Bacterioplankton growth responses to temperature and chlorophyll variations in estuaries measured by thymidine:leucine incorporation ratio
Inter- and intraspecific genetic relationships among and within three species of mackerels of the genus Scomber were investigated by restriction site analysis of the whole mitochondrial (mt) DNA genome and direct sequence analysis of the mitochondrial cytochrome b gene. A total of 15 samples, averaging 19 individuals each, were collected from geographically isolated populations throughout the ranges of S. scombrus (two samples), S, australasicus (five samples), and S. japonicus (eight samples). Restriction site analysis with 12 restriction enzymes revealed substantial genetic variation within each species. Sample haplotype diversities ranged from 0.28 to 0.95, and nucleotide sequence diversities from 0.13% to 0.76%. Spatial partitioning of genetic variation was observed in each of the species. Eastern and western North Atlantic samples of S. scombrus exhibited significant heterogeneity in the distribution of mtDNA haplotypes, but no fixed restriction site differences were observed between samples. Similarly, no fixed restriction site differences occurred among samples of S. japonicus in the Atlantic Ocean, although there were significant differences in the distribution of haplotypes among samples. In contrast, samples of S. japonicus from within the Pacific Ocean were characterized by fixed restriction site differences. North and South Pacific samples of S. australasicus were highly divergent, and one of two divergent mtDNA matrilines was restricted to samples from the South Pacific. A 420-bp segment of the cytochrome b gene was sequenced for representatives of each of the major mtDNA lineages identified by restriction site analysis. Scomber scombrus differed from S. australasicus and S. japonicus by more than 11% net nucleotide sequence divergence, considerably greater than the 3.5% sequence divergence between S. australasicus and S. japonicus. Levels of interspecific genetic divergences based on restriction site data were similar in pattern, but were approximately 20% lower in magnitude when based on the cytochrome b sequences. Parsimony analysis and neighbor-joining of restriction site data, and parsimony analysis of cytochrome b sequences showed similar paraphyletic patterns in both S, japonicus and S. australasicus. Levels of divergence among samples of S. japonicus were similar to those between samples of S. australasicus and S. japonicus. Complete partitioning of halpotypes among some samples of S. japonicus that are morphologically distinct suggests that Atlantic and Indo-Pacific populations of S. japonicus may need to be recognized as separate species.