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Evolutionary pathways for sex-determining mechanisms

Evolutionary pathways for sex-determining mechanisms based on X-chromosome elimination: the sciarid system. Lucas Sánchez Centro de Investigaciones Biológicas (C.S.I.C.), Madrid. Sex determination. Developmental process that gives rise to males and females,

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Evolutionary pathways for sex-determining mechanisms

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  1. Evolutionary pathways for sex-determining mechanisms based on X-chromosome elimination: the sciarid system Lucas Sánchez Centro de Investigaciones Biológicas (C.S.I.C.), Madrid

  2. Sex determination Developmental process that gives rise to males and females, which are distinct from the morphological, physiological and behavioural point of view If sex determination is based on chromosome differences, one sex being homomorphic and the other heteromorphic for ther sexual chromosomes (females = 2X and males X) then Dosage compensation Regulation of expression of the genes located in the sexual chromosomes to equalise the amount of their products to autosomal products in both sexes

  3. Life cycle of Sciara development of the soma X 0; A A male Xp; Ap Am Xp Xp X X; A testes Xm; Am meiosis I meiosis II sperm Xp Xm Xp Xp Am Ap germ cell Xm Xp Am Ap zygote Xp normal meiosis ovaries X; A oocytes X X; A A development of the soma female

  4. females males Putative evolutionary pathways XX X0 Primitive mechanism X X 0 females males ? Xs Xe Xc Xs Xe Xc females males XX X0 Xc Xs Xe Xe Xc Xs one male germ cell = one spermatocyte = gene s non-disjunction of X-chromatids = gene c Xe Xc Xs Xc Xs Xe X XX Sciara mechanism XXX XX X maternally controlled X-elimination = gene e XX X0 females males

  5. Population scenario - Infinitely large - Discrete generations - Effects on fitness - viability = no effect - reproduction - mating capacity = no effect & random mating - meiotic drive - No migration - Females accept only one mating - Males can mate more than one female

  6. extinction (c) XX/XXs/XsXs X0/Xs0 (s) (s) (c) XX/XXc X0/Xc0 XX/X0 (e) (e) (e) XX/XXe X0/Xe0 (c) (s) Gene “s” is expressed after 1º meiotic division XsXs/XsXes Xs0/Xes0 (c) Monogenic species Gene “s” is expressed after 2º meiotic division (c) XscXesc/XescXesc Xesc0 Monogenic species (Sciara coprophila)

  7. Conclusions - The sciarid sex determination system could evolved in the absence of positive selection upon the individual viability and mating capacity (absence of sexual selection). - The sex ratio acts as a driving force for the evolution of the sciarid sex determination system, yet it depends on the phenotypic effect of the new gene. - The most plausible evolutionary pathway of the sciarid sex determination system is: 1) maternal factor causing elimination of X chromosome 2) one male germ cell = one sperm (heterochronic regulation) 3) non-disjunction of X chromatids in spermatogenesis - The evolutionary pathway of the sciarid sex determination system shows constrictions; i.e. the fixation of a new gene in the population depends on its genetic structure (structure of the gene system)

  8. Maternal versus paternal imprinting - The elimination of X chromosomes depends on a maternal factor - Difference between maternal and paternal imprinting - maternal imprinting = the imprinted chromosome is not eliminated - paternal imprinting = the imprinted chromosome is eliminated Conclusion - The sex-determining system in sciara is compatible with maternal but not paternal imprinting

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