Related species share genetic and developmental backgrounds. Therefore, hidden genetic variation for sex determination may allow separate sex species - that share recent common ancestors with hermaphroditic species - to express flexible sex allocation and sex lability as a function of environmental factors. Worms of the polichaete species Ophryotrocha labronica have separate sexes whereas their congeneric species are hermaphroditic. O. labronica worms have a worldwide distribution and different populations may be subject to different selective pressures on sexual traits. Therefore, we exposed newly-mature O. labronica worms from three geographically-distant populations to different social conditions, that corresponded to different levels of mating opportunities. Worms were either isolated (i.e., had no mating opportunities), or kept in pairs (intermediate mating opportunities), or in promiscuous groups (high mating opportunities). After three weeks, we measured the sexual phenotype of the worms checking whether they had sperm, oocytes and nurse cells in their coeloms. The analyses showed that 55-95 % of the worms (depending on the population) had allocated to both sex functions after the experimental period. However, the sex allocation of the worms was influenced by mating opportunities in different ways depending on the population. These results 1) confirm the hypothesis that separate sex species that share recent common ancestors with hermaphroditic species adjust their sex allocation to current mating opportunities; 2) indicate that worms from different populations exhibit different levels of sex allocation plasticity and 3) suggest that intermediate steps exist along the evolutionary trajectories between hermphroditism and gonochorism.
Gonochorists or hermaphrodites? Gonochoric worms with flexible sex allocation
LORENZI, Maria Cristina;SELLA, Gabriella
2013-01-01
Abstract
Related species share genetic and developmental backgrounds. Therefore, hidden genetic variation for sex determination may allow separate sex species - that share recent common ancestors with hermaphroditic species - to express flexible sex allocation and sex lability as a function of environmental factors. Worms of the polichaete species Ophryotrocha labronica have separate sexes whereas their congeneric species are hermaphroditic. O. labronica worms have a worldwide distribution and different populations may be subject to different selective pressures on sexual traits. Therefore, we exposed newly-mature O. labronica worms from three geographically-distant populations to different social conditions, that corresponded to different levels of mating opportunities. Worms were either isolated (i.e., had no mating opportunities), or kept in pairs (intermediate mating opportunities), or in promiscuous groups (high mating opportunities). After three weeks, we measured the sexual phenotype of the worms checking whether they had sperm, oocytes and nurse cells in their coeloms. The analyses showed that 55-95 % of the worms (depending on the population) had allocated to both sex functions after the experimental period. However, the sex allocation of the worms was influenced by mating opportunities in different ways depending on the population. These results 1) confirm the hypothesis that separate sex species that share recent common ancestors with hermaphroditic species adjust their sex allocation to current mating opportunities; 2) indicate that worms from different populations exhibit different levels of sex allocation plasticity and 3) suggest that intermediate steps exist along the evolutionary trajectories between hermphroditism and gonochorism.
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