Genome diploidization associates with cladogenesis, trait disparity, and plastid gene evolution

Angiosperm genome evolution was marked by many clade-specific whole-genome duplication events. The Microlepidieae is one of the monophyletic clades in the mustard family (Brassicaceae) formed after an ancient allotetraploidization. Postpolyploid cladogenesis has resulted in the extant c. 17 genera and 60 species endemic to Australia and New Zealand (10 species). As postpolyploid genome diploidization is a trial-and-error process under natural selection, it may proceed with different intensity and be associated with speciation events. In Microlepidieae, different extents of homoeologous recombination between the two parental subgenomes generated clades marked by slow ("cold") versus fast ("hot") genome diploidization. To gain a deeper understanding of postpolyploid genome evolution in Microlepidieae, we analyzed phylogenetic relationships in this tribe using complete chloroplast sequences, entire 35S rDNA units, and abundant repetitive sequences. The four recovered intra-tribal clades mirror the varied diploidization of Microlepidieae genomes, suggesting that the intrinsic genomic features underlying the extent of diploidization are shared among genera and species within one clade. Nevertheless, even congeneric species may exert considerable morphological disparity (e.g. in fruit shape), whereas some species within different clades experience extensive morphological convergence despite the different pace of their genome diploidization. We showed that faster genome diploidization is positively associated with mean morphological disparity and evolution of chloroplast genes (plastid-nuclear genome coevolution). Higher speciation rates in perennials than in annual species were observed. Altogether, our results confirm the potential of Microlepidieae as a promising subject for the analysis of postpolyploid genome diploidization in Brassicaceae.