The role of KNOX genes in the evolution of morphological novelty in Streptocarpus

The genus Streptocarpus comprises species with diverse body plans. Caulescent species produce leaves from a conventional shoot apical meristern (SAM), whereas acaulescent species lack a conventional SAM and produce only a single leaf (the unifoliate form) or clusters of leaves from the base of more mature leaves (the rosulate form). These distinct morphologies reflect fundamental differences in the role of the SAM and the process of leaf specification. A subfamily of KNOTTED-like homeobox (KNOX) genes are known to be important in regulating meristern function and leaf development in model species with conventional morphologies. To test the involvement of KNOX genes in Streptocarpus evolution, two parologous KNOX genes (SSTM1 and SSTM2) were isolated from species with different growth forms. Their phylogenetic analysis suggested a gene duplication before the subgeneric split of Streptocarpus and resolved species relationships, supporting multiple evolutionary origins of the rosulate and unifoliate morphologies. In S. saxorum, a caulescent species with a conventional SAM, KNOX proteins were expressed in the SAM and transiently downregulated in incipient leaf primordia. The ability of acaulescent species to initiate leaves from existing leaves was found to correlate with SSTM1 expression and KNOX protein accumulation in leaves and to reflect genetic differences at two loci. Neither locus corresponded to SSTMI, suggesting that cis-acting differences in SSTM1 regulation were not responsible for evolution of the rosulate and unifollate forms. However, the involvement of KNOX proteins in leaf formation in rosulate species suggests that they have played an indirect role in the development of morphological diversity in Streptocarpus.