Gene duplication and relax from selective constraints of Gcyc genes create high floral diversity in Didymocarpoideae (Gesneriaceae)

<p>Floral bilateral symmetry is the most important trend for flower shape diversity in 200,000 angiosperm species. Zygomorphy and reversal back to actinomorphy independently evolved multiple times in angiosperm evolution. The genetic basis and the role of selection, however, remain lots of unknown. Yet the developmental mechanism which controls the floral bilateral symmetry relies on <em>CYCLOIDEA </em>(<em>CYC</em>), a member of TCP gene family. Meanwhile, <em>Gcyc </em>genes, <em>CYC</em>-like genes in Gesneriaceae, duplicated multiple times in evolutionary history. Members of Gesneriaceae consist of mainly zygomorphic flowers with only a few exceptions, providing a great natural experience opportunity for revealing the evolution patterns of symmetry evolution. Didymocarpoideae has the most number of actinomorphic species among Gesneriad subfamilies. Three duplication events lead to multiple copies of <em>Gcyc </em>ranging from two to four in each species. In this study, we detected the selective pressures of duplication <em>Gcyc </em>lineages, revealing a relaxation signals after duplication but without lineage-specific patterns. Expression of <em>Gcyc </em>genes of two zygomorphic and actinomorphic species indicates a species-specific pattern, which two zygomorphic species utilize different copies to retain bilateral symmetry. All <em>Gcyc </em>genes in actinomorphic species absent in corolla, stamens, and gynoecium which implies ventralization. Based on the relaxation of duplicated <em>Gcyc </em>genes and species-specific expression patterns, we suggested a possible "evolutionary flexibility" after <em>Gcyc </em>genes duplication which leads to diverse floral symmetry in Didymocarpoideae. This finding, therefore, supports the observation that lineages with floral bilateral symmetry have been able to generate more floral shape diversity.