Reference: Biol. Bull. 208: 60-68. (February 21)0?) 2005 Marine Biological Laboratory Diversification Before the Most Recent Glaciation in Balanus glandula JOHN P. WARES* AND CLIFFORD W. CUNNINGHAM Department of Biology, Duke University, Durham, North Carolina 27708 Abstract. A deep genetic cline between southern popula-tions of the barnacle Balanus glandula (from about Monterey Bay southward) and northern populations (from northern California through Alaska) has recently been de-scribed. If this pattern is due to historical isolation and genetic drift, we expect it to have formed recently and represent a transient, nonequilibrium state. However, this cline appears to have formed well before the last glacial maximum. Our assays of sequence diversity at a region of mitochondrial cytochrome oxidase I, combined with coales-cent estimators of the time of separation for these two regions, suggest that a late Pleistocene event more than 100 thousand years ago may be responsible for the initial sepa-ration. This suggests that either strong oceanographic mech-anisms or natural selection have maintained the cline. be-cause there has clearly been adequate time for this cline or polymorphism to resolve itself by genetic drift and migra-tion. However, reliance on only a single mitochondrial marker for which the substitution rate has been estimated still limits the resolution of our analysis. Introduction Patterns of genetic variation in some coastal species indicate the effects of changing paleoclimate on distribution and abundance (e.g., Edmands. 2001; Dawson, 2001; Hick-erson and Ross. 2001; Marko, 2004). These effects may lead to significant levels of population structure, where some regions harbor significantly different frequencies of partic-ular alleles or molecular markers (Grosberg and Cunning-Received 26 Max 2004; accepted 29 November 2004. * To whom convvH ..idcnce should be addressed, at Department of Genetics, University ol Georgia. Athens, Georgia 30602. E-mail:
[email protected] Abhrevialions: kya, thousand years ago; LGM, last glacial maximum, N e . effective population M/C: iniCOl, mitochondrial cytochrome oxidase I. ham, 2001). Predictions range from extreme divergence of regions, indicated by reciprocal monophyly on a gene tree of regional samples (see Avise, 2000), to gradients in over-all genetic diversity. Patterns suggesting divergence be-tween populations currently distributed to the north and south of the glacial margin at the last glacial maximum (LGM) about 18 thousand years ago (kya) are often explained as representing populations that survived Pleisto-cene glaciation in "unglaciated refugia" (Holder et ai, 1999; Hickerson and Ross, 2001; Wares, 2002). A finding of lower genetic diversity in populations north of the LGM glacial margin is typically interpreted as post-glacial expan-sion from a southern refugial distribution (see Marko. 2004). In marine organisms, the location and persistence of paleoceanographic phenomena must also be considered, such as the location of extant currents and their persistence through Pleistocene climate change (Herbert et ai. 2001; Wares, 2002). Sotka et ai (2004) characterized the genealogical rela-tionship among individuals of Balanus glandula from coastal British Columbia to southern California, using DNA sequence data from two loci (mitochondrial cytochrome oxidase I [mtCOI] and nuclear elongation factor la). The analysis of these data indicates a clinal pattern of allelic class frequencies, suggesting that there are surprisingly low levels of realized migration (gene flow) along the California coast from Point Arena to Monterey Bay. The divergence between the northern and southern lineages does not repre-sent a classical "phylogeographic break" in which the two groups are genealogically distinct (class I phylogeographic status; Avise 2000). Instead, the pattern is consistent with secondary contact between historically isolated lineages, which has proceeded slowly because of selection or physi-cal oceanographic forces. One way to distinguish among the mechanisms that may be maintaining this pattern is to estimate how long ago the cline formed. If the cline is quite 60
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