Coadaptation: lessons from the brine shrimp Artemia, "the aquatic Drosophila" (Crustacea; Anostraca)

During the fifties Brncic and the Dobzhansky's school, using the fruit fly Drosophila as a test organism, introduced the term integration of the genot ype. or genetic coadaptation, which has had great impact on thinking in tod ay's evolutionary biology. In this work we use the brine shrimp Artemia - i n many respects a sort of aquatic Drosophila as a model organism to evaluat e the relationship between population structure, potential for divergence a nd the degree of morphological and/or genetic change. These aspects, tightl y linked with the organization of the genotype, are important to understand ing how recombination and adaptive release of genetic and phenotypic variat ion affect the speciation process in Artemia. Analysis of genetic (allozyme , diploid and chromocentre numbers), morphological (Mahalanobis distances) and reproductive data (cross-fertility tests) available for populations of the bisexual, endemic species of the Americas, Artemia franciscana and A. p ersimilis, indicate that: (i) A. franciscana and A. persimilis are morpholo gically distinct in correspondence with observed genetic differences (D Nei > 1; 2n = 42 and 44; 12.5 and 1.5 mean chromocentre numbers, respectively) ; (ii) populations from Chile and other South American localities (mainly A , franciscana) display high levels of genetic variability and a trend to de velop large genetic distances between populations: (iii) the plasticity of Artemia gene pool is associated, at least in part, with ecological heteroge neity. Hence an adaptive divergence mode is thought to best define the spec iation process in Artemia; (iv) the succesful production of laboratory hybr ids in the allopatric Artemia populations studied in the Americas, a featur e seen in other anostracods. could be explained by the fact that formerly a llopatric populations have not achieved later sympatry, as required by the allopatric speciation paradigm.