medrano.et.al.2005.heredity (1).doc

medrano.et.al.2005.heredity (1).doc - Mating system sex...

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Mating system, sex ratio, and persistence of females in the gynodioecious shrub Daphne laureola L. (Thymelaeaceae) M Medrano, C Alonso and CM Herre ra Estacio´n Biolo´gica de Do n ˜ ana, CSIC. Apartado 1056, E-41080, Spain Although in gynodioecious populations male steriles require a fecundity advantage to compensate for their gametic disadvantage, southern Spanish populations of the long- lived shrub Daphne laureola do not show any fecundity advantage over hermaphrodites in terms of seed production and early seedling establishment. By using allozyme markers, we assess the mating system of this species in five populations differing in sex ratio, and infer levels of inbreeding depression over the whole life cycle by comparing the inbreeding coefficients at the seed and adult plant stages. Extremely low outcrossing rates (0.001 o t o 0.125) were consistently found for hermaphrodites in all populations, whereas, as expected, female progeny were entirely outcrossed. In most populations, offspring were much more inbred than their parents, and heterozygosity of adults was greater than expected from outcrossing rate estimates, with very few selfed progeny appearing to reproduce in the field. The combination of extensive selfing in hermaphrodites and a strong inbreeding depression expressed late in the life cycle (and thus, only estimable by indirect measures based on genetic markers) may explain the persistence and high frequency of D. laureola females in southern Spanish populations. Keywords: allozymes; female advantage; gynodioecy; inbreeding coefficient; outcrossing rates; sex ratio variation Introduction Gynodioecy is a reproductive system characterized by the coexistence of male steriles (‘females’ hereafter) and hermaphrodites in plant populations. Despite its r elative rarity among flowering plants (5–10% of species; Charlesworth, 2002), this reproductive system has been the subject of a considerable number of ecological, evolutionary, genetic, and theoretical investigations. Gynodioecy stands out as a particularly inter esting study system (Charlesworth, 2002) because of the combination of: (i) theoretical challenges posed by the spread and persistence of female individuals in popula- tions despite their gametic disadvantage relative to hermaphrodites (Lloyd, 1976; Charlesworth and Charles- worth, 1978; Frank, 1989; Bailey et al, 2003); (ii) its significance as an intermediate step in the evolution of dioecy from hermaphroditism (Barrett, 2002); and (iii) the genetic and evolutionary implications of the inher ent nucleocytoplasmic genomic conflict when male sterility is determined by cytoplasmic genes (Saumitou-Laprade et al, 1994; Budar et al, 2003). Irrespective of the mode of inheritance of male sterility , most theoretical models predict that, to spread and persist in populations, female plants must have some consistent fecundity advantage that cancels their gametic Correspondence: M Medrano, Estacio´n Biolo´gica de Do n ˜ ana, CSIC. Apartado 1056, E-41080, Spain.
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