Reproduction in the brine shrimp Artemia:: evolutionary relevance of laboratory cross-fertility tests

The brine shrimp Artemia, a typical inhabitant of hypersaline environments and characterized by a highly subdivided population structure, was used as a model to evaluate, under standardized laboratory conditions (at 65 ppt), primary reproductive traits (offspring quality and quantity) along with levels of reproductive isolation and degrees of divergence among populations. Intrapopulation experimental crosses and cross-fertility tests were evaluated in five populations (mostly A. franciscana) from coastal and inland environments in Chile, and in reference samples of A. franciscarza (San Francisco Bay, U.S.A.) and A. persimilis (Buenos Aires, Argentina), which are the species likely to be found in Chile. The populations compared displayed significant variability in fecundity (total offspring, brood size) as well as in the ratio encystment/oviviparity. Hybrid offspring, produced abundantly in cross-fertility tests with reference populations, showed a pronounced switch to the encystment mode, particularly in crosses with A. persimilis. Exposure to a broad range of ecological conditions seems to have optimized a generalist reproductive strategy in the Artemia populations studied that combines variation in both the quantity and quality of zygotes. Laboratory cross-fertility tests evaluated prime reproductive characteristics in individual crosses with fair repeatability, as well as testing barriers to laboratory reproductive isolation. The lack of efficient mechanisms for reproductive isolation in the allopatric Artemia populations studied follows a trend often seen in other anostracods. Formerly allopatric populations have not achieved sympatry later as required by the allopatric speciation paradigm, and this is a probable explanation for production of the laboratory hybrids.