A widespread and distinctive form of amphipod intersexuality not induced by known feminising parasites

Intersexuality occurs in a diverse range of animals, and its study offers insights into basic reproductive biology. Investigations in amphipods suggest intersexuality results from incomplete feminisation caused by sex-distorting parasites. It has also been noted that 2 intersex phenotypes occur in males of the amphipod Echinogammarus marinus, an external phenotype, in which males possess rudimentary brood plates, and an internal phenotype, in which only an ovotestis is present. This study examines the relationship between these phenotypes and finds their prevalences are independent. In addition, a cross-species microarray reveals the testicular transcriptomes of the intersex phenotypes are distinct from that of normal males and, most crucially, each other. Furthermore, the internal intersex phenotype, unlike the external phenotype, shows no correlation with infection by known feminising parasites. These findings suggest the male intersex phenotypes should not be considered stages on a single spectrum of intersexuality. Rather, they support the hypothesis that internal and external intersexuality are divergent phenotypes with separate causal mechanisms and point to the existence of a distinct and geographically widespread form of amphipod intersexuality caused by an unknown factor.