Chloroplast and ITS phylogenies to understand the evolutionary history of southern South American Azorella, Laretia and Mulinum (Azorelloideae, Apiaceae)

Abstract

Azorella, Laretia and Mulinum are taxonomically complex, and good candidates to study evolutionary radiations in the Andes and the importance of hybridizations. Previous phylogenetic studies of subfamily Azorelloideae agree that Azorella and Mulinum as currently conceived are not monophyletic, and hence a revision of their circumscription is necessary. However, these phylogenies were based only on chloroplast DNA sequence data. Here, phylogenetic relationships within Azorelloideae were inferred using sequence data from five chloroplast DNA (rps16 intron, trnQ-rps16, rps16-trnKUUU 5′ -exon, trnGGCC-trnSGCU and rpL32-trnLUAG), and from nuclear rDNA ITS regions to assess the monophyly of Azorella and Mulinum and discuss generic re-circumscriptions, determine hybridization and radiation events, identify and characterize important lineages, and propose hypotheses on evolution of key morphological characters. In total, 121 accessions of Azorelloideae were analyzed. Phylogenetic analyses of the different genomes were conducted separately and combined, with and without indels, using maximum parsimony, maximum likelihood, and Bayesian methods. To analyze the incongruence between plastid and nuclear-derived trees a consensus network from strongly supported nodes from cpDNA and ITS trees was constructed. Internode certainty values were calculated to evaluate the reliability of the relationships estimated from the individual cpDNA and ITS data sets and to examine the degree of conflict within the total evidence data set. Azorella and Mulinum were confirmed as not monophyletic. Except three Azorella species, the remaining azorellas, all species of Mulinum, and Laretia form a monophyletic group, designated here as Andean-Patagonian. The three species of Azorella that are not part of the Andean-Patagonian lineage are grouped together with Huanaca and Schizeilema in another lineage, designated here as Austral. Within the Andean-Patagonian clade, three major lineages can be recognized: Diversifolia, Trifurcata, and Spinosum. Each of these lineages have different leaf morpho-anatomies, Diversifolia species being more mesomorphic compared to species of Trifurcata, and species of Spinosum being the most xeromorphic. Hybridizations have been important in the evolution of the group, especially within Diversifolia, with at least six reticulation events resulting in putative homoploid and allopolyploid hybrid species. Evidence from branch lengths and low sequence divergences suggest a rapid radiation in the Spinosum group, probably associated with the acquisition of wings in the fruits.