Self-fertilization, sex allocation and spermatogenesis kinetics in the hypodermically inseminating flatwormMacrostomum pusillum.
The free-living flatworm genusMacrostomumis an emerging model system for studying the links between sex allocation, sexual selection and mating system evolution, as well as the underlying developmental and physiological mechanisms responsible for wide intra- and inter-specific variability in reproductive phenotypes. Despite compelling comparative morphological evidence of sexual diversity, detailed experimental work on reproductive behaviour and physiology inMacrostomumhas so far been largely limited to just two species,M. lignanoandM. hystrix, an obligate and a preferential outcrosser, respectively. In this study, we establish that a third species,M. pusillum, exhibits a combination of reproductive traits strikingly different from both of its congeners. UnlikeM. lignano, we demonstrate thatM. pusillumdoes not adjust sex allocation or the speed of spermatogenesis to the prevailing social group size.Macrostomumpusillum's relatively simple sperm morphology likely explains the short spermatogenesis duration we report, and is linked to a hypodermically inseminating mode of fertilization, which we show also means that these worms are capable of self-fertilization. Surprisingly, and unlikeM. hystrix, selfing in isolated worms commences after only a short (if any) delay compared with the onset of reproduction in grouped individuals, with little evidence of differential inbreeding depression in 'isolated' progeny. These combined results suggest that, in nature,M. pusillummay be regularly selfing, in contrast to the congeners studied to date. Our findings highlight the rapid and correlated evolution of reproductive traits, and reinforce the utility of the genusMacrostomumfor understanding the evolutionary and developmental mechanisms responsible for this diversity.
Evolutionary Biology, Bielefeld University, Morgenbreede 45, Bielefeld 33615, Germany.
Athina
A
Giannakara
Evolutionary Biology, Bielefeld University, Morgenbreede 45, Bielefeld 33615, Germany steven.ramm@uni-bielefeld.de.
Steven A
0000-0001-7786-7364
SA
Ramm
D000818:Animals; D005075:Biological Evolution; D057850:Hermaphroditic Organisms; D010986:Platyhelminths / Q000502:physiology*; D057945:Self-Fertilization; D013091:Spermatogenesis
Hermaphroditism; Hypodermic insemination; Self-fertilization; Sex allocation; Sperm competition; Spermatogenesis