Genetic analysis of pollination-related floral shape in Streptocarpus (Gesneriaceae)

The great diversity of floral shape arouses the natural curiosity to explore the underlying genetic control. Floral zygomorphy as a key innovation to promote plant-pollinator specialization has been proposed to facilitate floral shape diversification. CYCLOIDEA (CYC), RADIALIS (RAD), and DIVARICATA (DIV) have been identified to establish floral zygomorphy. With the ability of regulating cell division and cell growth differentially along the dorsoventral axis, these zygomorphy genes are proposed to be involved in the establishment of different pollination syndromes. In addition, MIXTA-LIKE (MYBML) genes, which are identified to act on the floral architecture via controlling cell shape, are also proposed to be involved in this process. However, no genetic analysis on the phenotypic effects of these genes on floral shape variation related to different pollination syndromes has ever been carried out. Species of the genus Streptocarpus (Cape Primrose, Gesneriaceae) exhibit flowers with pollination syndromes ranging from butterfly flowers with keyhole type corollas, fly flowers with open tube corollas to bird flowers with narrow tubular red corollas. The open tube flowered S. rexii and the keyhole flowered S. johannis were crossed to produced F1 and 119 F2 plants. Association analysis among 119 F2 individuals revealed that StrRAD1 had phenotypic effects on the overall corolla tube length and total stamen length, whereas StrDIV2 affected the diameter of the undilated part of the corolla tube(, an important trait likely correlated with the proboscis length of potential pollinators). In addition, StrCYC1A and StrMYBML2 formed a strong linkage group, and significantly influenced the diameter of the tube opening and the dilated part of the corolla tube, the key characters that define the size of the ‘insect visiting chamber’, and hence effective pollinator. Both traits are critical for the establishment of different pollination syndromes in Streptocarpus. This finding, the first reported genetic link between zygomorphy genes and the differentiation of pollination syndromes, implied that the zygomorphy genes not only act on the establishment of zygomorphy but also on the evolution of floral diversification.