On a polyploid complex of freshwater snails (Planorbidae: Bulinus) in Ethiopia


  • D. S. Brown,

    1. Experimental Taxonomy Unit, Zoology Department, British Museum (Natural History), London SW 7
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    • *External Scientific Staff, Medical Research Council, Fieldwork carried out at the Institute for Pathobiology, Haile Sellassic I University, Addis Ababa, Ethiopia.

  • C. A. Wright

    1. Experimental Taxonomy Unit, Zoology Department, British Museum (Natural History), London SW 7
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In Bulinus as in other Planorbidae the basic haploid chromosome number is 18 and B. tropicus belongs to a group of diploid forms (2n=36) distributed predominately in southern Africa. B. truncates belongs to a group of tetraploid forms (2n=72) occurring mainly in northern Africa and extending northwards beyond the limit of the diploid forms. Hexaploid (2n= 108) and octoploid (2n= 144) populations occur in Ethiopia. Observations were made on snails belonging to this polyploid complex collected from 69 localities in Ethiopia. Chromosome numbers were determined in gonad tissue and in embryos. Morphological features studied are the shell (spire length, columella shape, umbilicus, costulation), radula (size of the first lateral tooth and the shape of its mesocone), copulatory organ (presence or absence) and egg size. Biochemical features investigated are the body-surface mucus, egg-proteins and esterase iso-enzymes.

Certain diploid populations conform to B. natalensis and others to B. tropicus but these extremes are connected by intermediates preventing taxonomic subdivision and the entire diploid group is regarded as a northern part of the range of a B. natalensis/tropicus complex. Ethiopian tetraploid populations are identified as B. truncates because of similarity to Egyptian snails in morphology and egg-proteins and the susceptibility of some Ethiopian snails to infection with Egyptian Schistosoma haematobium. The shell spire is comparatively long in many hexaploid and octoploid populations, and distinctive egg-protein patterns are given by octoploids but these groups do not provide clear-cut taxonomic species.

Hexaploids and octoploids are associated with streams at high altitudes while diploids and tetraploids occur in various habitats, most frequently at lower levels. The comparative rarity of the tetraploid B. truncates probably is one of the factors that have apparently prevented the establishment of truncates-borne strains of S. haematobium in Ethiopia, though present developments are likely to increase the abundance of tetraploid snails and provide opportunities for transmission of the parasite.