The relationship between the pollen-ovule ratio and seed size: a comparative test of a sex allocation hypothesis

Hypothesis: Sex allocation theory predicts that the pollen-ovule ratio should increase linearly with increasing seed size among seed plants (Charnov, 1982). Data examined: We retrieved data for the pollen-ovule ratio, seed size, and possible confounding variables (ovule number, plant height, mating system) from a database and additional literature for 299 gymnosperm plant species of the German flora. Methods: We analysed uncorrected cross-species data as well as phylogenetically independent contrasts with Model II regressions and (partial) correlations. Results: A linear positive correlation between pollen-ovule ratio and seed size was found to exist across all plant species analysed and within different mating systems for phylogenetically corrected and uncorrected data. This positive correlation remained valid when we controlled for the effect of possible confounding variables. Conclusions: The interspecific variation of the pollen-ovule ratio depends, at least partly, on the allocation of resources to female sexual function.