Prezygotic barriers to hybridization were examined for two sympatric Gelsemium (Loganiaceae) species, G. sempervirens and G. rankinii, occurring in the southeastern U.S. Two populations in Lowndes County, Georgia were studied in 1999 and 2000. Both species have similar floral biologies and share pollinator species, with the apid bee Habropoda laboriosa the most important visitor to both species. Differences in temporal flowering patterns appear to be limiting potential hybridization events, with G. sempervirens flowering from late January to late March and G. rankinii flowering from late March to mid April. A consequence of this divergence in flowering time is much lower fruit set ratios in G. sempervirens than G. rankinii. This low fruit set is attributed to two factors: 1) colder temperatures limit the activity of pollinators of G. sempervirens but do not limit those of G. rankinii; 2) the proportion of males vs females of Habropoda laboriosa visiting Gelsemium flowers is more strongly male biased for G. sempervirens and males are less effective pollinators than females. One consequence of this lower fruit set may be the longer flowering period and greater flower production in G. sempervirens.

Key words: Gelsemium rankinii, Gelsemium sempervirens, Habropoda laboriosa, Loganiaceae, pollinators, reproductive isolation