Determinisme des anomalies de sex ratio á hérédité paternelle chez le crustacé amphipodeOrchestia gammarellusPallas

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Abstract
In the amphipod crustacea Orchestia gammarellus (heterogametic species: 2AXY male, 2AXX female), two kinds of sex ratio bias are recorded, hi the first category (thelygeny linked with intersexuality) a parasitic protozoa modifies the sexual phenotype of genetic males and can transform them into intersex males or functional females. This leads to the occurrence of viable 2AYY males and females. In a second kind of sex ratio bias, males cause hereditary shifts of sex ratio. These ‘paternal sex ratio’ (psr) traits are transmitted by the male at each generation. Psr-f males cause an excess of females, psr-m males an excess of males. The psr-m trait has a strictly patroclinous mode of transmission, but females from psr-f strains intervene in the expression of psr-f trait. Intra-sib matings are characterized by an excess of males. This characteristic seems to be linked with the age of the female. It disappears during successive brood. A relation between the psr-m and psr-f trait is observed: some psr- m males give psr-f males in the their progeny. The analysis of crosses between psr-f or psr-m males and YY females allows to discard meiotic drive or sex lethal mortality as causes for the psr traits. Our results are best explained if we suppose that psr-f and psr-m males are XX and that extrachromosomal hereditary factors or transposable genetic elements intervene in the determinism of the psr traits: a psr-m factor able to masculinize all the embryos and a psr-f factor able to masculinize embryos if present in a sufficient amount.