Resolving reticulate evolutionary histories of polyploid species of Azorella (Apiaceae) endemic to New Zealand

Abstract

Genera with species of multiple ploidy levels provide models to understand successive rounds of whole genome duplication leading to intricate reticulate relationships of polyploid plant species. Here, we studied 17 polyploid taxa (species, subspecies, or varieties) in Azorella (Apiaceae) sections Schizeilema and Stilbocarpa that are mostly endemic to New Zealand. Using phylogenomic approaches, our goals were to resolve species relationships, determine the origins of the higher-level polyploids (6x and 10x), and assess the biogeography of the New Zealand Azorella species. Phylogenomic analysis of Anigosperms353 baits-captured Hyb-Seq data, together with comparison of phylogenies reconstructed using genome-skimming retrieved nrDNA and plastome sequences, showed that species diversification within New Zealand may relate to multiple origins from South America, which has been further shaped by additional rounds of polyploidy as well as hybridization or introgression. The two Azorella sections in New Zealand likely resulted from different biogeographic events from South America − one to the subantarctic islands (section Stilbocarpa) and a second to the South Island (section Schizeilema). In addition, within section Schizeilema, species have dispersed from the South Island (New Zealand) to Australia, the subantarctic islands, and the North Island (New Zealand). Our combined approach of phylogenomic analyses of plastome and nuclear locus-based data, together with SNP-based network approaches allowed us to determine the origins of some higher-level polyploids in New Zealand Azorella and revealed a more complex picture of historical and ongoing polyploidy and hybridization within these lineages.