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Tuesday "Noon" Seminar: April 10, 2007Kathy Gerst on "Reproductive consequences of sex ratio variation in Bursera microphylla"
12:30-1:45 p.m. in Biosciences West (map of building location), Room 208 |
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Talk Abstract Bursera microphylla (Burseraceae) is a semi-succulent tree found in the Sonoran Desert and the cape region of Baja California. Of over 100 species in this genus, most of which are found in tropical dry forests, B. microphylla extends into the driest and most northern habitats and is uniquely adapted to desert ecosystems. In the center of its range this species is primarily dioecious and exhibits a sex ratio indistinguishable from 50:50. However, in at least one isolated population along the edge of its northeastern distribution in the Waterman Mountains near Tucson, Arizona, B. microphylla populations have been found to have skewed sex ratios. This population has an extremely high proportion of female individuals and very few male or hermaphroditic individuals, exhibiting a 92:8 sex ratio. This study aims to explore variation in floral traits between central and peripheral populations and differential allocation to male and female reproduction in populations with varying sex ratios. Results from the 2005 and 2006 flowering season indicate that plants are approximately seven times more pollen limited in the edge population with few male and hermaphrodite individuals compared to a more central population with even sex ratios located in the Pinacate Biosphere reserve in Sonora, Mexico. Using fluorescence microscopy we examined pollen tube growth on female and hermaphrodite stigmas and found significantly more pollen deposited on stigmas in the central population with even sex ratios compared to the edge population. In addition, fruit set on female individuals is significantly lower in the peripheral population compared to this more central population. Male flowers in the peripheral population are more similar in morphology to female flowers than to male flowers in the central population, and have a significantly smaller stamen number than central population males. Male flowers in the edge population with even sex ratios have significantly more pollen grains than male flowers in the skewed sex ratio population, but have similar pollen grain counts as hermaphrodites from the skewed sex ratio population. Finally, flowers in the central population have significantly greater nectar volumes than populations in the edge population. Despite the apparent reduction in reproductive success in the edge population with skewed sex ratios, this population appears to be reproducing each year and is not necessarily in decline. Future work will focus on the pollinator community shifts between populations and the genetics underlying the variation in breeding system. This work provides a context to integrate our understanding of range limits, sexuality, and breeding systems. |
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