Hye-Been KimP, and Seung-Chul Kim
Department of Biological Sciences, Sungkyunkwan University, Suwon, Korea
Macaronesian Islands include a collection of five oceanic archipelagoes in Atlantic Ocean: Azores, Madeira, Canaries, Salvagens, and Cape Verdes. The woody Sonchus alliance, which includes 6 genera and ca. 31 species, occurs exclusively on three islands, Madiera, Canaries, and Cape Verdes. Among them, four taxa of Sonchus are restricted to Madeira: small shrubs or tall trees in higher elevation (S. fruticosus and S. pinnatus) and caudex perennials on lower coastal area (S. ustulatus subsp. maderensis and S. ustulatus subsp. ustulatus). Although they originated from a single colonization event with an estimated crown age of < 3 million years, it is uncertain about their precise phylogenetic relationships relative to congeneric species in the Canaries. Also, very little is known for the plastome organization and variation within Sonchus on Madeira in comparison to the plastomes of the Canaries. Thus, we sequenced and assembled the plastomes of Sonchus on Madeira and conducted comparative phylogenomic analyses. As a result, we found highly conserved plastome sequences among four taxa of Sonchus on Madeira. Comparing with three available Sonchus species in the Canaries, their cp genome size is slightly larger without any changes in structural and content organization. We identified four mutation hotspot regions, i.e., trnK-rps16, petN-psbM, ndhF-Ψycf1, and ycf1, and found similar simple sequence repeat (SSR) motifs among four taxa. S. ustulatus subsp. maderensis showed different patterns in codon usage bias compared to the remaining species of Sonchus. In chloroplast phylogenomic analysis, Madeira species are monophyletic and sister relationship to the Canary Islands species is suggested. However, the precise position of monophyletic Madeiran Sonchus lineage should be investigated further in a broader phylogenetic framework of the woody Sonchus alliance. Also, a parallel evolutionary trend in the growth form (i.e., transition from caudex perennials to shrubs and small/tall trees) of Sonchus species between Madeira and Canaries require further study.