Intersexuality in Orchestia Mediterranea Costa, 1853, and Orchestia Aestuarensis Wildish, 1987 (Amphipoda): a Consequence of Hybridization or Parasitic Infestation?

An experimental study of intersexuality in Orchestia mediterranea and Orchestia aestuarensis shows that it is not related to hybridization between both species in overlapping populations. Interspecific crosses with individuals from populations of various geographic origin are either sterile or produce sterile individuals and an excess of males in the F, generation. Histological and ultrastructural study of the gonads of normal or intersex males reveals the presence of microsporidian cells attributed to the genus Nosema, and, in intersex males, of cells resembling Paramarteilia orchestiae, a parasite described in Orchestia gammarellus and responsible for intersexuality and sex ratio deviations in that species. Experiments of organ grafting between both species and Orchestia gammarellus demonstrate that this kind of intersexuality is inducible and related to the presence of Paramarteilia orchestiae, a transovarially transmitted parasite.