Evolution of the TCP gene family in Asteridae: cladistic and network approaches to understanding regulatory gene family diversification and its impact on morphological evolution

Abstract

In the plant subclass Asteridae, bilaterally symmetrical flowers have evolved from a radially symmetrical ancestral phenotype on at least three independent occasions: in the Boraginaceae, Solanaceae, and Lamiales. Development of bilateral flower symmetry has been shown to be determined by the early-acting cycloidea (cyc) and dichotoma (dich) genes in Antirrhinum, a member of the Lamiales. cyc and dich belong to the TCP gene family of putative transcription factors. TCP gene sequences were isolated from 11 Asteridae taxa using an array of degenerate PCR primers. Closely related species exhibiting either ancestral actinomorphic or derived zygomorphic flowers were sampled for each independent origin of bilateral flower symmetry. Cladistic and network-based analyses were performed to establish viable hypotheses regarding the evolution of bilateral symmetry in Asteridae. For the TCP gene family, the use of cladistic phylogenetic analysis to identify orthologous genes is complicated by a paucity of alignable data, frequent gene duplication and extinction, and the possibility of reticulate evolution via intergenic recombination. These complicating factors can be generalized to many regulatory gene families. As an alternative to cladistic analysis, we propose the use of network analysis for the reconstruction of regulatory gene family phylogenetic and functional relationships. Results of analyses support the hypothesis that the origin of bilaterally symmetrical flowers in the Boraginaceae and Solanaceae did not require orthologs or functional analogs of cyc or dich. This suggests that the genetic mechanism that determines bilateral flower symmetry in these taxa is not homologous to that of the Lamiales. Results of analyses are consistent with the hypothesis that the evolution of bilateral floral symmetry in the Lamiales required the origin of a novel gene function subsequent to gene duplication.