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Cytogeographic Evidence for the Evolution of Distyly from Tristyly in the North American Species of Oxalis Section Ionoxalis

Stephen G. Weller and Melinda F. Denton
American Journal of Botany
Vol. 63, No. 1 (Jan., 1976), pp. 120-125
Stable URL: http://www.jstor.org/stable/2441674
Page Count: 6
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Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
Cytogeographic Evidence for the Evolution of Distyly from Tristyly in the North American Species of Oxalis Section Ionoxalis
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Abstract

A survey of haploid chromosome numbers of 18 North American taxa of Oxalis section Ionoxalis was initiated to determine the relationship between ploidal level, geographic distribution, and the occurrence of tristyly and distyly. Although chromosome numbers in the section are variable, the majority of tristylous populations are diploid. Among the distylous taxa a greater diversity of ploidal levels exists, with the higher chromosome numbers predominating. In section Ionoxalis the majority of the tristylous taxa are geographically restricted endemics of southern Mexico, while the distylous taxa have more extensive distributions ranging to the north. The association of diploidy and geographic endemism in the majority of the tristylous taxa suggests that these species are relictual. A few widespread tristylous taxa are polyploid, and often somewhat weedy. The probable derivation of widespread polyploid species from the restricted diploid endemics of southern Mexico appears to have been accompanied by the evolution of distyly from tristyly. The frequent association of polyploidy and distyly in section Ionoxalis has apparently resulted from the concurrence of two evolutionary trends: increase in ploidal level and the derivation of distyly from tristyly.

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