Differential selection on pollen and pistil traits in relation to pollen competition in the context of a sexual conflict over timing of stigma receptivity.

Åsa Lankinen,Maria Strandh

doi:10.1093/aobpla/plw061

Abstract

Sexual conflict and its evolutionary consequences are understudied in plants, but the theory of sexual conflict may help explain how selection generates and maintains variability. Here, we investigated selection on pollen and pistil traits when pollen arrives sequentially to partially receptive pistils in relation to pollen competition and a sexual conflict over timing of stigma receptivity in the mixed-mating annual Collinsia heterophylla (Plantaginaceae). In this species the conflict is generated by early fertilizing pollen that reduces seed production, which is counteracted by delaying receptivity in the recipient. We performed sequential two-donor pollinations at early floral developmental stages involving two pollen deposition schedules (with or without a time lag of 1 day), using only outcross or self and outcross pollen. We investigated pollen and pistil traits in relation to siring success (male fitness) and seed production (female fitness). In contrast to previous findings in receptive pistils in C. heterophylla and in other species, last arriving pollen donors showed highest siring success in partially receptive pistils. The last male advantage was weaker when self pollen was the first arriving donor. Two measures of germination rate (early and late) and pollen tube growth rate of first arriving donors were important for siring success in crosses with a time lag, while only late germination rate had an effect in contemporary crosses. Curiously, late stigma receptivity was negatively related to seed production in our contemporary crosses, which was opposite to expectation. Our results in combination with previous studies suggest that pollen and pistil traits in C. heterophylla are differentially advantageous depending on stage of floral development and varying pollen deposition schedules. Variation in success of these traits over floral development time may result from sexually antagonistic selection.

Highlights

A main aim in biology is to understand the biological and genetic diversity within natural populations
The superiority of the second donor was evident for crosses with a time lag, i.e. when the first donor arrived to the stigma 1 day prior to the second donor
Given that seed set is impaired at early fertilization in C. heterophylla (Madjidian et al 2012a) the last-male advantage detected in the current study indicates that pistils at least partly control timing of fertilization

Summary

Introduction

A main aim in biology is to understand the biological and genetic diversity within natural populations. Sexual antagonism can result from sexual conflict between alleles at the same or different interacting loci (intra- vs interlocus, Parker 1979; Rice and Chippindale 2001). In the latter case the outcome of divergent evolutionary interests of males and females are expressed in sexual selection to maximize fitness in either sex at the expense of the mating partner, which can lead to sexually antagonistic coevolution between male and female traits (Parker 1979; Arnqvist and Rowe 2005; Kokko and Jennions 2014). Frequency dependent selection can favour polymorphism of antagonistic defence traits that can escape the negative impact of an antagonistic trait of the mating partner (Gavrilets and Waxman 2002; Gavrilets and Hayashi 2005), which appears to occur in some insect species (Svensson et al 2009; Green et al 2014)

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