POLYPLOIDY AND EVOLUTION IN THE GENUS BULINUS (MOLLUSCA: PLANORBIDAE)
Abstract
Species of the genus Bulinus occur in a polyploid series, a comparatively rare phenomenon in animals. Some species of Bulinus are important intermediate hosts of Schistosoma haematobium, a blood fluke of major public health importance. There appears to be a connection between ploidy level and ability to transmit schistosomes. Solid-stained and G-banded karyotypes were used to determine the mode of origin of tetraploid B. truncatus, and to clarify systematic problems in this genus. The closely-related genus, Biomphalaria, was studied to compare degrees of karyotypic diversity in the two groups. Differences in karyotype were demonstrated between two South African diploids, B. tropicus and B. natalensis. The latter shares several characteristics, including ability to transmit S. haematobium, with the tetraploids. B. tropicus and B. natalensis were shown to be karyotypically distinct, B. tropicus having 14 metacentric and 4 submetacentric pairs of chromosomes, and B. natalensis having 17 metacentric and 1 submetacentric pair. That the tetraploid B. truncatus evolved through hybridization of diploid taxa with dissimilar genomes was shown by comparing B. truncatus with B. tropicus karyotypes. Four marker chromosomes were present only in pairs rather than in sets of four in the tetraploid. Of these, only one was also found in B. tropicus, suggesting that the other karyotypic features arose from a diploid taxon unlike B. tropicus in these respects. Bulinus and Biomphalaria differ strikingly in that polyploidy has not been observed in Biomphalaria. This research also revealed that Biomphalaria is karyotypically more conservative than Bulinus. Contrasting rates of chromosome evolution between anurans and mammals has been explained as a consequence of small effective population size. There is evidence that Bulinus may also have small effective population size relative to Biomphalaria, providing a partial explanation for the contrasting degrees of karyotypic divergence observed.
Degree
Ph.D.
Subject Area
Genetics
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