HAZLE, THOMAS* and JUDITH M. CANNE-HILLIKER. Department of Botany, University of Guelph, Guelph, ON. N1G 2W1. - Ontogenetic floral evolution of Mimulus lewisii and M. cardinalis(formerly Scrophulariaceae, s.l.): Allometry and morphology.
Speciation in the bee-pollinated Mimulus lewisii and
bird-pollinated M. cardinalis group is hypothesized to have
occured through divergence in floral form that led to a change in
breeding system. This effectively reproductively isolates the species.
Previous studies have provided extensive genetic, phylogenetic and
natural selection data which allow for a rare opportunity to now study
and interpret ontogenetic changes as sources of evolutionary novelties
in floral form. Three populations of M. cardinalis and four
populations of M. lewisii(representing both described races)
were studied from initiation of floral apex to anthesis using SEM and
light microscopy. Allometric analyses were conducted on data derived
from floral organs. Sympatric populations of the species from Yosemite
National Park were compared. Calyces of M. lewisii initiate
later than those of M. cardinalis relative to the inner whorls,
and sepals are taller and more acute. Relative times of initiation of
petals, sepals and pistil are similar in both species. Petal shapes
differ between species throughout development. Corolla aperture shape
becomes dorso-ventrally narrow during development of M.
lewisii, and laterally narrow in M. cardinalis. In both
species, the adaxial filaments curve adaxially as they elongate. In
M. lewisii, the abaxial stamens curve away from the median
while those of M. cardinalis are directed adaxially. Allometric
analyses reveal that differences in corolla length dimensions between
species are controlled by alterations in rate of growth, while
differences in stamen and pistil dimensions are controlled by
alterations in timing of termination of growth relative to corolla
growth.
Key words: allometry, bee/bird pollination, evolution, Mimulus, ontogeny, Scrophulariaceae