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 lev els of reproductive isolation and degrees of divergence among populations. Intrapopulation experimental crosses and cross-fertility tests were evaluat ed in five populations (mostly A. franciscana) from coastal and inland envi ronments in Chile, and in reference samples of A. franciscarza (San Francis co Bay, U.S.A.) and A. persimilis (Buenos Aires, Argentina), which are the species likely to be found in Chile. The populations compared displayed sig nificant 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 sw itch to the encystment mode, particularly in crosses with A. persimilis. Ex posure to a broad range of ecological conditions seems to have optimized a generalist reproductive strategy in the Artemia populations studied that co mbines variation in both the quantity and quality of zygotes. Laboratory cr oss-fertility tests evaluated prime reproductive characteristics in individ ual crosses with fair repeatability, as well as testing barriers to laborat ory reproductive isolation. The lack of efficient mechanisms for reproducti ve isolation in the allopatric Artemia populations studied follows a trend often seen in other anostracods. Formerly allopatric populations have not a chieved sympatry later as required by the allopatric speciation paradigm, a nd this is a probable explanation for production of the laboratory hybrids.