Temporal variation in phenotypic gender and expected functional gender within and among individuals in an annual plant.
BACKGROUND AND AIMS
Adaptive explanations for variation in sex allocation centre on variation in resource status and variation in the mating environment. The latter can occur when dichogamy causes siring opportunity to vary across the flowering season. In this study, it is hypothesized that the widespread tendency towards declining fruit-set from first to last flowers on plants can similarly lead to a varying mating environment by causing a temporal shift in the quality (not quantity) of siring opportunities.


METHODS
A numerical model was developed to examine the effects of declining fruit-set on the expected male versus female reproductive success (functional gender) of first and last flowers on plants, and of early- and late-flowering plants. Within- and among-plant temporal variation in pollen production, ovule production and fruit-set in 70 Brassica rapa plants was then characterized to determine if trends in male and female investment mirror expected trends in functional gender.


KEY RESULTS
Under a wide range of model conditions, functional femaleness decreased sharply in the last flowers on plants, and increased from early- to late-flowering plants in the population. In B. rapa, pollen production decreased more rapidly than ovule production from first to last flowers, leading to a within-plant increase in phenotypic femaleness. Among plants, ovule production decreased from early- to late-flowering plants, causing a temporal decrease in phenotypic femaleness.


CONCLUSIONS
The numerical model confirmed that declining fruit-set can drive temporal variation in functional gender, especially among plants. The discrepancy between observed trends in phenotypic gender in B. rapa and expected functional gender predicted by the numerical model does not rule out the possibility that male reproductive success decreases with later flowering onset. If so, plants may experience selection for early flowering through male fitness.
['Austen|Emily J|EJ|', 'Weis|Arthur E|AE|']
D029741:Brassica rapa D035264:Flowers D005638:Fruit D008962:Models, Theoretical D056771:Ovule D010641:Phenotype D011058:Pollen D012098:Reproduction D013997:Time Factors
phenotypic gender sex allocation fruit-set pollen ovule Brassica rapa flowering time hermaphrodite hierarchical regression phenology within-plant variation