Gynodioecy and Ptilotus obovatus

Ptilotus obovatus
The habit of Ptilotus obovatus in the Gascoyne bioregion, Western Australia. Photo by T. Hammer.

Within the genus Ptilotus R.Br. (Amaranthaceae), one of the most common, yet enigmatic, species is the shrub Ptilotus obovatus (Gaudich.) F.Muell., sometimes referred to as ‘silver mulla mulla’ or ‘cotton bush’. Charles Gaudichaud-Beaupré (1829) originally described the species based on a specimen from near Shark Bay, Western Australia as Trichinium obovatum Gaudich. The species was later transferred to Ptilotus by Ferdinand von Mueller (1868). The species is widespread throughout the arid interior of Australia, and it is morphologically diverse.

An interesting phenomenon within the species is the presence gynodioecy, which is a dimorphic sexual system consisting of both hermaphrodite (i.e. having a functional androecium and a gynoecium) and female individuals (i.e. laking a functional androecium) in a population (Stewart & Barlow 1976). Within P. obovatus, the hermaphrodite individuals have three stamens and two yellow, appendage-like staminodes. The female individuals lack functional stamens, which instead take on the appearance of the staminodes in the hermaphrodite individuals and effectively having five staminodes instead of regularly two (see below images). Some populations have a majority of female individuals (pers. obs.).

Stewart and Barlow (1976) looked at polyploidy and a number of interesting features found in P. obovatus, including gynodioecy. Of the five localities counted for male sterility by Stewart and Barlow (1976) in WA and SA, four of the localities had more female individuals than hermaphrodites. The cytological samples examined found a lower sex ratio than field samples, in part because a number of plants that appeared to be hermaphrodite had anthers that did not produce pollen. Field observations of male-sterile plants could be underestimating them within the population, as it is difficult to determine in the field if plants are functionally hermaphrodite. Additionally, Stewart and Barlow (1976) attempted to look at self-compatibility within P. obovatus, but were unable to produce experimental crosses, which they “attributed to technical problems related to floral structure” (p. 244). They presumed that diploid P. obovatus was self-compatible due to the successful setting of two seeds from one plant. A study of self-compatibility within Ptilotus is warranted.

Ptilotus obovatus is not the only species within Ptilotus to possess gynodioecy. Ptilotus schwartzii (F.Muell.) Tate is another widespread arid species with gynodioecy (see images below), some populations having female individuals far outnumbering hermaphrodite individuals (pers. obs.). One species, P. crispus Benl, is known to be dioecious (i.e. separate male and female individuals in a population), and close relatives of Ptilotus also have dioecy (e.g. Aerva Forssk.).

Since Darwin, there has been much speculation as to why we find gynodioecious species. He perhaps did much to shape the view of gynodioecy shared by subsequent authors, that is a mid-way point between hermaphroditism and dioecy. Yet Darwin (1877) saw difficulty in seeing the need for this shift in sexuality, remarking (pp. 279–280):

There is much difficulty in understanding why hermaphrodite plants should ever have been rendered dioecious. There would be no such conversion, unless pollen was already carried regularly by insects or by the wind from one individual to the other; for otherwise every step towards dioeciousness would lead towards sterility. As we must assume that cross-fertilisation was assured before an hermaphrodite could be changed into a dioecious plant, we may conclude that the conversion has not been effected for the sake of gaining the great benefits which follow from cross-fertilisation. We can, however, see that if a species were subjected to unfavourable conditions from severe competition with other plants, or from any other cause, the production of the male and female elements and the maturation of the ovules by the same individual, might prove too great a strain on its powers, and the separation of the sexes would then be highly beneficial. This, however, would be effected only under the contingency of a reduced number of seeds, produced by the females alone, being sufficient to keep up the stock.

Darwin (1877) concluded by saying that the females of a gynodioecious species produce more seeds than if they were hermaphrodites (p. 345), thus justifying decreased pollen production.

  • Darwin, C. (1877). The different forms of flowers on plants of the same species. (John Murray: London)
  • Gaudichaud-Beaupré, C. (1829). Voyage Autour du Monde … sur les Corvettes de S.M. l’Uranie et la Physicienne. Botanique 11: 445.
  • Mueller, F.J.H. von (1868). Fragmenta Phytographiae Australiae 6(49): 228.
  • Stewart, D.A. & Barlow, B.A. (1976). Infraspecific polyploidy and gynodioecism in Ptilotus obovatus (Amaranthaceae). Australian Journal of Botany 24: 237–248.

Ptilotus actinocladus

Ptilotus actinocladus
The new species, Ptilotus actinocladus T.Hammer & R.W.Davis. Photo by G. Byne.

A new and rare species, Ptilotus actinocladus T.Hammer & R.W.Davis, has been recently published in volume 29 of the journal Nuytsia (Hammer & Davis, 2018). The new species is endemic to the Gascoyne bioregion of Western Australia and known from several collections, mostly around Doolgunna Station.

Ptilotus pseudohelipteroides
Ptilotus pseudohelipteroides Benl in South Australia. Photo by T. Hammer.

Specimens currently within P. actinocladus were previously assigned to P. pseudohelipteroides Benl, constituting the only specimens of this species from Western Australia. The type of P. pseudohelipteroides is from Queensland, with the typical variety of this species occurring in the Northern Territory, Queensland and South Australia. Ptilotus pseudohelipteroides was described by Gerhard Benl (1959), based on the variety Trichinium helipteroides F.Muell. var. minor J.M.Black described by John M. Black (1924) and latter the combination made within Ptilotus by Hansjörg Eichler (as P. helipteroides var. minor). Benl separated P. pseudohelipteroides from P. helipteroides (F.Muell.) F.Muell. based on it varying in floral and vegetative characters.

Ptilotus actinocladus map
Map the distribution of Ptilotus actinocladus (triangles), P. helipteroides (vertical shading) and P. pseudohelipteroides (horizontal shading).

When examining the specimens from Western Australia for the identification key to the Ptilotus (available on KeyBase), it was clear that these were different from P. pseudohelipteroides. The morphological variation and geographical disjunction between the two entities clearly showed that the specimens from Western Australia deserve to be placed within its own species. The new species does overlap with P. helipteroides, but is also morhologically distinct from that species in multiple floral and vegetative characters, including the habit, leaf and stem indumentum, and size of the floral parts (which are more similar to P. pseudohelipteroides). The specific epithet of the new species refers to the radiating (actino-) stems (cladus) that are prostrate and characteristic of this species. Please feel free to read it in more detail on FloraBase, at

  • Benl, G. (1959). New species and varieties of Ptilotus R.Br. (Amaranthaceae). Muelleria 1(2): 105–107.
  • Black, J.M. (1924). Casuarinaceae – Euphorbiaceae. Flora of South Australia 2: 212.
  • Eichler, Hj. (1965). Supplement to J.M.Black’s Flora of South Australia (Second Edition, 1943-1957): 130.
  • Hammer, T.A. & Davis, R.W. (2018). Ptilotus actinocladus (Amaranthaceae), a new and rare species from the Gascoyne bioregion, Western Australia. Nuytsia 29: 145–149. (pdf)