Abstract
Summary Understanding the mechanisms causing phenotypic differences between females and males has long fascinated evolutionary biologists. An extensive literature exists on animal sexual dimorphism but less information is known about sex differences in plants, particularly the extent of geographical variation in sexual dimorphism and its life‐cycle dynamics.Here, we investigated patterns of genetically based sexual dimorphism in vegetative and reproductive traits of a wind‐pollinated dioecious plant, Rumex hastatulus, across three life‐cycle stages using open‐pollinated families from 30 populations spanning the geographic range and chromosomal variation (XY and XY1Y2) of the species.The direction and degree of sexual dimorphism was highly variable among populations and life‐cycle stages. Sex‐specific differences in reproductive function explained a significant amount of temporal change in sexual dimorphism. For several traits, geographical variation in sexual dimorphism was associated with bioclimatic parameters, likely due to the differential responses of the sexes to climate. We found no systematic differences in sexual dimorphism between chromosome races.Sex‐specific trait differences in dioecious plants largely result from a balance between sexual and natural selection on resource allocation. Our results indicate that abiotic factors associated with geographical context also play a role in modifying sexual dimorphism during the plant life‐cycle.
Highlights
Trait differences between females and males reflect sex-specific optima related to their different reproductive roles (Darwin, 1871; Andersson, 1994)
We asked the following questions: (1) does sexual dimorphism in reproductive and vegetative traits vary among life-cycle stages in relation to the different reproductive roles of females and males? (2) Does sexual dimorphism vary among populations across its geographic range and between chromosome races? (3) Can demographic, geographical and bioclimatic variables explain among-population variation in sexual dimorphism? Having established the overall patterns of sexual dimorphism in R. hastatulus we investigated trait correlations within and between the sexes to ask if intra- and intersex correlations vary across the life-cycle for reproductive and vegetative traits
There was no sexual dimorphism at wk 2 before flowering (Fig. S2a), but at wk 4 males were significantly taller than females (% SD = À16.1; Figs 1a, 3 and Fig. S2b), and at wk 8 sexual dimorphism for height reversed, with females taller than males (%SD = 9.4; Figs 1b, 3 and Fig. S2c)
Summary
Trait differences between females and males (sexual dimorphism) reflect sex-specific optima related to their different reproductive roles (Darwin, 1871; Andersson, 1994). Mechanisms of pollen and seed dispersal may mediate the strength of sex-specific selection because female and male components interact indirectly through biotic or abiotic vectors (Lloyd & Webb, 1977; Moore & Pannell, 2011). Conspicuous sexual dimorphism is predicted to evolve for traits related to pollination success in anemophilous species including plant height, flower number and inflorescence deployment (Eppley & Pannell, 2007; Friedman & Barrett, 2009; Tonnabel et al, 2019). Sex-specific trait differences in anemophilous plants may reflect both wind-mediated selection for proficient pollen and seed dispersal, and optimal resource allocation between vegetative and reproductive structures
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