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Abstract Detail


Biogeography

Laport, Robert [1], Minckley, Robert [1], Ramsey, Justin [1].

Polyploidy and the Cytogeography of the North American Creosote Bush (Larrea tridentata, Zygophyllaceae).

The relative importance of ecological and non-ecological reproductive barriers is widely disputed, even among taxa that have been identified as classical examples of ecological or non-ecological speciation. Genome duplication, or polyploidy, is a common form of chromosome evolution that causes hybrid inviability (triploid block) in plants. Because polyploids are often geographically separated from diploids, however, some evolutionists have argued that polyploid speciation is a fundamentally ecological process. One widely-cited example of polyploid speciation is the creosote bush (Larrea tridentata, Zygophyllaceae), a dominant species of the North American desert flora exhibiting three geographically isolated chromosome races (diploids (2n=2x=26) occur in the Chihuahuan Desert, tetraploids (2n=4x=52) occur in the Sonoran Desert, and hexaploids (2n=6x=78) occur in the Mojave Desert). Despite being cited as an example of polyploid speciation, it remains unclear if the chromosome races of Larrea are distinct taxonomic units. This is due largely to uncertainty over whether they have divergent ecological distributions, and consequently to what degree they are reproductively isolated. Here, we use a combination of DNA molecular analyses and field observations to demonstrate the monophyly of the L. tridentata polyploid complex; determine the cytogeography of the chromosome races; and test the hypothesis that the chromosome races are divergent in ecological parameters. We find that the three cytotypes of North American creosote bush represent a distinct clade from South American relatives (0.3% cpDNA sequence difference), but display little molecular differentiation from each other. The cytotypes are largely allopatrically distributed with limited areas of overlap. However, significant overlap in range was observed between tetraploids and hexaploids throughout the western Sonoran Desert. Consistent with the hypothesis that the chromosome races occupy unique ecological niches, we find that the three cytotypes are distinct in climatic and edaphic parameters, and display differences in water use efficiency as measured by carbon isotope analyses. Taken together, these results suggest ecological isolation may play a significant role in reproductive isolation between the cytotypes.

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1 - University of Rochester, Department of Biology, 213 Hutchison Hall, River Campus, Rochester, NY, 14627, USA

Keywords:
Polyploidy
speciation
Biogeography
flow cytometry.

Presentation Type: Oral Paper:Papers for Topics
Session: 28
Location: 552B/Convention Center
Date: Tuesday, August 3rd, 2010
Time: 10:30 AM
Number: 28011
Abstract ID:210


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