Pollen tube development in two species of Trithuria (Hydatellaceae) with contrasting breeding systems

Trithuria (Hydatellaceae; Nymphaeales) is unique among early-divergent angiosperms in that its species are extremely small and most have exceptionally short, annual life histories. Given the evolution of these extremes of size and development, we sought to understand whether post-pollination processes still varied predictably with breeding system in Trithuria. To address this question, we studied two Western Australian species, Trithuria austinensis (dioecious, obligately outcrossing) and Trithuriasubmersa (bisexual, highly selfing). To document developmental timing, carpels were hand-pollinated, collected at sequential time points, and examined with light and fluorescence microscopy. In both species, pollen tubes first entered ovules <1 h after pollination, but the pollen tube pathway of outcrossing T. austinensis was almost four times longer and its pollen tube growth rates were up to six times faster (≤2,166 vs. 321 μm/h) than those of T. submersa. T. austinensis also exhibited greater male investment, slower pollen germination, and greater pollen tube attrition. These differences in male gametophyte development are predicted for outcrossers versus selfers in phylogenetically derived angiosperms. These new data for Hydatellaceae reinforce the idea that an acceleration of pollen tube development occurred in the Nymphaeales stem lineage, before the origin of Hydatellaceae. We infer that a recent evolutionary transition to selfing in T. submersa has been accompanied by predictable modifications to reproductive development, which, because of the ancient relationship between Hydatellaceae and all other angiosperms, suggests that traits underlying the lability of flowering plant post-pollination biology were present early in their history.