chromosome that is ‘floating’ around and is known as a selfish gene. When males and females mate, it destroys all the male chromosomes leaving only itself. The egg then become male as it only has the 1 set of chromosomes but does have the additional SR chromosome to pass onto the next generation. It destroys the fitness of all genes in the males except itself.

            Diploid organisms have 2 copies of chromosomes that separate during gamete production. This means only one chromosome from each homologous pair will get into the gamete. If one of the pairs can destroy the other pair, then only that chromosome will be passed on. This essentially doubles the fitness of the chromosome. Sometimes the dominant, or driving, chromosome will destroy the sperm that do not have the dominant chromosome in them. In gamete killing drive it I usually the male chromosomes that kill other chromosomes.

            In some populations of fruit flies, specifically Drosophila simulans, there is an x chromosome meiotic drive in the males. The x chromosome destroys the y chromosomes in gametes meaning any females that mate with males that have the drive will produce only females. This causes an in-balanced sex ratio of males to females. The drive was discovered in 2001 in a small island off of Madagascar, by 2016 it had reached Morocco and was found all over Africa. There was a 2^nd wave of repressor genes that emerged which could counteract the ratio in-balance and reset the ratio. This event showed that drivers can spread rapidly over large distances and cause rapid evolution.

            An experiment was performed using Drosophila pseudoobscura to determine if females selected males without the gene. It was determined they could not, and the males lose around half of the sperm cells. This means in competition with males that do not have the gene, they will lose out and parent very little offspring in the population. It was discovered populations that showed high polyandry showed a decline in the population percentage with the gene.

            Sometimes there is not always a repressor gene that surfaces. In Drosophila subobscura in North Africa, the populations are monandrous, but the drive does not spread like in D. simulans. Currently we have no idea why. This shows that although we understand much more about how genes work than we did a hundred years ago, there is still so much more we can learn from studying them