Do annual and perennial populations of an insect-pollinated plant species differ in mating system?

Background and Aims  Theory predicts that outcrossing should be
more prevalent among perennials than annuals, a pattern confirmed by
comparative evidence from diverse angiosperm families. However,
intraspecific comparisons between annual and perennial populations are few
because such variation is uncommon among flowering plants. Here, we test
the hypothesis that perennial populations outcross more than annual
populations by investigating Incarvillea sinensis, a wide-ranging
insect-pollinated herb native to China. The occurrence of both allopatric
and sympatric populations allows us to examine the stability of mating
system differences between life histories under varying ecological
conditions. Methods   We estimated outcrossing rates and
biparental inbreeding in 16 allopatric and five sympatric popula- tions in
which both life histories coexisted using 20 microsatellite loci. In each
population we measured height, branch number, corolla size, tube length
and herkogamy for ~30 individuals. In a sympatric population, we re-
corded daily flower number, pollinator visitation and the fruit and seed
set of annual and perennial plants. Key Results  As
predicted, outcrossing rates (t) were considerably higher in perennial
(mean = 0.76) than annual (mean = 0.09) populations. This difference in
mating system was also maintained at sympatric sites where plants grew
intermixed. In both allopatric and sympatric populations the degree of
herkogamy was consistently larger in outcrossing than selfing plants.
Perennials were more branched, with more and larger flowers than in
annuals. In a sympatric population, annuals had a significantly higher
fruit and seed set than perennials. Conclusions  Genetically
based differences in herkogamy between annuals and perennials appear to
play a key role in governing outcrossing rates in populations, regardless
of variation in local ecological conditions. The maintenance of mating
system and life history trait differentiation between perennial and annual
populations of I. sinensis probably results from correlated evolution in
response to local environmental conditions.