Sex-biased mortality associated with inbreeding in Drosophila melanogaster

BACKGROUND

One proposed consequence of inbreeding is a skewed sex ratio arising from sex specific mortality in the homogametic sex caused by inbreeding on the sex chromosome. However, recent work suggests that random distortions in sex ratio due to autosomal inbreeding may be of greater importance. In this study, we investigate the effect of biologically realistic levels of inbreeding on sex ratio and sex specific mortality in Drosophila melanogaster. We use two pedigree crossing designs to either maximise or minimise inbreeding on the X-chromosome whilst producing identical autosomal inbreeding.

RESULTS

We found increased female mortality and male biased sex ratios associated with inbreeding in our high, but not low, X-inbreeding pedigree. While our results are more consistent with being driven by inbreeding on the X-chromosome than on the autosomes, the marked difference between treatments does not fit closely the expectations of either model.

CONCLUSIONS

Our results are only partly consistent with the hypothesis that inbreeding on the X-chromosome can cause greater fitness reductions in the homogametic sex. Whilst the results of our study are not conclusive, they suggest that directional distortions in sex ratio due to inbreeding can occur, and highlight the need for further investigation on this topic.

X-chromosomal heterosis in Drosophila melanogaster

Population cages were set up containing anX-chromosome balancer, and either a single wild-type chromosome(homozygous cages) or a mixture of wild-type chromosomes(heterozygous cages). The balancer chromosome was eliminated more rapidly from the heterozygous cages, indicating that chromosome heterozygotes are at an advantage over chromosome homozygotes. The disadvantage ofX-chromosome homozygosity in the female is estimated to be about 40%. From earlier studies it is known that the average disadvantage of homozygosity for either of the two major autosomes ofD. melanogasteris approximately 80%. Since these autosomes are both about twice as long as theXchromosome, the disadvantage per unit length is similar for both chromosomal types.

BothX-chromosomal and autosomal heterosis can be explained by either dominance or overdominance at individual loci. However, a dominance model can only explain the similarity if many of theX-linked loci (about 50%) are limited in expression to the female.