Inbreeding depression in male reproductive traits

Inbreeding frequently leads to inbreeding depression, a general reduction
in trait values and loss of fitness, and it appears that some sexually
selected traits are especially sensitive to inbreeding, but sperm may be
an exception. Additionally, because inbreeding depression is always in the
direction of low fitness, it can reveal the direction of past selection
acting on trait values. Here, we experimentall manipulate levels of
inbreeding in a beetle (Tribolium castaneum) by full-sib mating for six
generations. This breeding design allowed us to track the effects of
increasing homozygosity on male reproductive traits (sperm and testes
size), male size and lifespan, and reproductive output within inbred
families, and on the heritability of these traits. All traits measured
showed significant inbreeding depression and heritabilities tended to
increase with inbreeding. Since inbreeding resulted in shorter sperm and
smaller testes, it suggests that longer sperm and larger testes confer
higher fitness in this beetle.