LARKIN, LEAH L.1*, JOHN L. NEFF2, and BERYL B. SIMPSON1. 1Section of Integrative Biology, University of Texas at Austin, Austin, TX 78712; 2Central Texas Melittological Insititute, 7307 Running Rope, Austin, TX 78731. - Evolution of floral specificity in a group of andrenid bees.
Callandrena is a subgenus of 80 described species in the bee
genus Andrena (Hymenoptera: Andrenidae). The group is delimited
mainly by branched scopal (pollen-carrying) hairs and shortened
mouthparts. All are specialists to varying degrees on pollen of the
Asteraceae, with females collecting pollen from plants in at least
five composite tribes. Some species are narrow specialists, using
pollen from one genus or a few related genera; others collect pollen
from plants in more than one tribe. Why bees should specialize on
flowers that are easily manipulable and morphologically similar is
unknown. We generate an hypothesis of phylogenetic relationships based
on both mitochondrial and nuclear DNA markers for 55 species of
Callandrena (eight of which are undescribed) and 40 additional
Andrena species, representing 23 subgenera, to investigate the
evolution of floral host choice in these bees. Host preference was
ascertained by analysis of pollen loads on museum specimens. Both
molecular data sets dispute the monophyly of Callandrena; there
are at least three distinct and strongly supported clades. They have
independently evolved a preference for pollen of the Asteraceae and
have also evolved convergent morphological traits to accompany this
preference. Within monophyletic groups, host shifts are generally
among plants within a tribe, however, shifts to other tribes have
occurred and may lead to adaptive radiation.
Key words: Asteraceae, Callandrena (Hymenoptera), floral specificity, phylogeny