High and Distinct Range-Edge Genetic Diversity despite Local Bottlenecks

Abstract

The genetic consequences of living on the edge of distributional ranges have been the subject of a largely unresolved debate. Populations occurring along persistent low latitude ranges (rear-edge) are expected to retain high and unique genetic diversity. In contrast, currently less favourable environmental conditions limiting population size at such range-edges may have caused genetic erosion that prevails over past historical effects, with potential consequences on reducing future adaptive capacity. The present study provides an empirical test of whether population declines towards a peripheral range might be reflected on decreasing diversity and increasing population isolation and differentiation. We compare population genetic differentiation and diversity with trends in abundance along a latitudinal gradient towards the peripheral distribution range of Saccorhiza polyschides , a large brown seaweed that is the main structural species of kelp forests in SW Europe. Signatures of recent bottleneck events were also evaluated to determine whether the recently recorded distributional shifts had a negative influence on effective population size. Our findings show decreasing population density and increasing spatial fragmentation and local extinctions towards the southern edge. Genetic data revealed two well supported groups with a central contact zone. As predicted, higher differentiation and signs of bottlenecks were found at the southern edge region. However, a decrease in genetic diversity associated with this pattern was not verified. Surprisingly, genetic diversity increased towards the edge despite bottlenecks and much lower densities, suggesting that extinctions and recolonizations have not strongly reduced diversity or that diversity might have been even higher there in the past, a process of shifting genetic baselines.

Closer to the rear edge: ecology and genetic diversity down the core‐edge gradient of a marine macroalga

“…Latitudinal gradients in air and sea temperatures, which are intensified by upwelling events that dominate north-western Iberia during summer periods (Ar ıstegui et al 2006), have been reported as the cause of species' distributional patterns. In particular, the western Iberian coastline studied here represents latitudinal clines in abundance and distributional edge limits for many coastal species (Boaventura et al 2002, Pereira et al 2006, Assis et al 2013.…”

“…Abundance patterns, and demographic parameters in general, appear to be driven by interacting fluctuations of several environmental factors and are more variable than originally thought (e.g., Gaston 2009, Viejo et al 2010. Relative to the core, southern edge populations of many species show lower intrapopulation genetic diversity and increased interpopulation genetic divergence due to habitat fragmentation, increased random genetic drift, bottleneck effects, and reduced gene flow (Arnaud-Haond et al 2006, Provan and Maggs 2011, Assis et al 2013. However, there is increasing doubt as to the generality of this correlation (reviewed in Eckert et al 2008).…”

“…However, there is increasing doubt as to the generality of this correlation (reviewed in Eckert et al 2008). Population demography and diversity can be asymmetrical at low latitudes as rear-edge populations might inhabit former refugial areas, and thus contain intrinsically greater genetic diversity, not just within-but also between-edge populations (Diekmann and Serrao 2012, Assis et al 2013). These contrast with leading edges resulting from expansions carrying founder effects (e.g., postglacial recolonizations; Marko et al 2010).…”

“…We focused on the Atlantic Iberian coastline because it is characterized by a strong latitudinal gradient in air and sea water temperature throughout the whole year (e.g., Lima et al 2007a, Assis et al 2013) and marks the geographic limit of many marine species (e.g., Pérez-Ruzafa et al 1993, Boaventura et al 2002, Lima et al 2007b, Diekmann and Serrao 2012. We selected the ecosystem-building brown macroalga Fucus guiryi as model species.…”

Seascape genetics of Indo-Pacific reef organisms: methodologies and case studies for an emerging discipline

“…Contrasting patterns at the northern and southern range peripheries for temperate species may not be surprising given the vastly different environmental conditions organisms are likely to experience (e.g. Hoban et al 2010, Assis et al 2013, Hasselman et al 2013). However for a tropical species such as the neon damselfish we expected similar genetic patterns as, presumably, individuals experience similar environmental conditions at both latitudinal range peripheries.…”