Supergene evolution via stepwise duplications and neofunctionalization of a floral-organ identity gene.

Abstract

Heterostyly represents a fascinating adaptation to promote outbreeding in plants that evolved multiple times independently. While l-morph individuals form flowers with long styles, short anthers, and small pollen grains, S-morph individuals have flowers with short styles, long anthers, and large pollen grains. The difference between the morphs is controlled by an <i>S</i>-locus "supergene" consisting of several distinct genes that determine different traits of the syndrome and are held together, because recombination between them is suppressed. In <i>Primula</i>, the <i>S</i> locus is a roughly 300-kb hemizygous region containing five predicted genes. However, with one exception, their roles remain unclear, as does the evolutionary buildup of the <i>S</i> locus. Here we demonstrate that the MADS-box <i>GLOBOSA2</i> (<i>GLO2</i>) gene at the <i>S</i> locus determines anther position. In <i>Primula forbesii</i> S-morph plants, <i>GLO2</i> promotes growth by cell expansion in the fused tube of petals and stamen filaments beneath the anther insertion point; by contrast, neither pollen size nor male incompatibility is affected by <i>GLO2</i> activity. The paralogue <i>GLO1</i>, from which <i>GLO2</i> arose by duplication, has maintained the ancestral B-class function in specifying petal and stamen identity, indicating that <i>GLO2</i> underwent neofunctionalization, likely at the level of the encoded protein. Genetic mapping and phylogenetic analysis indicate that the duplications giving rise to the style-length-determining gene <i>CYP734A50</i> and to <i>GLO2</i> occurred sequentially, with the <i>CYP734A50</i> duplication likely the first. Together these results provide the most detailed insight into the assembly of a plant supergene yet and have important implications for the evolution of heterostyly.