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Females show their masculine side
There is a widespread assumption that any feature exhibited by an organism must confer some selective advantage to that organism. However, this view has its critics and there are certainly some situations in which it is difficult to see what the advantage might be. Such a situation is described by Sandra Davis, University of Louisiana (pp. 119-126). She is working with Thalictrum pubescens, a member of the Ranunculaceae. This species is dioecious, i.e. the sexes are separate. Dioecy is seen as adaptive in that it restricts, in any one plant, the allocation of resources to the floral organs of only one sex. Furthermore, outbreeding is assured without the need for mechanisms to prevent selfing. However, in T. pubescens, the situation is rather more complex. While males show no vestiges of femaleness, females not only produce stamens and anthers, but also the anthers produce pollen, albeit sterile. This cryptic dioecy is regarded as a relic of the plant's hermaphrodite evolutionary origin. However, it is clear from this paper that cryptic dioecy is costly to female plants. Female plants produce only about 20 % of the number of stamens produced by male plants but those stamens contain more N and P per stamen than those in male plants and, even more surprisingly, each anther contains about 25 % more pollen than anthers on male plants. Overall, female plants allocate about 27 % of their floral dry matter to the production of sterile pollen. The author also obtained preliminary evidence that when plants are exposed to environmental stress during floral development, the number of sex organs is reduced and, very surprisingly, female plants produce a higher ratio of male to female organs, thereby ensuring less allocation to seeds. While this particular feature may be adaptive in times of stress, the overall picture is confusing in terms of selective advantage.
Professor J. A. Bryant
University of Exeter, UK
j.a.bryant{at}exeter.ac.uk
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