Delayed autonomous selfing and inbreeding depression in the Australian annual 'Hibiscus trionum' var. 'vesicarius' (Malvaceae)

Abstract

The Australian annual, 'Hibiscus trionum' var. 'vesicarius', produces large, showy flowers typical of an outcrossing species, yet flowers autonomously self-pollinate. We used experimental pollinations to examine self-compatibility, inbreeding depression and the efficiency and mechanism of autonomous selfing. Seed set of self- or cross-pollinated flowers did not differ, indicating that plants were fully self-compatible. Seed set following autonomous selfing varied among plants, and was 11–103% of that following hand-selfing. Autonomous selfing was delayed, and styles curved and stigmas contacted the anthers before flowers closed. Delayed selfing was facultative and curvature depended on the number of pollen grains on stigmas, with 50 or more grains preventing curvature. Both self- and cross-pollen prevented curvature. Similarly to unpollinated styles, styles that were pollinated with dead pollen curved fully, indicating that either pollen germination or pollen-tube growth prevents curvature. Within flowers, the five styles acted independently, depending on the amount of pollination that each received. Although plants exhibit a high potential for selfing, crossed progeny outperformed selfed progeny and cumulative inbreeding depression was 0.64, which is high for a self-compatible annual. Despite this high inbreeding depression, delayed selfing would be advantageous under variable pollinator conditions, providing reproductive assurance.