Strong genetic clines and geographical variation in gene flow in the rocky intertidal barnacleBalanus glandula

Abstract

A long-standing issue in marine biology is identifying spatial scales at which populations of sessile adults are connected by planktonic offspring. We examined the genetic continuity of the acorn barnacle Balanus glandula, an abundant member of rocky intertidal communities of the northeastern Pacific Ocean, and compared these genetic patterns to the nearshore oceanography described by trajectories of surface drifters. Consistent with its broad dispersal potential, barnacle populations are genetically similar at both mitochondrial (cytochrome oxidase I) and nuclear (elongation factor 1-alpha) loci across broad swaths of the species' range. In central California, however, there is a striking genetic cline across 475 km of coastline between northern and southern populations. These patterns indicate that gene flow within central California is far more restricted spatially than among other populations. Possible reasons for the steep cline include the slow secondary introgression of historically separated populations, a balance between diversifying selection and dispersal, or some mix of both. Geographic trajectories of oceanic drifters closely parallel geographical patterns of gene flow. Drifters placed to the north (Oregon; approximately 44 degrees N) and south (Santa Barbara, California; approximately 34 degrees N) of the cline disperse hundreds of kilometers within 40 days, yet over the long-term their trajectories never overlapped. The lack of communication between waters originating in Oregon and southern California probably helps to maintain strong genetic differentiation between these regions. More broadly, the geographical variation in gene flow implies that focusing on species-level averages of gene flow can mask biologically important variance within species which reflects local environmental conditions and historical events.

A biophysical perspective on dispersal and the geography of evolution in marine and terrestrial systems

“…Taylor & Hellberg 2003;Dawson & Hamner 2005) due to environmental heterogeneity (Gilg & Hilbish 2003;Sotka et al 2004;Hare et al 2005) a synthesis for marine and terrestrial systems seems increasingly tangible. Such a synthesis will necessarily consider a wide diversity of species and situations (e.g.…”

“…The growth of molecular biology, satellite oceanography and chemical tagging methods (among other technologies) since the early 1990s has provided alternative perspectives. Studies applying these techniques have often emphasized abundant cryptic taxa (Knowlton 1993(Knowlton , 2000Goetze 2003;Dawson 2004;Fukami et al 2004), considerable oceanographic structure with ecological and evolutionary consequences (Longhurst 1998;Sotka et al 2004;Hare et al 2005), interacting biological and physical restrictions on dispersal Cowen et al 2006), a high percentage of self-recruitment (Swearer et al 1999;Cowen et al 2000;Jones et al 2005) and strong phylogeographic structure on scales of tens of meters to tens of kilometres (Taylor & Hellberg 2003;Dawson & Hamner 2005). Re-evaluating all available data leads to the conclusion that a variety of processes, acting at various spatial and temporal scales, influence modern patterns of marine biogeography (see also Rosenblatt 1963).…”

Nonrandom Larval Dispersal Can Steepen Marine Clines

“…For example, in an eastern Pacific barnacle (Balanus glandula) allele frequencies shift strikingly over 475 km of coastline despite a capacity for dispersal at an equivalent scale during a 2ϩ week larval stage (Sotka et al 2004). The hydrography along the U.S. west coast is neither simple nor stable, but surface drifter tracks suggest that along-shore dispersal may be convergent over a broad geographic scale (Fig.…”

“…The hydrography along the U.S. west coast is neither simple nor stable, but surface drifter tracks suggest that along-shore dispersal may be convergent over a broad geographic scale (Fig. 1a, summarized from Sotka et al 2004). Similarly, eastern oysters (Crassostrea virginica) have larvae in the plankton for 2-3 weeks, yet populations along the Atlantic coast of Florida have a dramatic 50-75% shift in allele frequencies over just 20 km (Hare and Avise 1996).…”

“…In both cases mitochondrial DNA is representative of clinal patterns at one or more nuclear loci and the clines span broad environmental and ecological gradients. West coast solid arrows depict the latitudinal extent of multiple surface drifter trajectories over 40 days after release from within the open circles (for details see Sotka et al 2004). Dashed arrows show predominant drifter locations after 40 days.…”