Molecular Biology and Evolution 17:235-250 (2000)
© 2000 Society for Molecular Biology and Evolution
ARTICLES |
High DNA Sequence Variability at the 
1 Na/K-ATPase Locus of Artemia franciscana (Brine Shrimp): Polymorphism in a Gene for Salt-Resistance in a Salt-Resistant Organism
*Instituto de Investigaciones Biomédicas CSIC-UAM, Madrid, Spain;
and
Alfred Wegener Institute, Bremerhaven, Germany
We previously reported that the Na/K-ATPase 
1 subunit coding gene showed signs of being a very polymorphic locus in Artemia franciscana. This species is adapted to highly saline waters, and the Na/K-ATPase 1 isoform presumably plays a key role in this adaptation. Therefore, we were interested in further study of the 1 Na/K-ATPase polymorphisms to examine whether they might be due to an adaptation to salt resistance driven by natural selection. Using coding sequences from 10 genomic clones and 3 cDNAs, we observed that most substitutions are in synonymous positions (88.8%). The 12 nonsynonymous substitutions code for conservative amino acid replacements with an apparent scattered distribution across functional domains of the protein. Interspecific comparison between these sequences and two genomic clones from Artemia parthenogenetica containing 1,122 bp of the 1 Na/K-ATPase locus coding sequence showed independence of the synonymous/nonsynonymous ratio in the comparison within A. franciscana and between A. franciscana and A. parthenogenetica, which fits the neutral model of evolution. Since there were no previous studies on DNA polymorphism for other A. franciscana genes, we also studied variability at the Actin 302 locus for comparison. Both loci were amplified by reverse transcription–polymerase chain reaction, and 20 sequences were obtained for each. This study shows that the amplified region of the 1 Na/K-ATPase gene is 3.5 times as polymorphic as the Actin 302 gene and 2.9 times as heterozygotic. Interestingly, under a model of neutral evolution, the data observed would be expected with a probability of approximately 0.05, suggesting an excess of intraspecific variation of 1 Na/K-ATPase with respect to Actin 302. Restriction fragment length polymorphism studies show similar patterns of polymorphism along the ~41-kb span of the 1 Na/K-ATPase locus. Most of the nucleotide differences are linked in a few haplotypes, although recombination events are also inferred from the data. We propose a possible explanation for the high polymorphic levels at the 1 Na/K-ATPase locus which invokes positive selection acting tightly to the locus in transiently isolated or semi-isolated subpopulations.