The varied ways of being male and female

Our understanding of sexual reproduction is mainly informed by research on gonochorists (i.e. species with separate sexes), including insects, birds, and mammals. But the male and female sexes are not two types of individuals; they actually represent two different reproductive strategies, and in many organisms these two strategies are distributed among individuals in a population in a variety of ways. For example, sequential hermaphrodites (or sex-changers) exhibit one strategy early in life, and later switch to the other, while simultaneous hermaphrodites exhibit both strategies at the same time. There are also many intermediate sexual systems that mix gonochorists and hermaphrodites in the same species and within many organismal groups, shifts occur between these sexual systems. A fascinating collection of six articles in this special issue on Hermaphroditism & Sex Determination impressively documents some important challenges to our understanding of sex determination, and the specification of male and female reproductive function when these need to occur within the same individual rather than in two separate individuals. Moreover, hermaphroditism changes how we need to think about reproductive allocation to sexual functions, how such allocation can be specified, as well as how this sexual system affects sexual conflict and the resulting antagonistic coevolution. Our understanding of sexual reproduction will profit greatly from exploring the varied ways of being male and female.