On the Horizon: Generic changes for the amaranth family

The amaranth family (Amaranthaceae) are flowering plants with a cosmopolitan distribution. You may be familiar with some of the species used in agriculture (e.g. grain amaranth or Amaranthus species), horticulture and floriculture (e.g. globe amaranth or Gomphrena globosa). The Australian amaranth Ptilotus exaltatus is likewise popular in gardens. Like many of the other species in the genus Ptilotus (commonly mulla mullas, foxtails or pussytails), P. exaltatus is at home in the arid regions of Australia.

Ptilotus exaltatus (tall or pink mulla mulla) in South Australia. Photo by Tim Hammer.

Ptilotus contains more than 120 species, all of which are native to Australia. Ptilotus is one of the largest and most diverse genera in its family, but has been relatively under-studied despite being remarkable within the family for its diverse flowers and pollination strategies and for its interesting biogeography, genetics (e.g. relatively high chromosome number) and sexual strategies (e.g. gynodioecy, where some plants in a population have bisexual flowers and others have all female flowers). New species of Ptilotus are being discovered regularly, especially in arid Western Australia (e.g. P. actinocladus and P. yapukaratja from 2018).

A forthcoming study in the journal TAXON by Tim Hammer and colleagues from the University of Western Australia and Curtin University has produced a new evolutionary tree (or phylogeny) based on chloroplast genome sequences for Ptilotus and relatives, a group (or clade) of genera informally known as the 'aervoids'. The aervoid clade includes the Australian genera Kelita, Omegandra and Ptilotus and the African-Asian genera Aerva and Nothosaerva. Aerva javanica (kapok bush) also occurs in Australia, but is considered an invasive weed in parts of northern Australia. All individuals of A. javanica within Australia are female, as the plant spreads via seeds that develop without needing pollination. The other Australian genera of the aervoids, Omegandra, native to northern Queensland and Northern Territory, and Kelita, endemic in Queensland, were sequenced for the first time in this study.

Aerva javanica in the Pilbara, Western Australia. Photo by Tim Hammer.

The study revealed that the genus Aerva is polyphyletic with respect to the other genera in the clade. Kelita was found to be nested within one of the major clades of Ptilotus. The other genera were confirmed to be monophyletic.

Morphological characters were found to support the phylogeny and helped to diagnose a new generic taxonomy for the aervoids.

The polyphyly of Aerva was resolved in the study by splitting the genus into three separate genera, reinstating the genus Ouret for eight species and erecting the new genus Paraerva for two Socotran endemic species. Aerva is the now re-circumscribed in a narrow sense to include only the morphologically distinct species A. javanica.

Multiple morphological characters supported the recognition of these monophyletic clades as distinct from Aerva. Further work is required to properly delimit species boundaries within Ouret, particularly for the widespread and morphologically diverse species O. lanata (previously A. lanata).

The new phylogeny and generic taxonomy will enable future research into the evolution and biogeography of the aervoids and Ptilotus, potentially uncovering when Ptilotus arrived in Australia and what has driven its diversification in arid Australia.

Stigma morphology of three aervoid genera imaged by scanning electron microscopy. Image credit: Tim Hammer.

Article Abstract:

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

by Timothy A. Hammer, Xiao Zhong, Catherine Colas des Francs-Small, Paul G. Nevill, Ian D. Small, Kevin R. Thiele