Effects of ovule and seed abortion on brood size and fruit costs in the leguminous shrub Caesalpinia gilliesii (Wall. ex Hook.) D. Dietr

For several plant species, brood size results from the abortion of ovules and seeds. However, these processes have rarely been studied together in wild plants. In some of the leguminous species studied, seed abortion has been found to depend on pollen quality and on the position of the ovule or fruit. The direct consequence for the mother plant is that fruit costs increase as the seed:ovule ratio decreases. However, because ovule abortion occurs earlier than does seed abortion, the former can reduce the biomass invested per seed (i.e., fruit costs) more efficiently than does the latter. Here, the frequencies of aborted ovules and seeds were analyzed in relation to the type of pollination treatment (open pollination vs. hand cross-pollination) and ovule/fruit position within pods of the leguminous shrub Caesalpinia gilliesii. The influence of ovule and seed abortion on fruit costs was analyzed by comparing the pericarp mass per seed between fruits with different frequencies of aborted ovules and seeds. The rate of ovule abortion was similar between hand cross-pollinated and open-pollinated fruits but was higher than that of seed abortion in oneand two-seeded fruits, as well as in those at stylar positions and in distal fruits. Hand cross-pollination reduced seed abortion but did not increase the seed:ovule ratio. In addition, fruits that aborted ovules were found to be less costly than were those that aborted seeds. From the mother plant perspective, these results indicate that ovule abortion is a more efficient mechanism of reducing fruit costs than is seed abortion, because fertilization opportunities decrease with position, and show that brood size is significantly influenced by the fate of the ovule at the pre-zygotic stage.