The reduced fitness of progeny produced from selfing is an evolutionary force that helps shape the mating system of many self-compatible plant species. The rose-mallows (genus Hibiscus section Muenchhusia) bear large self-compatible hermaphroditic flowers that bloom for one day and possess simultaneously mature anthers and stigmas. An insect visitor is required for pollination and they have a mixed mating system. In tests to determine whether inbreeding depression may have influenced the maintenance of features that foster outcrossing, progeny produced by inbreeding were compared to progeny derived from outcrossing in several populations of two Hibiscus species: H. grandiflorus, which is restricted in distribution to the southeastern Atlantic and Gulf coasts, and the widespread H. moscheutos. Both taxa exhibited inbreeding depression in two components of fitness expressed later in the life-cycle: seed germination (in a few instances only), and (more frequently) progeny growth. The populations that were more strongly affected by inbreeding tended to be the more southerly ones, suggesting that the more northerly ones may have gone through bottlenecks and so been purged of the deleterious alleles and/or the beneficial polymorphisms as they re-established the region following glacial retreat. The selfing rate of one Ohio H. moscheutos population was estimated to be 0.44 using introduced plants with a foreign electrophoretic genotype. Inbreeding depression appears likely to be maintaining floral features which result in a substantial fraction of outcrossed offspring. A related study is underway of variation in inbreeding depression among populations of another rose-mallow, Hibiscus laevis, that differ in their ability to self-pollinate autonomously.

Key words: Hibiscus, Inbreeding depression, outcrossing, rose mallows