Delayed selfing and low levels of inbreeding depression in Hibiscus trionum (Malvaceae)

Abstract

We used experimental pollinations to examine the breeding system and inbreeding depression in Hibiscus trionum L., an annual herb of cultivated and disturbed sites in eastern Australia. Seeds were not produced asexually. Flowers were fully self-compatible and autonomously self-pollinating. Autonomous self-pollination was due to a delayed selfing mechanism that gave precedence to cross pollen but ensured that stigmas contacted the anthers at the end of floral life. Using selfed and crossed progeny from 10 maternal plants, we examined the magnitude and timing of inbreeding depression over a range of life-cycle stages, including seed production by maternal plants, and seed germination, seedling growth, survival, flowering and seed production by F₁ plants. Average cumulative inbreeding depression was 0.15, although there was considerable variation among maternal families (δ = –0.07–0.43). Inbreeding depression was not uniform across all life stages and was most prevalent late in the life cycle, affecting days to flowering and fruiting and flower production. Our results indicate that major lethal alleles have been purged from this population, probably through high levels of recurrent self-fertilisation. The flexibility in plant mating afforded by delayed selfing is likely to contribute to the invasiveness of H. trionum, particularly in annual cropping situations.