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Sex change involves one for one swap
Many of us were introduced to homeotic genes via homeotic mutants in drosophila in which, for example, legs grow in the place of antennae. It is tempting, given current emphasis in the literature, to imagine that the term was invented by molecular biologists. However, as we are reminded by the French-Canadian team led by Denis Barabé (pp. 579-578), it is actually based on the morphological phenomenon of homeosis, the replacement of one organ by another, first described in the 19th century. In the words of a certain film actor, ‘Not a lot of people know that’. The flowers of the genus Philodendron are ideal for investigating homeosis. In each inflorescence, female flowers form near the base and male flowers near the apex. However, the transition is not abrupt: there is a gradual transition from femaleness to maleness via several intermediate forms. Immediately below the male flowers are sterile males with non-productive stamens (staminodes); between these and the female flowers are atypical bisexual flowers with both carpels and staminodes. In these atypical flowers, staminodes tend to predominate towards the male end of the inflorescence and carpels towards the female end of the inflorescence. The authors have carried out a detailed analysis of the atypical flowers. They note that when carpels and staminodes occur in the same flower, they are in the same whorl. In progressing from femaleness to maleness, carpels are replaced by staminodes on a one for one basis. These findings are reinforced by the occasional presence of structures intermediate between carpels and staminodes. This is a fascinating example of a morphogenetic gradient and leads us to speculate on the nature of the positional information. Furthermore, the gradient involves clear examples of homeosis and provides the authors with an ideal system for studying expression of genes involved in floral organ identity.
Professor J. A. BryantUniversity of Exeter, UK
j.a.bryant{at}exeter.ac.uk
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