Discussion

Abstract: Within populations of the amphipod crustaceans Orchestia gammarellus and O. aestuarensis, a proportion of females produce thelygenic (female-only) broods which often contain intersexual individuals. This phenomenon is associated with the presence of two parasites, the paramyxid Paramarteilia orchestiae and the microsporidian Dictyocoela cavimanum, which frequently co-infect the same host. In order to determine which of the parasites causes feminisation, Orchestia were resampled from the type locality of P. orchestiae in France and from another population at Dale in the UK. Breeding experiments indicated that female O. gammarellus infected with P. orchestiae produced a significantly higher proportion of female and intersex offspring than uninfected females, even in the absence of D. cavimanum. Although D. cavimanum also displays a female-biased prevalence in Orchestia populations, this is due to co-infection with P. orchestiae, indicating that the paramyxid, rather than the microsporidian, is the cause of feminization in these Orchestia populations.