Inbreeding reveals mode of past selection: stabilizing selection for sperm length but directional selection for sperm competition success and male attractiveness in Drosophila melanogaster. Outi Ala-Honkola^1,2, David Hosken³, Mollie Manier², Stefan Lüpold², Elizabeth Droge-Young², Kirstin Berben², William Collins², John Belote², Scott Pitnick². 1) Biological and Environmental Science, University of Jyväskylä, Jyväskylä, Finland; 2) Department of Biology, Syracuse University, Syracuse, NY, USA 13244; 3) Centre for Ecology and Conservation, College of Life and Environmental Sciences, University of Exeter, UK.

   Directional dominance is a prerequisite of inbreeding depression. Directionality arises because selection fixes alleles increasing fitness and eliminates dominant deleterious alleles, whereas deleterious recessive alleles are hidden from selection and hence maintained at low frequencies. Thus, directional dominance should be high for traits under directional selection, but low for traits under stabilizing selection or for traits weakly linked to fitness, and hence such traits 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 use it to 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 competing male sperm and quantification of characteristics of ejaculate composition, performance and fate. We found that male attractiveness (i.e., mating latency) and competitive fertilization success (i.e., P2) both show some inbreeding depression, suggesting they have been directional selection, whereas sperm length showed no inbreeding depression suggesting it has been under stabilizing selection. Despite having measured several sperm quality (sperm viability in female reproductive tract, offspring viability, in vivo sperm swimming speed) and quantity (ejaculate size, the number of sperm in storage) traits, we were unable to discern the mechanism underlying the lowered competitive fertilization success of inbred males.