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Hybridization Experiments and the Taxonomy of Gammarus (Amphipoda): a Contribution To the Understanding of Controversial Results

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In the last two decades, hybridization experiments were frequently used to elucidate the taxonomic position of populations belonging to the Gammarus pulex-group. Since these experiments did not always give adequate answers, genetic differentiation based on enzyme polymorphisms was investigated. The differentiation recorded was apparently neither consistent with morphological data nor with results from cross-breeding experiments. In order to explain the inconsistency of hybridization levels and genetic differentiation, a more detailed knowledge of the mechanisms enhancing various levels of reproductive isolation is necessary. Two new series of interpopulational hybridization experiments were carried out. Unlike in previous experiments it was not our first aim to determine the final outcome (positive or negative) of the crosses, but to obtain a more detailed analysis of all stages leading towards the final result. The animals were daily followed from the formation of precopulations until the hatching of juveniles or expulsion of the eggs. Factors influencing the reproductive success are found in all pre- and post-zygotic stages, viz., the formation of precopulations, successful oviposition as well as all embryonic stages. Size differences and a longer duration of the precopulatory stage were shown to have a negative influence on the final success. The development of embryos appeared not to be a linear process; distinct stages could be observed and "breaking points" between those stages indicate instances of developmental failure. A correlation seems to exist between the genetic identity and the survival of embryonic stages. The role of structural genes and presumptive regularity genes in the process of embryonic development is discussed.


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Affiliations: 1: Institute of Taxonomic Zoology (Zoologisch Museum), University of Amsterdam, P.O. Box 94677, NL-1090 GT Amsterdam, The Netherlands


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