Inbreeding reveals mode of past selection on male reproductive characters in Drosophila melanogaster

Abstract

Directional dominance is a prerequisite of inbreeding depression. Directionality arises when selection drives alleles that increase fitness to fixation and eliminates dominant deleterious alleles, while deleterious recessives are hidden from it and maintained at low frequencies. Traits under directional selection (i.e., fitness traits) are expected to show directional dominance and therefore an increased susceptibility to inbreeding depression. In contrast, traits under stabilizing selection or weakly linked to fitness are predicted to exhibit little-to-no inbreeding depression. Here, we quantify the extent of inbreeding depression in a range of male reproductive characters and then infer the mode of past selection on them. The use of transgenic populations of Drosophila melanogaster with red or green fluorescent-tagged sperm heads permitted in vivo discrimination of sperm from competing males and quantification of characteristics of ejaculate composition, performance, and fate. We found that male attractiveness (mating latency) and competitive fertilization success (P2) both show some inbreeding depression, suggesting they may have been under directional selection, whereas sperm length showed no inbreeding depression suggesting a history of stabilizing selection. However, despite having measured several sperm quality and quantity traits, our data did not allow us to discern the mechanism underlying the lowered competitive fertilization success of inbred (f = 0.50) males.

Keywords: Attractiveness; Drosophila melanogaster; inbreeding depression; past selection; sperm competition; sperm length.

Figure 1

Drosophila melanogaster lines with red or green fluorescently tagged sperm heads allowed us to distinguish between the ejaculates of two males in competition within the female reproductive tract (here inside female seminal receptacle).

Figure 2

Breeding design to create flies with different inbreeding coefficients.

Figure 3

Proportion of offspring sired by the second male to mate (P₂; mean ± SE) when second males were inbred to different degrees and first males were outbred competitor males (N = 48 in f = 0 and f = 0.5, N = 46 in f = 0.25).