Maintenance of clinal variation for shell colour phenotype in the flat periwinkle Littorina obtusata

Abstract

Clines can signal spatially varying selection and therefore have long been used to investigate the role of environmental heterogeneity in maintaining genetic variation. However, clinal patterns alone are not sufficient to reject neutrality or to establish the mechanism of selection. Indirect, inferential methods can be used to address neutrality and mechanism, but fully understanding the adaptive significance of clinal variation ultimately requires a direct approach. Ecological model systems such as the rocky intertidal provide a useful context for direct experimentation and can serve as a complement to studies in more traditional genetic model systems. In this study, we use indirect and direct approaches to investigate the role of environmental heterogeneity in the maintenance of shell colour polymorphism in the flat periwinkle snail, Littorina obtusata. We document replicated clines in shell colour morph frequencies over thermal gradients at two spatial scales, contrasting with patterns at previously reported microsatellite loci. In addition, experimental results demonstrate that that shell colour has predictable effects on shell temperature and that these differences in temperature, in turn, coincide with patterns of survivorship under episodic thermal stress. Direct manipulation of shell colour revealed that shell colour, and not a correlated character, was the target of selection. Our study provides evidence that spatially varying selection via thermal regime contributes to the maintenance of shell colour phenotype variation in L. obtusata in the sampled areas of the Gulf of Maine.

Morph-specific and sex-specific temperature effects on morphology in the colour polymorphic damselfly Ischnura elegans

“…Colour polymorphism is usually associated with correlated differences in multiple traits (McKinnon and Pierotti, 2010), and ecological differences between morphs have been found even in species where colour polymorphism was long-thought to be neutral (e.g., Schemske and Bierzychudek, 2007). In species where morphs differ in the darkness or melanization of their colouration, temperature or light effects are often determined to be a proximate cause for geographic variation in morph frequencies (de Jong and Brakefield, 1998;Galeotti et al, 2003;Phifer-Rixey et al, 2008). However, in colour morphs that do not differ in any systematic way along a "dark-light" axis, other factors may be more important in determining spatial clines, such as visibility in the water column (Terai et al, 2006) or UV resistance (Cooper, 2010).…”

Predation and the Maintenance of Color Polymorphism in a Habitat Specialist Squamate

“…Geographic separation of a population combined with limited dispersal also reduces gene flow, which in turn favors natural selection for background matching and avoidance of predation. Most analyses of geographic variation in color patterns focus on prey species [7], [11]. For example, deer mice of the genus Peromyscus are one of the few color polymorphic vertebrates that have been studied extensively [12], [13], [14], [15].…”

Biogeographical Survey Identifies Consistent Alternative Physiological Optima and a Minor Role for Environmental Drivers in Maintaining a Polymorphism

“…One possibility to explore the relative contribution of these mechanisms is to compare genetic variability of neutral loci with geographic and temporal variation in morph frequencies [9]–[11]. Alternatively, one may evaluate to what extent spatiotemporal variation in morph frequencies and morph fitness correlates can be explained by variation in ecological parameters [12], [13].…”