SYSTEMATIC RELATIONSHIPS WITHIN GELSEMIUM (LOGANIACEAE) - EVIDENCE FROM ISOZYMES AND CLADISTICS

We used horizontal starch-gel electrophoresis to assess the degree of genetic divergence between four populations each of Gelsemium sempervi rens and G. rankinii from the southeastern United States and to test t he hypothesis that G. sempervirens is an autotetraploid derivative of G. rankinii. Levels of genetic variation in both taxa are somewhat hig her than the mean for long-lived, woody perennials. There was no indic ation that the alleles of one species represent a subset of the variat ion present in the other nor was there any indication of tetrasomic ex pression in G. sempervirens. Moreover, chromosome counts from root tip squashes confirm that both species are diploid, with 2n = 16. There i s, however, unusual mitotic behavior that may explain apparently erron eous counts of lower numbers by some previous workers, who also report ed larger and fewer stomates in the presumably autotetraploid G. sempe rvirens. Sharp genetic differences between the two species (mean genet ic identity, IBAR = 0.48) parallel their morphological discontinuities in many characters, and there was no evidence of hybridization or int rogression in regions of sympatry. A cladistic analysis using eight mo rphological characters supports the view that G. sempervirens and C. r ankinii are sister species, more closely related to each other than ei ther is to the third species in the genus, G. elegans from southeast A sia. Genetic evidence suggests that the lineage leading to G. rankinii and G. sempervirens split in the late Tertiary, about 3-3.5 million y ears ago. It is possible that G. rankinii adjusted to life in Coastal Plain swamps by loss of winged seeds, becoming adapted for dispersal b y water, rather than wind.