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Rivuline studies taxonomic studies of rivuline cyprinodonts from tropical atlantic africa rivulinae cyprinodontidae atheriniformes pisces



Rivuline studies taxonomic studies of rivuline cyprinodonts from tropical atlantic africa rivulinae cyprinodontidae atheriniformes pisces



Koninklijk Museum voor Midden Afrika Tervuren Belgie Annalen Zoologische Wetenschappen (211): 1-150



The geographical distribution of the major rivuline phenotypes in tropical Atlantic Africa and the biology of selected representatives of Aphyosemion are discussed. An attempt is made to reconcile the old systematics based on phenotypes with the new systematics based on reproductive isolation of populations. The study of the zoogeorgraphy of cyprinodonts in tropical Atlantic Africa brought forth a rather simple pattern with 5 major faunal groups in the forest and 1 similar group in the savannahs. In some areas derived subgroups have developed in the adjacent savannahs. Zoogeography based on phenotypy appears to reflect the natural conditions for cyprinodonts in this part of Africa. There are no reasons to believe that in most cyprinodont taxa species discrimination may be based on phenotypy alone. Karyotypes and corresponding genotypes have stabilized or exhibit little variation. This is true for the procatopodine forms and for most phenotypes of the genus Aplocheilus with Aplocheilus fasciolatus as the only known exception. The condition among the major phenotypes of the genus Aphyosemion is different. Karotype and genotype variations (results of crossings) occur with little or no change of corresponding phenotype. This may be due to exchange of genes between different populations being limited, particularly across large rivers, and being insufficient to counteract karyotype and genotype changes which occur as a response to local ecological conditions. Superficially the karyotypic dynamics in Aphyosemion resemble the evolution of a young group of animals during an explosion. The picture is rather that of group which insufficiently fitted to ecological conditions and tryping to adjust karyotype and genotype in order to become better fitted. There are no indications that all this karyotypic change and confusion has created anything new. Probably, the different major phenotypes of this taxon and the closely related genus Nothobranchius differentiated a long time ago, when the karyotype was not very specialized and had .apprx. 20 haploid elements and many chromosomal arms. Subsequent parallel evolution of the karyotype, characterized by 3 features took place in several distinct lines of descent. The number of chromosome arms increased associated with similar increase in cell diameter. This produced gene linkages followed by an increasing uniformity of subsequent generations (specialization), which seems to represent an irreversible condition. This pattern may encourage polyploidy because of the enlarged cells, the reduced number of chromosomes and the absence of sex chromosomes. This pattern is not unique for Aphyosemion and Nothobranchius and is also evident in the South American Pterolebias. The zoogeographic importance of this cryptic speciation in Aphyosemion should not be overestimated. The zoogeographic patterns of the sympatric Aphyosemion bivittatum and Aphyosemion calliurum are different when based on karyotype, genotype and reproductive isolation. Development of sibling species in Aphyosemion was probably accidental in spite of the possible influence of large rivers and different soil types.

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