Reproductive Biology of the Granite Outcrop Endemic Sedum pusillum (Crassulaceae)

Abstract

Sedum pusillum, a succulent winter annual endemic to granitic outcrops in the south- eastern United States, is self-compatible. Comparisons of F1 progeny from artificial crosses in a population from South Carolina demonstrated either no difference in success for outcrossed versus selfed plants (percentage germination, pollen stainability) or slightly greater success for selfed progeny (survival to adult size, survival to flowering). Outcrossing may be promoted in the field by protandry, extrorse dehiscence of the anthers, and spatial separation of the stamens and pistils. Although S. pusillum has some pollinators in common with Diamorpha smallii, another outcrop endemic and supposed close relative, hybrids between the two taxa could not be produced in artificial crosses. In addition to these primary cytological and genetic barriers to crossing are a number of ecological and phenological differences that should result in complete reproductive isolation in natural populations.