Monkeyflowers
Working
in a university department with a long-standing research interest in Mimulus, I noticed immediately the paper by Cooley et al. (Durham, USA
and Santiago, Chile (pp. 641–650) who are interested in speciation mechanisms and reproductive
isolation. In a recent paper (Hybridization as an invasion of the genome. Trends
in Ecology and Evolution 20: 229–237; 2005),
Mallet states that at least 25 % of plant species, mostly younger in
evolutionary terms, are involved in hybridization. How then do reproductive
barriers actually arise? Cooley et al. carried
out a very careful, well-designed and thorough study based on the possibility
that pollinator preference plays a role in floral diversification. They worked
on Chilean populations of four Mimulus (sub)-species,
M. luteus luteus, M. l. variegatus, and M. naiandinus and M. cupreus. The latter has orange flowers rather
than the classic yellow with variable red spotting of the ‘monkeyflower’.
In the field, pollinator visits were studied for large numbers of plants.
Laboratory-based observations included floral morphology, floral anthocyanins and nectar contents. The three yellow (sub)species were all visited almost exclusively by the same
generalist pollinator, a bumble bee, Bombus dahlbomii, which made no overall distinction
between flowers based on the amount of red anthocyanin
pigment spots when species were growing separately. However, observations in an
area where both M. l. luteus and M. naiandinus (and hybrids) grow
revealed that individual pollinators had different preferences but, overall,
there was a bias towards luteus-type
flowers. Mimulus cupreus differs from the other three, both
in overall plant morphology and in possessing orange flowers. It received very
few pollinator visits and, indeed, it exhibits a high degree of selfing. Low pollinator visitation was not caused by
discrimination against orange flowers: a rare yellow morph was equally
neglected. The authors have thus shown that the appearance of red-pigmented
flowers in ‘yellow monkeyflower’ Mimulus species has not led to changes in
pollinators and that we need to know more about non-pollinator mechanisms in
the generation of floral diversity.
