Sex-biased mortality associated with inbreeding in Drosophila melanogaster

Abstract

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.

Figure 1

Predicted sex ratio. Predicted relationship between sex ratio and autosomal inbreeding coefficient in the two pedigrees. The high X–inbreeding pedigree is shown as black diamonds. The low X-inbreeding pedigree is shown as white squares.

Figure 2

Sex ratio. Variation in sex ratio between the two pedigrees plotted against autosomal inbreeding coefficient. The high X-inbreeding pedigree is shown as black diamonds with black error bars. The low X-inbreeding pedigree is shown as white squares with grey error bars. Data has not been corrected for block. Error bars indicate 95% confidence intervals.

Figure 3

Female mortality. Variation in female mortality between the two pedigrees plotted against autosomal inbreeding coefficient. The high X-inbreeding pedigree is shown as black diamonds with black error bars. The low X-inbreeding pedigree is shown as white squares with grey error bars. Data has not been corrected for block. Error bars indicate 95% confidence intervals.

Figure 4

Male mortality. Variation in male mortality between the two pedigrees plotted against autosomal inbreeding coefficient. The high X-inbreeding pedigree is shown as black diamonds with black error bars. The low X-inbreeding pedigree is shown as white squares with grey error bars. Data has not been corrected for block. Error bars indicate 95% confidence intervals.

Figure 5

Pedigree crossing design. All individuals without origins within the pedigrees were sourced from the mass population. Females marked with a * indicate the focal female group in each pedigree, with which males were crossed to produce the desired levels of inbreeding. The blue, green and brown colourings trace the inheritance of X-chromosomes that may be shared between the focal females and their mates. Same pair indicates that this same pair of individuals was shared in common between the two pedigrees.