Breeding systems of hermaphroditic and gynodioecious populations of the colonizing species Trifolium hirtum All. in California.

A multilocus procedure was used to estimate outcrossing rates in ten roadside populations of Trifolium hirtum in California. Three groups of populations were studied: cultivars, hermaphroditic, and gynodioecious (sexually dimorphic) populations. The multilocus outcrossing rate (tm) varied from 0.05 to 0.43 among populations. Population level tm estimates were significantly correlated with the observed heterozygosity in gynodioecious populations but not in hermaphroditic populations. The outcrossing rate of hermaphrodites and females was estimated in three gynodioecious populations; the estimates of tm varied from 0.09 to 0.23 for hermaphrodites and from 0.73 to 0.80 for females. The distribution of outcrossing rates in gynodioecious populations is bimodal. Our results indicate that for the levels of selfing observed among hermaphrodites, inbreeding depression is likely to be a major factor in the maintenance of females in gynodioecious populations.