Turbellarian taxonomic database

Abstract / Notes

"Hermaphroditic animals face the fundamental evolutionary optimization problem of allocating their resources
to their male vs. female reproductive function (e.g. testes and sperm vs. ovaries and eggs), and this optimal
sex allocation can be affected by both pre- and post-copulatory sexual selection. For example, local sperm
competition (LSC) - the competition between related sperm for the fertilization of a partner's ova - occurs in
small mating groups and can favour a female-biased sex allocation, because, under LSC, investment into sperm
production is predicted to show diminishing fitness returns. Here, we test whether higher testis investment
increases an individual's paternity success under sperm competition, and whether the strength of this effect
diminishes when LSC is stronger, as predicted by sex allocation theory. We created two subsets of individuals
of the simultaneously hermaphroditic flatworm Macrostomum lignano - by sampling worms from either the highest
or lowest quartile of the testis investment distribution - and estimated their paternity success in group
sizes of either three (strong LSC) or eight individuals (weak LSC). Specifically, using transgenic focal
individuals expressing a dominant green-fluorescent protein marker, we showed that worms with high testis
investment sired 22% more offspring relative to those with low investment, corroborating previous findings in
M. lignano and other species. However, the strength of this effect was not significantly modulated by the
experienced group size, contrasting theoretical expectations of more strongly diminishing fitness returns
under strong LSC. We discuss the possible implications for the evolutionary maintenance of hermaphroditism in
M.lignano."

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