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Tropical Biology Section

Cozza, John [1].

Sex expression in a rainforest herb, Begonia urophylla, correlates with plant size and environment.

Monoecious plants may vary their ratio of female to male flowers according to plant size or environmental conditions. The theories of Size Dependent Sex allocation (SDS) and Environmental Sex Determination (ESD) suggest that larger plants or those growing under favorable conditions will tend to be more female, while smaller plants or those growing under less-favorable conditions will tend to be more male. I measured the sex expression of Begonia urophylla in a premontane, seasonal rainforest at Las Cruces, Costa Rica in 2003-2004. Gender was calculated as the proportion of a plant's buds (and later, flowers) that were female. Bud gender was constrained by inflorescence architecture to range from 0% female (all male) to about 50% female. In 2004, smaller plants were more variable in bud and bloom gender than larger plants were. Larger plants tended to be highly female in bud gender. While some smaller plants were just as highly female, many others had male-biased bud genders. Plants whose newer leaves were smaller than the older leaves tended to be more female, suggesting a cumulative determination of gender and a trade-off between vegetative and reproductive allocation. In 2004, gender correlated with light and nutrients. Plants growing under more open canopy in 2003 tended to be less variable, and less male, in bud gender in 2004 than more shaded plants. In a natural experiment, small plants growing on a sunny bank were more female in bud gender and (marginally) bloom gender, compared to plants growing on a shady bank. In a gender series harvested after blooming, plants that had been more female in bud gender had greater concentrations of nitrogen and calcium. Experiments currently underway with plants growing under lights in the lab should help to clarify the relationships between gender allocation, plant size, light, and nutrients.


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1 - University of Miami, Department of Biology, Po Box 249118, Coral Gables, Florida, 33124, USA

Keywords:
sex expression, gender, monoecy, Begonia.

Presentation Type: Oral Paper
Session: 8-4
Location: 410/Hilton
Date: Monday, August 15th, 2005
Time: 11:00 AM
Abstract ID:484


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