Symmetry genes and shifts in corolla bilateral symmetry

Repeated losses of mature corolla bilateral symmetry are not uncommon. Zhong et al. focus on repeated and diverse losses of bilateral symmetry testing whether similar or distinct developmental genetic mechanisms underlie shifts in corolla symmetry in Lamiaceae.

Ontogenetic origins of mature radially symmetrical corollas in (A–E) *Mimulus guttatus*, (F–J) *Callicarpa cathayana*, (K–O) *Lycopus americanus* and (P–T) *Mentha longifolia*. Cartoons (E), (J), (O) and (T) depict mature developmental stages for four focal species. *Mimulus* corollas are bilaterally symmetrical with five petals from early (B–D) to late (E) developmental stages; *Callicarpa* corollas are tetramerous and radially symmetrical from petal initiation to late development (G–I); corollas of *Lycopus* (L–N) and *Mentha* (P) are initially pentamerous and bilaterally symmetrical with two adaxial petals being fused during development (*Lycopus*, M, N; *Mentha*, Q, R), and the final corolla forms become tetramerous and radially symmetrical (*Lycopus*, O; *Mentha*, S, T). However, *Lycopus* has only two adaxial fertile stamens (yellow) with the other two abaxial (green) being sterile (O), while *Mentha*has four fertile stamens (yellow) (T); all species have two carpels (centre, blue). Scale bars = 50 μm except (I) = 100 μm and (S) = 200 μm. Sepal, s; petal, p; stamen, st; gynoecium, g; adaxial sepal, ds; abaxial sepal, bs; lateral sepal, ls; adaxial petal, dp; abaxial petal, bp; lateral petal, lp; lateral stamen, lst; abaxial stame

Using phylogenetic, SEM-based developmental and gene expression analyses, repeated losses of mature corolla bilateral symmetry in Lamiaceae are shown to result from various different mechanisms and changes to symmetry genes, including losses of a CYC2 clade and a RAD-like gene from the genome.