Sex allocation plasticity in response to resource and pollination availability in the annual plant Brassica rapa (Brassicaceae)

Premise of researchHermaphroditic plants reproduce as females by maturing seeds from fertilized ovules and as males by fertilizing the ovules of other plants. Sex allocation theory predicts a trade-off between investment in male and female function. Thus, to maximize fitness, selection should favor plasticity in resource allocation among individuals or flowers of the same plant in response to environmental conditions. As female reproduction is typically more costly while male reproduction is mate-limited, we predict greater investment in female function when resources are plentiful and in male function when pollination is limited. MethodologyWe investigated plasticity in sex allocation in the rapid cycling lineage of the hermaphroditic mustard species, Brassica rapa, in response to resource availability (altered pot size) and the pollination environment (unpollinated or fully pollinated). We assess investment in male function (anther length) and female function (ovary length) in flower buds produced at the onset of reproduction and in buds produced approximately 15 days later. We also measured traits often correlated with increased allocation to female (plant size) and male (flower size) function. Pivotal ResultsLarger plants had longer anthers, longer ovaries, and larger flowers at the onset of reproduction, resulting in similar anther:ovary length ratios across plants of different sizes. Independent of plant-size, plants produced smaller anthers at the onset of reproduction in the low resource treatment and larger flowers over the course of reproduction in the pollen-absent treatment. Furthermore, larger plants produced increasingly longer ovaries over the course of reproduction compared to smaller plants. ConclusionsOur findings underscore the influence of condition on changes in sex allocation and correlated traits over time. Furthermore, we provide some additional supporting evidence that resource availability and the pollination environment can influence sex allocation and contribute cautionary advice on effective methods for experimentally eliciting and measuring sex allocation plasticity.