Genic similarity of American and European species of the lobster Homarus.

European lobsters (Homarus gammarus) from the Norway coast and from the Irish Sea are examined for electrophoretically detectable genetic variation in seventeen functionally different proteins. Forty-one loci encoding these proteins are homologous with loci studied in a previous survey of eight populations of H. americanus. Progeny hatched from ovigerous Norway females show variation in three enzymes, but Mendelian inheritance is confirmed only for triosephosphate isomerase and for one of the phosphoglucose isomerases. Complex PGI phenotypes are described. The average amounts of genetic variability in European and American lobster populations appear to be equivalent. More than one allele is detected at 20% of the loci, the average number of alleles detected per locus is 1.2 and the average proportion of loci heterozygous per individual is 4.0%. While much less genically variable than other invertebrates, Homarus is not atypical when compared with eleven decapod species that average 5.8% heterozygosity. This is consistent with hypotheses relating genetic variability to adaptive strategy. At thirty loci H. gammarus is monomorphic for the common H. americanus allele. Two acid phosphatase systems are fixed or nearly fixed for alternative alleles in the two species while the remaining polymorphic loci show various degrees of interspecific divergence. Unique H. gammarus alleles are detected at five loci but only contribute significantly to species differences at the Acph-5, Me, and Pgi-4 loci. Acph-1, Est-2, Pgi-3, and Pgm-1 are polymorphic for the same alleles in both species, but again, with various differences in allelic frequencies. In sum, average genetic identity and average genetic distance are: I = 0.896 +/- 0.007 and D = 0.110 +/- 0.007, respectively. Compared to the values for conspecific population comparisons, I = 0.994 +/- 0.001 and D - 0.006 +/- 0.001, it is clear that a small but significant amount of genetic divergence separates the European and American lobster. Based on the premise that protein differences between existing species reflect the amount of time since they shared a common ancestor, it can be speculated that the European and American lobsters were isolated during the Pleistocene. The apparent weakness of reproductive isolating barriers suggests that these populations have evolved allopatrically. Finally, quantification of species' genetic differences, together with recent successes in interspecific laboratory matings, implicates species hybridization as a potentially important breeding practice in lobster aquaculture.