Diadromy and genetic diversity in Nearctic and Palearctic fishes: a reply

graphic structure of Nearctic and Palearctic fishes (Bernatchez & Wilson 1998), McDowall (1999) highlights the contribution of diadromous species to our data set. Using linear regression, McDowall reanalysed data from fig. 3 in our study and found no significant relationship between median distributional latitude of diadromous species and nucleotide diversity (McDowall 1999; fig. 1), which apparently contradicts an important conclusion of our study. He goes on to discuss genetic variation of diadromous fish as well as their contribution to boreal fish assemblages, and closes with the suggestion that future phylogeographic studies should explicitly include ecological hypotheses. We acknowledge the contribution of diadromous species to our data set Ð as stated in the description of the methodology (Bernatchez & Wilson 1998; p. 438); anadromous species were included despite their potential for marine dispersal, because of their dependence on a freshwater spawning habitat. The fact that the majority of our species can be classified as ÔdiadromousÕ, however, cannot be used to Ôsupport the hypothesis that the high degree of diadromy at high latitudes results from relatively easier dispersal by diadromous fishesÕ (McDowall 1999). Rather, our summary reflects the published data that were available and fit our criteria, i.e. studies of intraspecific mtDNA diversity that encompassed ranges of several hundred square kilometres or more. Although our sampling did not target species a priori, and despite our attempt at broad taxonomic coverage, the data set nonetheless carries inherent taxonomic bias from researcher interest and funding realities (e.g. 16 out of 42 species listed in our Appendix 1 are salmonids). Thus, our data set cannot be used to support, let alone examine, hypotheses regarding the community contribution of diadromous species to boreal fish assemblages. Despite these limitations, we agree with McDowallÕs statement that comparisons between diadromous and nondiadromous species should be informative. Although we appreciate the fact that differences in life history may affect patterns of genetic diversity, we chose not to separate diadromous and freshwater species in our analyses for several reasons. Subdivision of the limited data available would have reduced resolution power, and could potentially have prevented us from being able to draw conclusions relative to the major issues we were interested in; that is, quantifying general trends on the effects of Pleistocene glaciations on genetic diversity. Furthermore, definitions of diadromy are still partly speculative, as demonstrated by McDowallÕs discussion of Morone americana, and his exclusion of Coregonus artedii from his diadromous species set, despite the main source of data for the latter species resulting from a study on anadromous James BayÐHudson Bay populations (Bernatchez & Dodson 1990). In addition, several species in our data set encompass both truly anadromous and strictly freshwater populations, and the published data available could have come from freshwater populations only (e.g. Salvelinus fontinalis), anadromous populations only (e.g. Salmo salar), or a combination of both types (e.g. Coregonus lavaretus, Coregonus clupeaformis, Osmerus mordax, Salmo trutta, Salvelinus alpinus). Under these circumstances, we could not justify subdividing our data set by life-history types. We support McDowallÕs effort to discriminate between patterns of latitudinal diversity gradients for diadromous vs. strictly freshwater fishes, and for the sake of argument have reanalysed our data set, discriminating between diadromous and freshwater fishes as defined by McDowall but including C. artedii in the first category. As pointed out by McDowall, there is no significant relationship between mean distributional latitude and nucleotide diversity based on a linear regression for all diadromous species in our data set. An examination of McDowallÕs fig. 1 (redrawn from fig. 3 of Bernatchez & Wilson 1998) Molecular Ecology (1999) 8, 529Ð530