Doctor of Philosophy
Date of Defense
Elizabeth A. Kellogg
Bilaterally symmetrical corollas have evolved independently numerous times from radially symmetrical ancestors and are thought to represent adaptation to specific pollinators. However, evolutionary losses of bilateral symmetry have occurred sporadically in different lineages. CYC2-like and RAD-like are genes needed for the normal development of bilateral symmetry in snapdragon corollas. However, exactly how changes in the floral symmetry patterning genes correlate with the origin and loss of floral bilateral remains poorly known. To address this question, a densely sampled phylogeny of CYC2-like genes across the order Lamiales was inferred and calibrated. The expression patterns of these genes in early diverging and higher core clades were also examined. The phylogeny indicated at least four independent duplications of CYC2-like genes in four major lineages of Lamiales around the Cretaceous-Paleogene (K-Pg) boundary, coinciding with the initial diversification of bumble bees and euglossine bees. Losses of CYC2-like paralogs were common, but did not correlate with a corresponding loss in floral symmetry. Relaxed positive selection occurred concurrently with retention of duplicates. CYC2-like paralogs showed differential expression, and asymmetrical expression of individual CYC2-like genes in adaxial and lateral petals correlated with the independent origins of floral zygomorphy in core Lamiales. The expression patterns of CYC2-like genes have evolved in a stepwise fashion. CYC2-like was expressed only very early in development in Oleaceae, while persistent expression of CYC2-like in petals originated in the common ancestor of Tetrachondraceae and core Lamiales. Asymmetrical expression in adaxial and lateral petals appeared later with the common ancestor of the core Lamiales. Similarly, expression of RAD-like in petals appeared in early diverging Lamiales or earlier, while asymmetrical expression in adaxial and lateral petals appeared later with Plantaginaceae and Gesneriaceae. Flowers of three radially symmetrical members of Lamiaceae were studied in detail, and the results indicated that each achieved radial symmetry in a different way. Development and expression of CYC2-like genes in Lycopus were similar to those of their bilaterally symmetrical relatives. However, expanded expression of CcCYC2A correlated with a radially symmetrical corolla in Callicarpa. Finally, loss of CYC2A and altered expression of CYC2Bs may account for the early bilateral symmetry but late radial symmetry in Mentha.
Zhong, Jinshun, "The evolution of floral symmetry across the plant order Lamiales" (2013). Dissertations. 271.