Gene flow patterns of the euphausiid, Meganyctiphanes norvegica, in the NW Atlantic based on mtDNA sequences for cytochrome b and cytochrome oxidase I

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Abstract
Population genetic structure and patterns of gene flow are described for the euphausiid, Meganyctiphanes norvegica, in the NW Atlantic Ocean based on DNA sequence variation of two regions of mitochondrial DNA (mtDNA). DNA sequences were determined for portions of cytochrome oxidase I (COI; 400 base pairs; 76 individuals) and cytochrome b (CYB; 300 base pairs; 101 individuals) for euphausiids collected from the Gulf of Maine in 1991, the Gulf of St Lawrence in 1993 and 1994, the Scotian Shelf in 1994, and Georges Bank in 1994. For comparison, M.norvegica collected from the fjords of western Norway in 1992 were sequenced for COI (20 individuals) and CYB (18 individuals). COI was less variable than CYB, based on both haplotype (h=0.6847 for COI; 0.9077 for CYB) and nucleotide diversities (π = 0.0038 for COI; 0.0182 for CYB). Haplotype frequencies among all samples of M.norvegica were significantly heterogeneous for CYB (P < 0.001 by χ2 COI/CYB (P < 0.034) and for the 37 sites in the COI/CYB sequence at which there were multiple substitutions (MSUB; P < 0.0001). Haplotype frequencies among regional populations (i.e. pooled samples collected within each region) in the NW Atlantic were different for MSUB haplotypes (P < 0.021), but not for any other gene portion. Comparison of NW Atlantic versus Norwegian fjord populations (based on pooled samples) revealed significant differences in haplotype frequencies for CYB (P < 0.004) and MSUB (P < 0.0001), and by analysis of molecular variance (AMOVA) for CYB (P < 0.01) and COI (P < 0.01). All evidence considered together indicated that: (i) genetic heterogeneity among samples reflected high levels of molecular genetic diversity and, perhaps, under-sampling of the population genetic structure; (ii) gene flow of M.norvegica within the NW Atlantic was sufficient to prevent the formation of distinctive geographical populations, except in the case of the Gulf of St Lawrence sample collected in 1994; (iii) genetic differentiation of NW Atlantic and Norwegian fjord populations indicated highly restricted gene flow across the N Atlantic Ocean. We conclude that mtDNA genes may vary significantly in the amount and pattern of variation, and that this variation affects the results and conclusions from molecular population genetic analyses. Consideration of genetic traits with small numbers of moderately frequent variants, such as the MSUB haplotypes, markedly increased the power of statistical analyses. Resolution of the population genetic structure of marine zooplankton at spatial scales smaller than ocean basins will require the analysis of numerous individuals (i.e. thousands), samples collected at appropriate temporal and spatial resolutions, and molecular markers with appropriate levels and patterns of variation.

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