Size-dependent sex allocation and the expression of andromonoecy in a protogynous perennial herb: both size and timing matter

Abstract

The optimal life history and sex allocation of perennial hermaphrodites should depend on both their size and the relative costs and benefits of reproducing through male versus female functions. Theory predicts that insect-pollinated perennials should increase their allocation to female function with size, while the 'mating environment' hypothesis predicts that allocation to male function should track mating opportunities over the course of flowering. We test these two predictions by inferring male and female reproductive success in the protogynous perennial herb Pulsatilla alpina by tracking the patterns and dynamics of sex allocation over time for marked individuals over a range of sizes. We found that small individuals tend to produce only male flowers and that both small and larger individuals produced male flowers at the beginning of the flowering season when mating opportunities were high. By considering within-population variation in life history and phenology jointly rather than separately, and by considering both the tradeoff costs and benefits of allocation to male versus female functions, our results provide new insights into the evolution of both gender diphasy and andromonoecy in perennial plants that are constrained by a dichogamous flowering strategy.

Unisexual flowers as a resolution to intralocus sexual conflict in hermaphrodites

“…However, plants may achieve similar ends by separating their male and female functions in time, either within the same season or in different seasons. In P. alpina , small (and presumably young) individuals produce only male flowers and graduate as they become larger and older to producing hermaphrodite flowers [35]—a strategy known as ‘gender diphasy’ [66]. In P. alpina and several other species (e.g.…”

“…Immediately after the snowmelt in spring (early May to July), several vegetative and/or reproductive shoots emerge from the ground. Depending on their size and resource status, individuals produce from zero to approximately 20 flowers, each on its own reproductive shoot [ 35 ]. Flowers may be either male or bisexual.…”

Effects of floral sex allocation and phenology on the within-flower selfing rate and female reproductive success inPulsatilla alpina, a perennial herb with strong inbreeding depression

“…Male flowers bear only stamens, whereas bisexual flowers bear stamens and one to a few hundred uni-ovulate pistils (Figure 1). Bisexual and male flowers bear a similar number of stamens (Chen & Pannell, 2023a). Bisexual flowers are strongly protogynous, with pistils receptive before pollen is dispersed from stamens.…”

“…The species is self-compatible and likely capable of self-fertilizing autonomously without pollinators (Figure S2). Individuals have size-dependent sex allocation, with larger plants allocating absolutely and proportionally more resources to their female function (Chen & Pannell, 2023a). Small individuals often produce only a single male flower and thus function as pure males in the respective flowering season (Chen & Pannell, 2023a), leading to 'gender diphasy' (Schlessman, 1988).…”

“…Individuals have size-dependent sex allocation, with larger plants allocating absolutely and proportionally more resources to their female function (Chen & Pannell, 2023a). Small individuals often produce only a single male flower and thus function as pure males in the respective flowering season (Chen & Pannell, 2023a), leading to 'gender diphasy' (Schlessman, 1988). Both male and bisexual flowers are visited predominantly by dipteran insects, including houseflies and syrphid flies (Chen & Pannell, 2022).…”