Sex‐specific selection on plant architecture through “budget” and “direct” effects in experimental populations of the wind‐pollinated herb, Mercurialis annua

Abstract

Sexual selection may contribute to the evolution of plant sexual dimorphism by favoring architectural traits in males that improve pollen dispersal to mates. In both sexes, larger individuals may be favored by allowing the allocation of more resources to gamete production (a “budget” effect of size). In wind‐pollinated plants, large size may also benefit males by allowing the liberation of pollen from a greater height, fostering its dispersal (a “direct” effect of size). To assess these effects and their implications for trait selection, we measured selection on plant morphology in both males and females of the wind‐pollinated dioecious herb Mercurialis annua in two separate experimental common gardens at contrasting density. In both gardens, selection strongly favored males that disperse their pollen further. Selection for pollen production was observed in the high‐density garden only, and was weak. In addition, male morphologies associated with increased mean pollen dispersal differed between the two gardens, as elongated branches were favored in the high‐density garden, whereas shorter plants with longer inflorescence stalks were favored in the low‐density garden. Larger females were selected in both gardens. Our results point to the importance of both a direct effect of selection on male traits that affect pollen dispersal, and, to a lesser extent, a budget effect of selection on pollen production.

Do metrics of sexual selection conform to Bateman's principles in a wind-pollinated plant?

“…Our study is based on a recently published dataset that estimated male fitness through marker-assisted paternity analyses in two common gardens [ 21 ]. Seeds were collected from 35 populations located in northern Spain that were bulked and grown for three generations in a common garden in Lausanne.…”

“…Sexual selection in plants likely occurs through between-male competition to fertilize a limited pool of ovules and may consequently affect the evolution of traits involved in pollen production, export and competitiveness [ 13 – 15 ]. Such potentially sexually selected traits include large flowers and floral displays that enhance pollinator attraction [ 15 , 17 – 19 ]; increased pollen production [ 20 , 21 ]; male flowering phenology that tracks that of females [ 22 ]; vegetative architectures that enhance pollen dispersal [ 21 ]; evolution of horn weapons that prevent the attachment of pollen-bearing structure from additional males [ 23 ] and high pollen-grain performance [ 24 ]. Floral strategies that affect the distribution of pollen on pollinators' bodies could also be under sexual selection [ 25 ].…”

“…However, it is also conceivable that a plant's traits (e.g. [ 17 , 21 , 32 ]) may influence mating success at the plant level, rendering estimates of mate number at the plant level potentially useful.…”

“…High pollen production may also allow the competitive exclusion of pollen from other males, for instance, by saturating stigmas with pollen [ 33 ]. On the other hand, sexual selection may occur through the placement of flowers on elongated branches or inflorescences that favour pollen dispersal, especially in wind-pollinated plants; this has been termed a ‘direct effect’ of plant traits [ 21 , 33 ]. Analogous ideas have been proposed for animals with a sessile life-form and external fertilization, e.g.…”

Influence of local density and sex ratio on pollination in an ambophilous flowering plant

“…, right panel). It seems unlikely that wind pollination would have contributed to reduce seed set in 3:1 arrays, because the only conceivable effect of increasing male density is increased pollen transfer to females; that is, seed set is usually regarded as an increasing function of pollen production and density (production per area) in wind‐pollinated species (Schoen and Stewart, ; Allison, ; Smith et al., ; Cox, ; Friedman and Barrett, ; Nakahara et al., ; Tonnabel et al., ). Thus, it is unclear why seed set was reduced in male‐biased arrays compared to those in which the sexes were equally represented.…”