Interfertility between Hawaiian Ecotypes of Sida fallax (Malvaceae) and Evidence of a Hybrid Disadvantage

The flora of the Hawaiian Islands is widely recognized for its spectacular adaptive radiations, yet factors maintaining or reinforcing the evolutionary divergence associated with radiation have been little studied. In the Hawaiian Islands, Sida fallax occurs as two genetically distinct forms, a beach ecotype, which grows as a prostrate shrub with small, pubescent leaves, and a mountain ecotype, which grows as an erect shrub with larger, nearly glabrous leaves. Plants with intermediate morphology have only occasionally been reported. We made experimental crosses between the beach and mountain forms to test for fertility barriers and to assess the morphology and fitness of hybrids. Among beach-mountain crosses, average fruit set (83%) and germination (80%) were not statistically different from intra-ecotype crosses. Furthermore, pollen stainability among beach-mountain hybrids (96%) was also not significantly lower than in intra-ecotype crosses, indicating a lack of fertility barriers. Among eight traits that differed statistically between beach and mountain ecotypes, four (50%) were intermediate in the hybrids, three (37.5%) showed affinity toward one parental type, and one (12.5%) was extreme. The survival and growth rate of beach-mountain hybrids did not differ from intraecotype crosses under well-watered or drought greenhouse conditions; however, the beach ecotype had a higher flowering frequency under drought conditions in comparison with the hybrids and the mountain ecotype. In the field, low water availability in the beach environment probably selects against hybrids, reinforcing differentiation between beach and mountain ecotypes. Furthermore, human land use has reduced intermediate habitats, decreasing opportunities for natural hybridization.