FRESHWATER SNAILS OF THEBULINUS TRUNCATUS/TROPICUSCOMPLEX IN KENYA: TETRAPLOID SPECIES

摘要

Studies in recent years on chromosome number in freshwater snails belonging to theBulinus truncatus/tropicuscomplex demonstrated the occurrence in Kenya of one diploid species,B. tropicus, and 2 tetraploid species,B. truncatusandB. permembranaceus. Further observations are now described on the morphologies of the tetraploid species, and their distributions.Observations were made onB. truncatusfrom 26 localities (11 newly reported) andB. permembranaceusfrom 26 localities (7 newly reported), in respect of chromosome number, egg proteins, enzymes (5 systems), shell (10 variables measured), male genital system (normal or aphallic) and radula (1st lateral tooth, size and mesocone shape).B. permembranaceusdiffers most clearly fromB. truncatusin egg proteins, enzyme-loci and in lacking aphallic individuals. Mesocone shape is generally less angular inB. permembranaceus. The shell ofB. permembranaceusgrows larger, has the columella more commonly concave and its spire is proportionally higher and more acute. To distinguish these tetraploid species from the partly sympatricB. tropicus, observations on chromosome number and biochemical characters are necessary.The tetraploids are allopatric,B. truncatusbeing found rarely as high as 1900 m altitude, whereasB. permembranaceusoccupies the altitude range 1940–2760 m.B. truncatusmay be expanding its distribution in Kenya in man-made waterbodies, whileB. permembranaceuspossibly is restricted by adaptation to cool conditions and by interaction withB. tropicus. Differences betweenB. truncatusandB. permembranaceus, both morphological and biochemical, indicate their origins lie in independent episodes of tetraploid