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On Speciation by Evolution of Gametic Incompatibility: A Model Case in Chlamydomonas

Lutz Wiese and Waltraud Wiese
The American Naturalist
Vol. 111, No. 980 (Jul. - Aug., 1977), pp. 733-742
Stable URL: http://www.jstor.org/stable/2460327
Page Count: 10
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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.
On Speciation by Evolution of Gametic Incompatibility: A Model Case in Chlamydomonas
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Abstract

Sexual isolation between dioecious isogamous syngens of the clade Chlamydomonas moewusii can be ascribed to the nonoccurrence of initial contact between noncompatible gamete types. As demonstrated by its sensitivity to enzymes and concanavalin, sex-cell contact depends on a molecular complementarity between glycoproteinaceous components on the gametic flagella tips, the mating-type substances. In related syngens gamete contact is mediated by a carbohydrate ligand on one gamete type interacting with a trypsin-sensitive, sugar-binding component on the other. Modulating features render this basic complementarity taxon specific and account for cell-to-cell recognition and the gametic isolation between syngens. The polymorphism of the bipolar contact mechanism seems to be conditioned by a specifying complexity of the carbohydrate compound on the (+) substance and its selective recognition by a discriminating contact site on the (-) substance. A comparative study between several related taxa suggests a simple concept on the evolution of complex breeding systems, i.e., on speciation, by mutative alterations on either side of the contact-causing complementarity. The model presented explains readily (1) the frequent coexistence of sexually compatible and incompatible taxa within species with asexual reproduction; (2) the common occurrence of strains without a known sex partner; and (3) peculiarities in the geographic distribution of syngens and of individual sexual strains.

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