Molecular analysis of the anthocyanin2 gene of petunia and its role in theevolution of flower color

The shape and color of flowers are important for plant reproduction because they attract pollinators such as insects and birds. Therefore, it is thoug ht that alterations in these traits may result in the attraction of differe nt pollinators, genetic isolation, and ultimately, (sympatric) speciation. Petunia integrifolia and P axillaris bear flowers with different shapes and colors that appear to be visited by different insects. The anthocyanin2 (a n2) locus, a regulator of the anthocyanin biosynthetic pathway, is the main determinant of color differences. Here, we report an analysis of molecular events at the an2 locus that occur during Petunia spp evolution. We isolat ed an2 by transposon tagging and found that it encodes a MYB domain protein , indicating that it is a transcription factor. Analysis of P axillaris sub species with white flowers showed that they contain an2(-) alleles with two alternative frameshifts at one site, apparently caused by the insertion an d subsequent excision of a transposon. A third an2(-) allele has a nonsense mutation elsewhere, indicating that it arose independently. The distributi on of polymorphisms in an2(-) alleles suggests that the loss of an2 functio n and the consequent changes in floral color were not the primary cause for genetic separation of P. integrifolia and P. axillaris. Rather, they were events that occurred late in the speciation process, possibly to reinforce genetic isolation and complete speciation.