Anomalous apetaly: localized character displacement in Ranunculus eschscholtzii

Abstract

Apetaly is usually a rare variation in populations of the widespread hexaploid alpine Ranunculus eschscholtzii Schlecht. In certain high rainfall sections of the western Cascade Mountains of Washington, where populations of the hexaploid have been found in mixed association with those of a diploid relative, R. suksdorfii (elsewhere ecologically well separated though sympatric), the apetalous condition almost completely replaces the normal form. Any petal-bearing flowers of the hexaploid attract the flying pollen vectors (mainly syrphids) which also visit the overwhelmingly more numerous diploid species. Any resulting hybrids between the two species are sterile. Apetalous flowers are avoided by the syrphid pollinators. These flowers, nevertheless, regularly produce full sets of viable fruits. This may be attributed to the regular foraging visits of large terrestrial ants. The apetalous condition of R. eschscholtzii appears to have replaced the normal form in the high rainfall areas because only it can continue to reproduce and maintain the species in the presence of R. suksdorfii and the syrphid pollinators. This regional intensification of reproductive barriers between species already genetically isolated by their difference in ploidy, and between which hybrids have zero fitness, is interpreted as a case of character displacement involving both competition for a resource (pollinators) and hybrid sterility.