Thirty-four years of climatic selection in the land snail Theba pisana

Abstract

Few continuous, long-term studies have measured the intensity and variability of natural selection within a framework of clear adaptive hypotheses. In the snail Theba pisana, the proportion of effectively unbanded shells is higher in exposed habitats than in adjacent acacia thickets, which has been explained by microclimatic selection. Comparisons across an ecotone for 34 consecutive years determined the combined effects on morph frequencies of habitat and changes in weather conditions during summer. The long-term average ( ± s.e.) frequency of effectively unbanded shells was 0.577 ± 0.011 in the open habitat when compared with 0.353±0.005 in the acacia. The persistent association of shell banding with habitat accounted for 34% of the variation in morph frequencies. Differences among years were also large, representing 23% of the variation. Higher proportions of effectively unbanded snails were associated with hotter, sunnier summers. Thus, temporal variation supports the hypothesis of microclimatic selection, consistent with the spatial association with habitat. Based on observed rates of change, the mean annual selection on this polymorphism was about 0.13, but with a large variance: s was as high as 0.5, but p0.05 in about 40% of the years. The large variance and frequent reversals in direction of selection indicate a potential for rapid genetic change, but with little net change in morph frequencies over three decades, highlighting the value of long-term continuous studies of populations facing natural environmental variation.

Hsp70 and lipid peroxide levels following heat stress in Xeropicta derbentina (Krynicki 1836) (Gastropoda, Pulmonata) with regard to different colour morphs

“…Asterisks show significant differences between the colour categories within an exposure group: 0.01< p≤0.05 (*), 0.001<p≤0.01 (**); p≤0.001 (***) derbentina populations change their composition of colour morphs during the years and how different local temperatures may influence the morph frequencies within a population of these snails. However, the phenomenon of morph frequency fluctuations throughout the years has been reported for other helicoid land snails before (Cowie 1992;Johnson 2011;Silvertown et al 2011). As summarized by Ozgo and Schilthuizen (2012), the shell colour of Cepaea nemoralis was found to be associated with a gene locus coding for the different background shell colours in this species, while the banding was found to be associated with another locus, linked to the colour coding one.…”

Phenotypic diversity, population structure and stress protein-based capacitoring in populations of Xeropicta derbentina, a heat-tolerant land snail species

“…Various authors have investigated in respect to how phenotypic diversity in polymorphic snail populations are influenced by genetic traits and environmental conditions (Jones et al 1982;Baur 1988;Baur and Raboud 1988;Cowie 1990;Johnson 2011;Di Lellis et al 2012;Köhler et al 2013). Shell colour and pattern are highly variable in pulmonate snail species such as Xeropicta derbentina (Krynicki, 1836) and Theba pisana (Müller, 1774) (Johnson 1981;Cowie 1990;Köhler et al 2009).…”

“…This might be particularly true for populations subjected to strong bottle-necks caused by, for example, high ambient temperatures. In T. pisana, high temperatures emerged either as a selective force or an epigenetically acting component of the environment (Johnson 2011(Johnson , 2012Köhler et al 2013): organisms possess various mechanisms to deal with high temperatures.…”

“…Several gene loci primarily control shell colour, presence or absence of shell banding, intensity of pigmentation, et cetera (Jones et al 1982;Cowie 1984). Habitat structure (such as shaded/exposed) and ambient temperature influence the composition of phenotypes in snail populations (Johnson 2011;Köhler et al 2013). These conditions, in turn, might also be reflected in their population structure (as inferred from their genotypic diversity).…”

Epistasis, phenotypic disequilibrium and contrasting associations with climate in the land snail Theba pisana

“…The land snail Theba pisana is genetically polymorphic for shell banding, and variation of morph frequencies indicates adaptation to local climate: snails with paler, unbanded shells are favoured in exposed habitats (Johnson, 1980(Johnson, , 2011aHeller, 1981) and in hot, sunny summers (Johnson, 2011a), and they select more exposed aestivation sites in summer than their banded counterparts (Johnson, 1981;Hazel and Johnson, 1990). The banding polymorphism is complex, and effectively unbanded (EUB) shells may be either genetically unbanded or banded with genetic suppression of bands on the upper portion of the shell (Cain, 1984;Cowie, 1984).…”