Polychaete Worms on the Brink Between Hermaphroditism and Separate Sexes

Abstract

Polychaetes (i.e. non-clitellate annelids) display one of the highest diversities of reproductive traits among marine invertebrates, due to the wide variety of habitats they have colonized and the relative simplicity of their reproductive system. Among polychaetes (i.e. non-clitellate annelids), the genus Ophryotrocha is one of the best-studied models in sexual system investigations and is providing several clues as to how the evolutionary transition from hermaphroditism to gonochorism may have occurred. Within the genus, there are both hermaphroditic and gonochoric species, but many species exhibit traits intermediate between the two sexual systems. Hermaphroditic species are able to plastically adjust their sex allocation to current mating opportunities, by diminishing the proportion of resources invested into the female function and increasing those invested into the male function, when mating opportunities increase (although the enhanced investment in the male function is expressed in behavioural traits—such as mate competition—rather than increased sperm production). Occasionally, hermaphrodites specialized in the male function are found in hermaphroditic populations (functional-male hermaphrodites), which might represent a first step towards gender specialization. On the other hand, multiple sexual phenotypes have been described in gonochoric (sexually dimorphic) species, with males which produce oocytes and females which produce sperm, which are likely to be vestigial hermaphroditic traits. The existence of functional-male hermaphrodites in hermaphroditic populations and of males and females which produce both eggs and sperm in gonochoric species suggest that in Ophryotrocha the transition from hermaphroditism to gonochorism occurred via androdioecy, that is, through an evolutionary trajectory involving an intermediate stage where both hermaphrodites and males are present.