The Chromosomes of Polygyrid Snails

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Abstract
18 spp. and subspp. in 3 genera of the Polygyridae have 58 as the typical chromosome number. The chromosomes of Mesodon (Polygyrinae) resemble more closely those of Triodopsis (Triodopsinae) than those of Stenotrema (Polygyrinae). Two species were extensively sampled. In T. fraudulenta the chromosome number varies. Individuals have 58, 59, 60, 61(?) or 62 (diploid) chromosomes. It is suggested that snails can tolerate recurrent duplication of chromosome elements, and that in the Pulmonata as a whole, the nearly unbroken seriation in chromosome number may be attributed to a gradual increase from 34 to 62. In T. fraudulenta snails with extra chromosomes show no increase in the vol. of their nuclei nor can they be distinguished morphologically from those with the chromosome number characteristic of the family in Virginia. In 2 spp. (T. fraudulenta and T. tridentata) a heteromorphic bivalent occurs in approx. 40-47% of the individuals examined. In T. fraudulenta the frequency of this bivalent, and also of extra chromosomes, appears to be a locality characteristic. In this sp. the occurrence or non-occurrence of the heteromorphic bivalent is correlated with a taxonomic distinction which may also be justified on morphological and ecological grounds. Subspecific categories of T. tridentata, which by other criteria are of questionable validity, show no striking difference in frequency of the heteromorphic bivalent.
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