Reproductive Costs of Self‐pollination in Ipomopsis Aggregata (Polemoniaceae): Are Ovules Usurped?

In common with many cosexual angiosperms, the hummingbird‐pollinated montane herb Ipomopsis aggregata (Polemoniaceae) is likely to experience self‐pollination. Does this incur a fecundity cost even in such a highly self‐sterile (presumably self‐incompatible) species? Histological studies showed that self pollen germinates, and its tubes penetrate ovules almost as often as those of outcross pollen. Ovules penetrated by self tubes are especially likely to show milky callose occlusion 24–48 hr after pollination, and several observations suggest an association between occlusion and degeneration. Compared to flowers receiving only outcross pollen, seed set was reduced by 42% on average when self pollen was applied along with outcross, either by hand (in emasculated flowers) or by natural autodeposition (in unemasculated flowers). Reductions were statistically indistinguishable whether self pollen was applied 9 hr before outcross pollen, or at the same time. Unemasculated flowers accumulated substantial self pollen loads, and this autodeposition persisted when flowers were probed to mimic hummingbird visitation. Geitonogamy also is substantial, judging from field estimates of pollen transfer. Thus natural self pollen deposition may be sufficient to “usurp” ovules that otherwise could mature. In this light, late‐acting self‐rejection in I. aggregata seems decidedly less efficient than an early‐acting system that would block pollen germination or tube growth.