FLORAL DISPLAY IN PHYLA INCISA : CONSEQUENCES FOR MALE AND FEMALE REPRODUCTIVE SUCCESS

Abstract

Abstract. -The inflorescences of Phyla incisa consist of flowers in two phases: younger, nectarcontaining flowers that have yellow corolla throats and older, nectar-lacking flowers that have dark purple corolla throats. Observations of pollinator visitation patterns to both natural and manipulated inflorescences were made to determine the role ofeach flower phase in pollinator attraction. The effect of older-phase flowers on male and female reproductive success was determined by comparing stigmatic pollen loads and estimates of pollen removal from inflorescences having different numbers of these flowers.The pollinators of Phyla selected larger inflorescences more often than expected based upon the size distribution of inflorescences available to them. Both younger-and older-phase flowers contributed to the attraction of pollinators, but the latter were less effective in this function. The presence of older-phase flowers significantly increased the visitation rate to inflorescences and the amount of pollen removed but had little effect on pollen deposition on stigmas. The lack of correspondence between pollen deposition and pollinator-visitation rate was not due to stigma saturation, since stigma loads varied greatly. The data indicate that the deposition of pollen on stigmas in this species is a relatively stochastic process, whereas pollen removal from inflorescences occurs at a much more regular rate. Old-phase flower retention appears to contribute to reproductive success through increased pollen donation when pollinator activity is high and may also increase the probability of seed set when pollinators are rare.

Sexual selection and mating systems

“…Debate on the possibility that sexual selection can operate within plants (5,(26)(27)(28)(29)(30) has identified three primary contexts in which sexual selection might occur: (i) differential pollen transfer, (ii) differential pollen tube growth, and (iii) maternal control of seed set. However, although male and female fitness may vary within each of these contexts in plants, the extreme variance in male fitness seen in animals appears to be constrained by limited opportunities for pollen dispersal, by inconsistent success of particular pollen tubes among different stigmas, or by large within-individual variation in seed size and seed set, often due to pollen limitation (31,32).…”

Ovule Packaging in Stochastic Pollination and Fertilization Environments

“…The effect this has on fer tilizations per flower is variable, however, because increased visitation rates need not keep pace with increases in flower number (Pyke 1978(Pyke , 1982Klinkhamer and de Jong 1989). Thus, mean female success per flower may decline with in creasing inflorescence size (Andersson 1988;Robertson 1992), remain the same (Geber 1985;Cruzan et al 1988;Schmid-Hempel and Speiser 1988), or increase (Dudash 1991).…”

“…Each flower on a plant is a "trial" at capturing pollen, and counts of stigmatic pollen loads indicate wide variation in success (table 1 ; see also Koptur 1984;Garwood and Horvitz 1985;Cruzan et al 1988;Elam and Linhart 1988;Bertin and Sholes 1993). Given limited resources for making ovules and other floral parts, selection favoring efficient reproductive allocation, including ovule packaging that matches, in some way, the probable levels of pollen receipt and fertilization, would be expected.…”

Heterogeneity Among Floral Visitors Leads to Discordance Between Removal and Deposition of Pollen

“…The differences may be manifest in their effect on reproductive success either through female or through male function (Lloyd and Yates 1982, Bell 1985, Campbell 1989, Snow 1989. A number of recent studies have measured pollen removal as a component of male function (Snow and Roubik 1987, Cruzan et al 1988, Galen and Stanton 1989, Harder and Thomson 1989, Wolfe and Barrett 1989, Harder 1990, Murcia 1990, Young and Stanton 1990; however, high pollen removal need not result in high subsequent deposition of the removed pollen or in high success at siring seeds. Here we provide an example in which pollen-collecting bees remove more pollen but deposit less of it than nectar-collecting bees.…”