Understanding the evolution and function of the heterostyly supergene in Amsinckia (Boraginaceae)

• Applicants: Dr. Axel Himmelbach; Professor Dr. Michael Lenhard
• Subject Area: Plant Genetics and Genomics ; Evolution and Systematics of Plants and Fungi
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 464595150

Heterostyly is a complex adaptation to promote outbreeding in flowering plants. In the simplest form, distyly, individuals either form flowers with long styles and low anthers (L-morph) or with short styles and high anthers (S-morph). In addition, the morphs are often distinguished by ancillary polymorphisms. The difference between the morphs is determined by a supergene, i.e. a chromosomal region with strongly suppressed recombination that contains several individual genes controlling the variation in the different traits. In general, the S-morph is heterozygous for a dominant and a recessive supergene haplotype, while the L-morph is homozygous for the recessive haplotype. Heterostyly has evolved independently in more than 25 families, and even within some families several independent origins have been found, raising questions about the basis of such convergent evolution. Molecular studies have begun to suggest commonalities in the molecular basis of heterostyly between different taxa. First, in the cases studied in detail up to now the heterostyly supergene is a hemizygous region, explaining the lack of recombination. Second, the reciprocal positions of the reproductive organs appear to be due to modulation of phytohormone pathways.The Boraginaceae family offers an excellent system to address the molecular basis of the repeated evolution of heterostyly, as this breeding system has arisen multiple times independently in the family. We have begun to study heterostyly in the genus Amsinckia. In contrast to previously studied model systems, the heterostyly supergene in Amsinckia is not hemizygous, but rather a biallelic chromosome segment of more than 20 megabases in length with strongly suppressed recombination. Based on this preliminary work, this project will pursue the following four main objectives. (1) We will establish high-quality genome assemblies of three heterostylous species in Amsinckia and use these to define the heterostyly supergene in more detail. (2) We will retrace its evolutionary origin by generating a genome assembly of an ancestral non-heterostylous species closely related to Amsinckia and by comparing the results from Amsinckia with those of a closely related genus with independently evolved heterostyly. (3) Based on the above sequences, we will test a number of population-genetic predictions about non-recombining supergenes for the Amsinckia heterostyly supergene. (4) We will identify and characterize the causal loci within this supergene that are responsible for the differences in style length, anther height and pollen size between the morphs. Together, these studies will provide insight into the repeated evolution of a fascinating breeding system in flowering plants.

• DFG Programme: Research Grants
• Co-Investigator: Dr. Christian Kappel