Quantitative measures and 3D shell models reveal interactions between bands and their position on growing snail shells

Abstract

The nature of shell growth in gastropods is useful because it preserves the ontogeny of shape, colour, and banding patterns, making them an ideal system for understanding how inherited variation develops, is established and maintained within a population. However, qualitative scoring of inherited shell characters means there is a lack of knowledge regarding the mechanisms that control fine variation. Here, we combine empirical measures of quantitative variation and 3D modeling of shells to understand how bands are placed and interact. By comparing five‐banded Cepaea individuals to shells lacking individual bands, we show that individual band absence has minor but significant impacts upon the position of remaining bands, implying that the locus controlling band presence/absence mainly acts after position is established. Then, we show that the shell grows at a similar rate, except for the region below the lowermost band. This demonstrates that wider bands of Cepaea are not an artifact of greater shell growth on the lower shell; they begin wider and grow at the same rate as other bands. Finally, we show that 3D models of shell shape and banding pattern, inferred from 2D photos using ShellShaper software, are congruent with empirical measures. This work therefore establishes a method that may be used for comparative studies of quantitative banding variation in snail shells, extraction of growth parameters, and morphometrics. In the future, studies that link the banding phenotype to the network of shell matrix proteins involved in biomineralization and patterning may ultimately aid in understanding the diversity of shell forms found in molluscs.

Deep structure, long‐distance migration and admixture in the colour polymorphic land snail Cepaea nemoralis

“…One benefit of working with these species is that variation in colour and banding (Figure 1) shows straightforward inheritance (Cook, 2017; Jones et al, 1977; Richards et al, 2013), being controlled by a series of at least nine loci, of which five make a single ‘supergene’ containing tightly linked colour, banding and other loci. In these two species, we now have some understanding of the pigments and shell proteome (Affenzeller et al, 2020; Mann & Jackson, 2014; Williams, 2017) and some knowledge underlying the development of the banding pattern (Jackson et al, 2021) and have begun to use genomic methods to try to map and identify the genes involved (Kerkvliet et al, 2017; Richards et al, 2013).…”