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A Genetic Approach to the Systematics of Planorbid Molluscs

W. Lobato Paraense
Evolution
Vol. 10, No. 4 (Dec., 1956), pp. 403-407
DOI: 10.2307/2406999
Stable URL: http://www.jstor.org/stable/2406999
Page Count: 5
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A Genetic Approach to the Systematics of Planorbid Molluscs
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Abstract

Recessive albino mutants occur in natural populations of several species of planorbid molluscs. These mutants have been used as genetic markers in experiments planned to place the systematics of this family on a sound biological basis. These experiments have yielded the following general conclusions. 1. A snail grown in isolation freely reproduces by self-fertilization, but this process is readily and completely replaced by cross-fertilization if the snail is placed with another conspecific individual from the same population. 2. Cross-fertilization does not entirely replace self-fertilization in mates between conspecific allopatric individuals. Such crosses result in a lowered production of hybrid offspring. However, the fertility of the snail is kept unimpaired, owing to self-fertilization of those eggs which failed to be cross-fertilized. This fact indicates that the lowered interfertility depends on variable degrees of gametic isolation rather than on any cytogenetic mechanisms. 3. All the recognizable morphological species so far studied by us have shown absolute reproductive isolation. 4. In spite of the planorbids being hermaphrodites perfectly capable of self-fertilizing, their colonies must be regarded as Mendelian populations in the same sense as those of dioecious organisms. 5. It seems that geographic speciation is paramount also in the evolution of the planorbids. However, it is possible that the faculty of self-fertilization may favor in some special cases also sympatric speciation.

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