Protogyny, Pollination, and Sex Expression of AndromonoeciousPseudocymopterus montanus(Apiaceae, Apioideae)

Abstract

Studies of dichogamy seldom address its possible effects on other floral traits. An exception has been work on the apioid umbellifers (Apiaceae, subfamily Apioideae), which typically exhibit andromonoecy and both intra‐ and interfloral dichogamy. Several students of apioid umbellifers have suggested that variation in proportions of perfect and staminate flowers among sequentially blooming inflorescences (umbels) of individual plants has evolved in response to dichogamy. Here, we report a 2‐yr study of dichogamy, pollination, and sex expression in Pseudocymopterus montanus, a common wildflower of the Rocky Mountains. We recorded floral phenology, determined the timing of pollination, collected and identified floral visitors, documented variation in sex expression and fecundity within and among plants, and examined possible reasons why some perfect flowers did not set fruit. Floral visitors included andrenid and halictid bees and anthomyid, muscoid, syrphid, and tachinid flies. Autogamy was prevented by strong intrafloral protogyny of the perfect flowers, which were pollinated before their anthers dehisced. Flowering individuals produced a single primary (terminal) umbel, one or two secondary (lateral) umbels, and occasionally a tertiary (sublateral) umbel. Umbels of different orders matured sequentially, producing multiple cycles of protogyny. Weak interfloral protogyny within and among umbels allowed the possibility of geitonogamous selfing. All individuals expressed the same pattern of variation in sex expression among sequentially blooming umbels. Over 90% of flowers in primary umbels were staminate, but the frequency of staminate flowers fell to 65% or less in secondary and tertiary umbels. Primary and secondary umbels differed significantly in sex expression, even though their prepollination reproductive efforts (floral biomass) were the same. We favor the view that this distinctive pattern of within‐plant sex expression, i.e., decreasing proportions of staminate flowers in sequentially blooming umbels, evolved in response to protogyny. In apioid umbellifers, protogyny appears to have been derived from protandry. The selective forces involved in the derivation of protogyny may have included more specialized interactions with pollinators and the ability to geitonogamously self‐pollinate if cross‐pollination failed. Sex allocation theory predicts that protogyny is a sufficient condition for the evolution of increasing relative allocations to female function in sequentially blooming flowers or inflorescences. Plants with higher reproductive effort had higher absolute allocations to both male and female reproductive functions as well as higher relative allocations to female function (higher phenotypic femaleness). The more female‐biased gender of larger plants may result from higher frequencies of geitonogamy in inflorescences with more flowers.