Do female Drosophila melanogaster adaptively bias offspring sex ratios in relation to the age of their mate?

Abstract

Modification of offspring sex ratios in response to parental quality is predicted when the long-term fitness returns of sons and daughters differ. One factor that may influence a mother's sex allocation decision is the quality (or attractiveness) of her mate. We investigated whether the sex ratios of offspring produced by female Drosophila melanogaster are biased with respect to the age of the males to which they are mated, and whether there is an adaptive basis for this phenomenon. We found that females mated to old males (13 d post-eclosion) initially produced a greater proportion of daughters than did females mated to young males (1 d post-eclosion). This pattern does not appear to be due to a systematic difference in the numbers or mortality of the X- and Y-bearing sperm originating from old and young fathers, as the overall sex ratios of all offspring produced from a single copulation did not differ between broods fathered by the two types of males. The sons of older males fared worse in competitive mating assays than did the sons of younger males, while daughters of old and young males were of comparable fitness. These results suggest that there is an adaptive basis for the observed sex ratio modification.

Figure 1

Mean sex ratios (±s.e.) of offspring produced in the first 24 h following copulation by groups of female D. melanogaster that were mated to males of various ages. For each column, the number of vials is shown in brackets.

Figure 2

Mean sex ratios (±s.e.) of offspring produced in the first 18 h following copulation by individual female D. melanogaster that were mated to either young (1 d post-eclosion) or old (13 d post-eclosion) males in two replicate laboratory fly populations (LH_M and LH_M-b). Columns that share a letter are not significantly different from each other according to a Tukey's HSD test. For each column, the number of test tubes is shown in brackets. See table 1 for effects of mate age and population origin on offspring sex ratios.

Figure 3

The mean proportion of broods (±s.e.) laid by LH_M-b females that are predominantly red-eyed from each of two combinations where both red-eyed and brown-eyed males of different sire age compete for fertilizations (i.e. old-LH_M versus young LH_M-b and young-LH_M versus old-LH_M-b).

Figure 4

Fecundity of females (mean eggs laid per female over 18 h±s.e.) that were sired by either young (1 d post-eclosion) or old (13 d post-eclosion) fathers in two replicate laboratory fly populations (LH_M and LH_M-b). Columns that share a letter are not significantly different from each other according to a Tukey's HSD test.