Journal Article

Sexual Dimorphism in the Andromonoecious <i>Euphorbia nicaeensis</i>: Effects of Gender and Inflorescence Development

Eduardo Narbona, Pedro Luis Ortiz and Montserrat Arista

in Annals of Botany

Published on behalf of The Annals of Botany Company

Volume 101, issue 5, pages 717-726
Published in print April 2008 | ISSN: 0305-7364
Published online February 2008 | e-ISSN: 1095-8290 | DOI: http://dx.doi.org/10.1093/aob/mcn011
Sexual Dimorphism in the Andromonoecious Euphorbia nicaeensis: Effects of Gender and Inflorescence Development

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  • Evolutionary Biology
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Background and Aims

In andromonoecious taxa with separate floral types along the inflorescence, architectural or plastic effects can simulate floral sexual dimorphism. Both the primary and secondary sexual characteristics of the cyathia of the protogynous andromonoecious species Euphorbia nicaeensis were analysed according to their sex and arrangement on the inflorescence.

Methods

The production of male and hermaphrodite cyathia at each inflorescence level was surveyed in two natural populations. The longevity, size, pollen production and viability, and nectar secretion of both types of cyathia were checked between inflorescence levels and between sexes at the only level at which they occur together. This sampling method makes it possible to know whether differences between cyathia types are based on sex or are attributable to inflorescence development.

Key Results

Male cyathia were produced predominantly at the first and second inflorescence levels, whereas at levels 3–5, the cyathia were almost exclusively hermaphrodite. Viable pollen production by male cyathia at the second inflorescence level was higher than that of hermaphrodite cyathia at the third level but, when males and hermaphrodites at the same level were compared, their pollen production was similar. Male and hermaphrodite cyathia were similar in size, irrespective of the inflorescence level, although the exclusively hermaphrodite cyathia of the last level were smaller. Both cyathium types produced similar amounts of sugar. However, male cyathia produced nectar during their whole lifespans, whereas hermaphrodites produced it exclusively during their male phase. Moreover, the nectary activity of male cyathia started earlier in the day than that of hermaphrodites.

Conclusions

An apparent floral dimorphism exists in the primary sexual characteristics of Euphorbia nicaeensis because differences in pollen production between cyathium types are due to theirs positions. Similarly, differences affecting most secondary sexual characteristics are only apparent between the two cyathium types. However, E. nicaeensis shows a true but slight floral dimorphism in some of the secondary sex characters related to nectar secretion. The lack of nectar production by the female phase of the hermaphrodite cyathia of E. nicaeensis indicates that this is a deceit-pollinated species.

Keywords: Andromonoecy; cyathia longevity; deceit pollination; Euphorbia nicaeensis; Euphorbiaceae; inflorescence architecture; nectar secretion; positional effects; protogyny; viable pollen production; sexual dimorphism

Journal Article.  6834 words.  Illustrated.

Subjects: Ecology and Conservation ; Evolutionary Biology ; Plant Sciences and Forestry

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