Inbreeding depression in male traits and preference for outbred males in Poecilia reticulata

Inbreeding depression often affects male reproductive traits. Here, we measure inbreeding depression in sperm, body size, ornamental color patterns, and attractiveness in male guppies and ask whether different environmental conditions play a role in the severity of inbreeding depression. Male sperm traits and coloration were measured in inbred and outbred fish in 2 experiments under different environmental conditions (large mixed-sex greenhouse tanks and small individual laboratory tanks). Overall, we found that inbreeding leads to significant declines in sperm numbers. Moreover, males raised under greenhouse conditions showed a much stronger decline in sperm numbers with inbreeding compared with those raised under laboratory conditions. These differences might have been caused by a more competitive environment in the greenhouse. The decline in total sperm numbers with inbreeding was due to declining numbers of sperm bundles (spermiozeugmata), not to variation in numbers of spermatozoa within spermiozeugmata. Some, but not all ornamental traits including tail size and the areas of solid black as well as iridescent coloration, were significantly influenced by inbreeding, whereas the environment only affected differences in body size. Mating trials conducted on laboratory-raised fish revealed significantly greater attractiveness of outbred than inbred males. Female choice of outbred over inbred mates may yield direct benefits in the form of greater fertilization efficiency.