A molecular framework phylogeny for Ptilotus (Amaranthaceae): Evidence for the rapid diversification of an arid Australian genus

Abstract

Ptilotus (Amaranthaceae) is an Australian genus with over 100 species, most of which occur in arid Western Australia. Ptilotus has been a taxonomically difficult genus; despite rigorous morphological studies into the genus over many years, previous workers have found it difficult to delimit infrageneric groups due to inconsistent morphological variation. With the goal to establish a phylogenetic framework for the genus, 100 taxa were sampled, including 87 Ptilotus spp., and the ITS nrDNA and matK cpDNA were sequenced. The phylogeny was reconstructed using Bayesian, maximum likelihood and maximum parsimony analyses on separate and concatenated datasets. Morphological characters were assessed and compared to clades on the phylogeny to identify synapomorphies and aid in the construction of an infrageneric classification. A diversification rate analysis was used to identify rate shifts in speciation across the phylogeny. Four major clades of the monophyletic Ptilotus were resolved, three small clades together comprising 27% of sampled taxa and a large, diverse clade comprising the remaining 73%. Four floral synapomorphies were identified as uniquely occurring within the latter, although none were common to all taxa in the clade. The diversification rate analysis identified a probable rate shift at the base of Ptilotus, indicating that the genus may have undergone a rapid diversification early in its evolution. This rapid diversification provides a plausible explanation for the lack of consistent variation in morphology among the major clades.

Resolving intergeneric relationships in the aervoid clade and the backbone of Ptilotus (Amaranthaceae): Evidence from whole plastid genomes and morphology

“…The phylogenies of Hammer & al. () and Hammer & al. (), based on Sanger sequencing of a small number of markers, provided a good understanding of major clades within the aervoids.…”

“…The Sanger sequencing section of this study incorporated all ITS and matK sequences from Hammer & al. (), some of the outgroup species used in Hammer & al. (2017), and an additional 26 species not previously sequenced.…”

“…Hammer & al. () reconstructed the phylogeny of 87 species of Ptilotus using ITS and matK . Sampling covered the morphological and geographic diversity of the genus, and included three species of Aerva , two species of Gomphrena , Achyranthes aspera L. and Amaranthus caudatus L. (on which the tree was rooted).…”

“…Hammer & al. () speculated that Ptilotus had radiated rapidly, probably after its ancestor arrived on the Australian continent following a divergence in Southeast Asia from a common ancestor with one of the clades of Aerva .…”

Pollen characters and DNA sequence data converge on a monophyletic genus Iresine (Amaranthaceae, Caryophyllales) and help to elucidate its species diversity

“…The plastid trees as compared to ITS trees tend to be better supported at deeper nodes. This is probably due to high levels of homoplasy in ITS and has been observed in other datasets in a similar way (e.g., Fuentes & al., 2012;Hammer & al., 2015).…”