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. http://dx.doi.org/10.1071/BT9760237