High DNA sequence variability at the alpha 1 Na/K-ATPase locus of Artemia franciscana (brine shrimp): Polymorphism in a gene for salt-resistance in asalt-resistant organism

We previously reported that the Na/K-ATPase alpha 1 subunit coding gene sho wed signs of being a very polymorphic locus in Artemia franciscana. This sp ecies is adapted to highly saline waters, and the Na/K-ATPase al isoform pr esumably plays a key role in this adaptation. Therefore, we were interested in further study of the alpha 1 Na/K-ATPase polymorphisms to examine wheth er 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 ob served that most substitutions are in synonymous positions (88.8%). The 12 nonsynonymous substitutions code for conservative amino acid replacements w ith an apparent scattered distribution across functional domains of the pro tein. Interspecific comparison between these sequences and two genomic clon es from Artemia parthenogenetica containing 1,122 bp of the al Na/K-ATPase locus coding sequence showed independence of the synonymous/nonsynonymous r atio in the comparison within A. franciscana and between A. franciscana and A. parthenogenetica, which fits the neutral model of evolution. Since ther e were no previous studies on DNA polymorphism for other A. franciscana gen es, we also studied variability at the Actin 302 locus for comparison. Both loci were amplified by reverse transcription-polymerase chain reaction, an d 20 sequences were obtained for each. This study shows that the amplified region of the alpha 1 Na/K-ATPase gene is 3.5 times as polymorphic as the A ctin 302 gene and 2.9 times as heterozygotic. Interestingly, under a model of neutral evolution, the data observed would be expected with a probabilit y of approximately 0.05, suggesting an excess of intraspecific variation of alpha 1 Na/K-ATPase with respect to Actin 302. Restriction fragment length polymorphism studies show similar patterns of polymorphism along the simil ar to 41-kb span of the alpha 1 Na/K-ATPase locus. Most of the nucleotide d ifferences are linked in a few haplotypes, although recombination events ar e also inferred from the data. We propose a possible explanation for the hi gh polymorphic levels at the alpha 1 Na/K-ATPase locus which invokes positi ve selection acting tightly to the locus in transiently isolated or semiiso lated subpopulations.