Ecological speciation and convergent evolution in the North American columbine radiation (Aquilegia, Ranunculaceae).

摘要

Ecological speciation is a driving force in evolution, yet we know little about the process of how adaptations to the environment lead to reproductive isolation. I have chosen the columbine radiation (Aquilegia, Ranunculaceae) to investigate the patterns of ecological speciation. I first confirmed the recent and rapid radiation in Aquilegia by developing a novel method for efficiently isolating orthologous intron-containing nuclear loci in non-model taxa. A survey of four of these loci suggests low levels of sequence variation confirming the radiation hypothesis. I then generated a highly resolved AFLP phylogeny for the North American Aquilegia clade which revealed similar contributions of two ecological forces, pollinator shifts and habitat adaptations, during this adaptive radiation. Significant directionality during pollinator shifts from bee to hummingbird to hawkmoth was coupled with independent origins of the hummingbird and hawkmoth pollination syndromes. For ten quantitative floral traits, we found significant correlations among most traits and evidence for increasing spur length during the North American Aquilegia radiation. I then investigated the degree of molecular convergence during the convergent adaptations to hawkmoth pollination focusing on the multiple losses of floral anthocyanins. Using the AFLP phylogeny for the North American Aquilegia clade, I identified at least six independent losses of floral anthocyanins and significantly higher rates of losses to gains. To determine the degree of molecular convergence during these parallel losses, I isolated single genes expressed in spurs and sepals for the six structural loci in the anthocyanin biosynthetic pathway. In a preliminary expression survey using rtPCR for five independently evolved anthocyanin-less lineages, we found five distinct expression profiles. Constraints on the loss of expression in the ABP are consistent with an HPLC survey of flavonoid intermediates. Several interspecific crosses among independently derived A-lineages failed to restore floral anthocyanins suggesting the cause might be different mutations in the same regulatory locus. Overall, a comparative approach to understanding the columbine radiation has revealed a significant role of directional ecological adaptation and convergence driving the diversification. Eventually, understanding the molecular basis for these traits may partly explain the mechanism underlying these patterns.