Fertilization by proxy: rival sperm removal and translocation in a beetle

Competition between different males' sperm for the fertilization of ova has led to the evolution of a diversity of characters in male reproductive beh aviour, physiology and morphology. Males may increase sperm competition suc cess either by enhancing the success of their own sperm or by negating or e liminating the success of rival sperm. Here, we find that in the flour beet le Tribolium castaneum, the second male to mate gains fertilization precede nce over previous males' sperm and fertilizes approximately two-thirds of t he eggs. It is not known what mechanism underlies this pattern of last-male sperm precedence; however, the elongate tubules of the female sperm storag e organ may encourage a 'last-in, first-out' sperm use sequence. Here we pr esent an additional or alternative mechanism of sperm precedence whereby pr eviously deposited sperm are removed from the female tract by the mating ma le's genitalia. In addition to providing evidence for sperm removal in T. c astaneum, we also show that removed, non-self sperm may be translocated bac k into the reproductive tracts of new, previously unmated females, where th e translocated sperm go on to gain significant fertilization success. We fo und that, in 45 out of 204 crosses, sperm translocation occurred and in the se 45 crosses over half of the offspring were sired by spermatozoa which ha d been translocated between females on the male genitalia. In the natural e nvironment of stored food, reproductively active I: castaneum adults aggreg ate in dense mating populations where copulation is frequent (we show in th ree naturally occurring population densities that copula duration and inter mating intervals across three subsequent matings average 1-2 min). Selectio n upon males to remove rival sperm may have resulted in counter-selection u pon spermatozoa to survive removal and be translocated into new females whe re they go on to fertilize in significant numbers.