Mechanisms Governing Sex-Ratio Variation in Dioecious Rumex Nivalis

Abstract

Sex ratios of flowering individuals in dioecious plant populations are often close to unity, or are male biased owing to gender-specific differences in flowering or mortality. Female-biased sex ratios, although infrequent, are often reported in species with heteromorphic sex chromosomes. Two main hypotheses have been proposed to account for female bias: (1) selective fertilization resulting from differential pollen-tube growth of female- versus male-determining microgametophytes (certation); (2) differences in the performance and viability of the sexes after parental investment. Here we investigate these hypotheses in Rumex nivalis (Polygonaceae), a European alpine herb with female-biased sex ratios in which females possess XX, and males XY1Y2, sex chromosomes. Using field surveys and a glasshouse experiment we investigated the relation between sex ratios and life-history stage in 18 populations from contrasting elevations and snowbed microsites and used a male-specific SCAR-marker to determine the sex of nonflowering individuals. Female bias among flowering individuals was one of the highest reported for populations of a dioecious species (mean female frequency = 0.87), but males increased in frequency at higher elevations and in the center of snowbeds. Female bias was also evident in nonflowering individuals (mean 0.78) and in seeds from open-pollinated flowers (mean 0.59). The female bias in seeds was weakly associated with the frequency of male flowering individuals in populations in the direction predicted when certation occurs. Under glasshouse conditions, females outperformed males at several life-history stages, although male seeds were heavier than female seeds. Poor performance of Y1Y2 gametophytes and male sporophytes in R. nivalis may be a consequence of the accumulation of deleterious mutations on Y-sex chromosomes.

Sex ratio and genetic diversity in the dioecious Pistacia atlantica (Anacardiaceae)

“…This causes damage to the female plants, leading to a male-biased population. Therefore, sex ratios in populations of dioecious flowering species often deviate from 1:1 (Stehlik and Barrett 2005), and in most cases male populations have greater individual numbers (Vandepitte et al 2010). A review survey of the sex ratios in dioecious flowering plants has confirmed this idea by showing that 29% of species had a ratio 1:1 while 57% of species were male-biased populations (Rottenberg 1998, Delph 1999).…”

“…Producing more male seeds always reduces local resources competition because pollen grains are dispersed to further distances than the seed (Hu and Ennos 1997). Moreover, another reason for male-biased sex ratios especially in long-lived dioecious species has been attributed to differences in the costs of reproduction in different genders (Lloyd and Webb 1977) since a greater proportion of resources in females are usually allocated to reproduction than males (Stehlik and Barrett 2005). This can make female individuals more susceptible to environmental stress, and consequently could result in higher mortality in females (Meagher 1981).…”

“…Several factors cause differences in the levels of genetic diversity between male and female populations of dioecious plants including distribution patterns of individuals across the populations (De Jong and Klinkhamer 2005), sex ratio (Vandepitte et al 2010), and in small isolated populations, stochastic events also (Engen et al 2003). The variation of sex ratio in the dioecious plant populations is influenced by environmental conditions since these conditions have a different affect on distribution, survival and growth rates of male and female individuals (Eppley 2001, Stehlik and Barrett 2005, Dudley 2006). According to the ecological causation hypothesis for secondary sexual dimorphism in dioecious plant species, sexes perform differently under environmental stresses and resource availability since males and females have adapted to different ecological niches (Shine 1989, Dudley 2006.…”

Influences of clonality on plant sexual reproduction

“…Data on sex ratios largely come from sampling flowering individuals because the sex of individuals can usually be determined only at reproductive maturity (but see ref. 38). Also, clones are difficult to identify without genetic markers, and sex ratio estimates are usually based on the sampling of ramets rather than genets.…”

Sex determination in dioecious Mercurialis annua and its close diploid and polyploid relatives

“…Mechanisms that bias the sex ratios of individual seed families are well documented in other dioecious species and can operate in spite of strict genetic sex-determining systems (reviewed in De Jong and Klinkhamer, 2002;Barrett et al, 2010). For example, Rumex acetosa (Rychlewski and Zarzycki, 1975), Rumex nivalis (Stehlik and Barrett, 2005;Stehlik et al, 2008) S. latifolia (Taylor, 1994) and Urtica dioica (De Jong and Klinkhamer, 2002;De Jong et al, 2005;De Jong, 2007, 2009) all display family-level sex ratio heterogeneity despite the presence of chromosomal or, in the case of dioecious U. dioica (De Jong et al, 2005), evidently single-locus sex-determining systems.…”