PASCARELLA, JOHN B. Department of Biology, Valdosta State University, Valdosta, GA 31698. - Causes and consequences of reproductive isolation between two sympatric Gelsemium species.
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