Females make tough neighbors: sex-specific competitive effects in seedlings of a dioecious grass

Abstract

If males and females of a species differ in their effect on intraspecific competition then this can have significant ecological and evolutionary consequences because it can lead to size and mortality disparities between the sexes, and thus cause biased population sex ratios. If the degree of sexual dimorphism of competitive effect varies across environments then this variation can generate sex ratio variation within and between populations. In a California population of Distichlis spicata, a dioecious grass species exhibiting extreme within-population sex ratio variation (spatial segregation of the sexes), I evaluated the intraspecific competitive effects of male and female D. spicata seedlings in three soil types. The sex of seedlings was determined using a RAPD-PCR marker co-segregating with female phenotype. Distichlis spicata seedlings, regardless of sex, were six times larger when grown with male versus female conspecific seedlings in soil from microsites where the majority of D. spicata plants are female, and this sexual dimorphism of competitive effect was weaker or did not occur in other soil types. This study suggests that it is not just the higher costs of female versus male reproduction itself that cause spatial segregation of the sexes in D. spicata, but that differences in competitive abilities between the sexes--which occur as early as the seedling stage--can generate sex ratio variation.

Sex-specific plant responses to light intensity and canopy openness: implications for spatial segregation of the sexes

“…Additionally, variation in edge pore density in males was signiWcantly associated with several variables in the path diagram, indicating an importance of edge pore density to male responses. The pattern of males being in more stressful patches relative to females is consistent with patterns found in seed plants (Freeman et al 1976;Bierzychudek and Eckhart 1988;Dawson and Ehleringer 1993;Dawson and Geber 1999;Eppley 2001Eppley , 2006Dudley and Galen 2007), in the moss genera Splachnum (Cameron and Wyatt 1990), but inconsistent with the moss S. caninervis, where males and females co-occur in the shade but males are absent in open, high-light sites (Stark et al 2005a). However, males of S. caninervis were recently found to regenerate better than females after heat stress (Stark et al 2008).…”

“…The underlying mechanisms to account for sex-speciWc demographies are thought to result from the diVering male vs. female sexual reproductive functions and the tradeoVs associated with these functions and other life history traits Obeso 2002;Case and Ashman 2005). Nevertheless, few studies have linked SSS to variation in sex-speciWc ability to occupy diVerent ranges of the gradients [but see Eppley (2006) for an example of sex-speciWc competitive ability that varies with habitat]. Here, we test for sex-speciWc physiological and related morphological responses that are linked to observed patterns of SSS in a dioecious bryophyte, Marchantia inXexa.…”

Changes in sex ratios of a dioecious grass with grazing intensity: the interplay between gender traits, neighbour interactions and spatial patterns

“…plastic alteration of sex expression in relation to its environment) (Cox 1981;Bierzychudek & Eckhart 1988;Eppley, Stanton & Grosberg 1998). Spatial segregation of the sexes can also be the result of interactions with other species such as competition for limited resources (Freeman, Klikoff & Harper 1976;Cox 1981;Eppley 2006;Mercer & Eppley 2010). Inter and intrasexual competition has been proposed as one of the probable mechanisms behind local spatial segregation of the sexes (Bertiller, Sain & Carrera 2002b;Eppley 2006;Mercer & Eppley 2010).…”

“…Spatial segregation of the sexes can also be the result of interactions with other species such as competition for limited resources (Freeman, Klikoff & Harper 1976;Cox 1981;Eppley 2006;Mercer & Eppley 2010). Inter and intrasexual competition has been proposed as one of the probable mechanisms behind local spatial segregation of the sexes (Bertiller, Sain & Carrera 2002b;Eppley 2006;Mercer & Eppley 2010). Nevertheless, there has been a conspicuous lack of investigation into sex-related effects on the competitive interaction with other plant species (S anchez-Vilas, Turner & Pannell 2010).…”

Female versus male: Rumex thyrsiflorus Fingerh. under in vitro conditions. Does sex influence in vitro morphogenesis?

“…Results obtained by Marciniuk et al (2010) demonstrated that female individuals of Aruncus sylvestris had markedly higher habitat requirements compared with the males. Eppley (2006) revealed that in Distichlis spicata a variation in the sex ratio could be caused by the differences in competitive abilities at the seedling stage, with female seedlings exhibiting a stronger environment dependent competitive ability than male seedlings. It was suggested that sex-specific competitive differences could have a greater impact on seedlings, and research was needed to explain if such competitive interactions occured at the vulnerable seedling stage.…”