Plant size effects on allocation to male and female functions in pearl millet, a hermaphroditic wind-pollinated species

In hermaphroditic higher plants, reproductive fitness can be achieved through male and female functions. The relative allocations to reproduction by maternal and paternal investments define the gender of each individual. In wind-pollinated species, the theory predicts that gender should evolve with plant size, the largest plants investing the most in male function. More specifically, the tallest plants should exhibit the highest degree of maleness, because pollen dispersal is most effective when the release point is high. We searched for potential variation in gender with plant size in pearl millet (Pennisetum typhoides), where each plant produces several shoots. Size was estimated by using four parameters, namely height, stem diameter, total leaf area, and vegetative weight, for shoots and for plants. Gender was estimated as the pollen to ovule (P:O) ratio for each shoot or plant. We found that allocation to male function is very variable, both between shoots within a plant and between plants. Most of the difference observed is correlated with differences in the weight of the shoots or plants, the heaviest ones having the highest P:O ratios. The theoretical prediction that maleness should increase with plant height was not fulfilled. The parameter used to estimate plant size seems to have a great influence on the ability to detect gender variation with size. The allocation to reproduction (defined as the weight of seeds, stamens, floral parts, and rachis of the spike) represented a decreasing proportion of total plant weight; also, the heavier the plant, the higher the proportion of this allocation devoted to male function. Since pearl millet is a cultivated species, this may be due to the fact that the domestication process, by selecting for larger plants, involuntarily selected for maleness by favoring the largest plants.