An analysis of progeny sex ratios in dioecious Silene latifolia

Much work on sex ratios in dioecious plants has involved the genus Silene. The classic literature indicates that female-biased sex ratios result from excess pollen because pollen tubes from female-determining pollen reach the ovary more quickly, on average, than pollen tubes from male-determining pollen. I proposed the weak male hypothesis to account for female-biased ratios from limited amounts of pollen. This hypothesis assumes that male-determining pollen either germinates at a lower rate or experiences greater pollen tube attrition than female-determining pollen. To test this hypothesis, limited numbers of pollen grains were placed at one of three positions along the style. According to the hypothesis, if male-determining pollen is handicapped in germination, seed set and progeny sex ratios, which should all be female biased, should not be affected by where on the style the pollen is placed. If handicapping occurs during pollen tube growth, seed set should decrease and sex ratios should become increasingly female biased as the distance traveled by the pollen tubes increases. Seed set exhibited a nonsignificant decrease as style distance increased; none of the sex ratios was female biased. Therefore, the weak male hypothesis is rejected. In another experiment, pollen from individual anthers was separated into nonoverlapping small and large size classes. Sex ratios resulting from the two size classes did not differ, indicating that there is no sex-related bimodality for pollen size. In two other experiments, excess pollen was placed at one of two positions on the style. Of 48 resulting sex ratios, only six deviated significantly from 1:1; of these six, three were female biased and three were male biased. Style position had a significant effect in one experiment (progeny from distant pollinations contained excess males), but not the other. Sex ratios were not affected by either parent in these experiments. The data indicate that female-biased sex ratios do not necessarily result from excess pollen. Therefore, gametophytic competition cannot be assumed to explain the preponderance of females in natural populations. Reasons for the differences between my results and results of early experiments are discussed