Phenological selection mosaic of predispersal seed predation affects gender variation in an andromonoecious plant

Abstract

Predispersal seed predation could influence the sexual expression of plants with sexually heteromorphic flowers. The timing of snowmelt determines the reproductive phenology of alpine plants at a given site, and it can also cause variation in the intensity of seed predation at a local level in alpine ecosystems. We investigated the associations between predation intensity and the floral sex allocation and fitness in an andromonoecious alpine herb, Peucedanum multivittatum (Apiaceae), along the natural gradient of flowering phenology in northern Japan over 4 years. The developing fruits of the early‐flowering populations were heavily predated by moth caterpillars, whereas seed predation was negligible in the late‐flowering populations because predator moths concentrated their oviposition in early summer. The moths tended to oviposit on umbels with more perfect flowers, fewer male flowers and taller flower stems. Plants with male‐biased umbels predominated in the early‐flowering populations. Selection differential and selection gradient analyses showed that greater production of perfect flowers accelerated seed predation, resulting in lower female fitness as seed producers. In contrast, male flower production contributed to a reduction in predation damage, but this was independent of male fitness as a pollen donor. Therefore, having a male‐biased floral gender is advantageous in minimising seed predation and maintaining the floral display. The proportion of perfect flowers per umbel increased in the late‐snowmelt populations, where higher production of perfect flowers led to greater fruit production due to low levels of seed predation. Along the phenological gradient, natural selection for perfect flower production shifted from negative to positive values, indicating heterogeneous local‐scale selection. Synthesis: Our study shows that spatiotemporal variation in the intensity of seed predation acts as a selective force, promoting the local evolution of floral sex expression in andromonoecious plants.