Environmental variation in sex ratios and sexual dimorphism in three wind‐pollinated dioecious plant species

Abstract

Variation in plant sex ratios is often attributable to sex-specific mortality in heterogeneous environments that differentially limit male and female plant reproduction. Yet sexual dimorphism and plastic responses to environmental heterogeneity are common and may co-vary with variation in sex ratios. Here, we show that the sex ratio and the degree of sexual dimorphism for a number of plant traits varied along climatic and elevation gradients in three wind-pollinated dioecious species, Rumex lunaria, Urtica dioica and Salix helvetica. Some of the observed sex-specific responses to climatic variation are consistent with greater sensitivity of females to water scarcity, but most responses rather point to the greater sensitivity of males to ecological stress, consistent with larger male reproductive effort, as has been commonly reported for wind-pollinated plants.In contrast, we found no evidence for variation in either sex ratios or sexual dimorphism expected under sexual selection. Interestingly, sex ratios and sexual dimorphism varied both along distinct and the same ecological axes of variation, suggesting that the evolution of sexual dimorphism in the measured traits was not sufficient to prevent sex-specific mortality.

Female-biased population sex ratios caused by genetic rather than ecological mechanisms

“…The sex ratio of flowering plants of Salix herbacea was strongly female-biased, with a proportion of females of 0.71-0.82 in three natural populations in its alpine environment above the treeline. Strong female bias (0.65-0.80) has also been observed in S. herbacea populations in Iceland (Crawford & Balfour, 1983) and in natural populations of many other Salix species in various habitats, including dwarf or small shrubs in arctic or alpine habitats (Crawford & Balfour, 1990; Dawson & Bliss, 1989; Myers-Smith & Hik, 2012) and small to medium shrubs in boreal or montane/sub-alpine habitats (Bürli et al, 2022; Che-Castaldo et al, 2015; Dudley, 2006; Elmqvist et al, 1988; Hroneš et al, 2019) as well as in temperate habitats (Alliende & Harper, 1989; de Jong & van der Meijden, 2004; Ueno et al, 2007). In our study, we further cultivated crosses of S. herbacea under controlled conditions, at temperatures near maximum daily temperatures in the field (4-16 °C, 3-4 months growth period).…”

“…Willows ( Salix ) are one of the plant genera in which sex ratios have been frequently investigated. Almost all studies report female-biased secondary sex ratios in natural populations (Alliende & Harper, 1989; Bürli et al, 2022; Che-Castaldo et al, 2015; Crawford & Balfour, 1983, 1990; Dawson & Bliss, 1989; de Jong & van der Meijden, 2004; Dudley, 2006; Elmqvist et al, 1988; Hroneš et al, 2019; Hughes et al, 2010; Myers-Smith & Hik, 2012; Ueno et al, 2007). In part of these studies, sex ratios were associated with environmental conditions such as altitude, drought or the intensity of herbivory, and ecological mechanisms were implicated in the generation of secondary sex ratio bias (e.g., Bürli et al, 2022; Dawson & Bliss, 1989; Dudley, 2006; Elmqvist et al, 1988; Ueno et al, 2007).…”

“…Almost all studies report female-biased secondary sex ratios in natural populations (Alliende & Harper, 1989; Bürli et al, 2022; Che-Castaldo et al, 2015; Crawford & Balfour, 1983, 1990; Dawson & Bliss, 1989; de Jong & van der Meijden, 2004; Dudley, 2006; Elmqvist et al, 1988; Hroneš et al, 2019; Hughes et al, 2010; Myers-Smith & Hik, 2012; Ueno et al, 2007). In part of these studies, sex ratios were associated with environmental conditions such as altitude, drought or the intensity of herbivory, and ecological mechanisms were implicated in the generation of secondary sex ratio bias (e.g., Bürli et al, 2022; Dawson & Bliss, 1989; Dudley, 2006; Elmqvist et al, 1988; Ueno et al, 2007). Studies on other Salix species, in contrast, did not find evidence for ecological differences between sexes and concluded that sex bias could be present from an early stage (Che-Castaldo et al, 2015; de Jong & van der Meijden, 2004; Hroneš et al, 2019; Myers-Smith & Hik, 2012).…”

Allelochemicals and soil microorganisms jointly mediate sex‐specific belowground interactions in dioecious Populus cathayana

“…Sexual conflict, initially conceptualized by Parker as a ‘conflict between the evolutionary interests of individuals of two genders’ (Parker, 1979), occurs when selection pressures operate in opposite directions for males and females. Sex conflict arises in some dioecious plants, which have reproduction restrictions frequently influenced by flowering phases and pollinators' dependence (Wilcock & Neiland, 2002; Delph et al ., 2011; Bürli et al ., 2022; Soininen & Kytöviita, 2022). Female plants bear additional resource costs associated with fruiting, which are not experienced by male plants (Barrett & Hough, 2013; Hultine et al ., 2016; Lei et al ., 2017).…”

Covariations and trade‐offs of phosphorus (P) acquisition strategies in dioecious Populus euphratica as affected by soil water availability

“…Females tend to bear greater reproductive costs than males due to the necessity to develop seeds and fruits (Juvany & Munne‐Bosch, 2015). In addition to the absolute costs of reproduction that may differ between the two sexes, the seed and fruit production tend to rely heavily on water and phosphorus (P), whereas pollen production requires large amounts of nitrogen (N) (Bürli et al, 2022; Lei et al, 2017; Yu et al, 2022).…”