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.
Highlights
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 and is established and maintained within a population (Johnson et al, 2019)
Studies on the shell polymorphism of the snail Cepaea have played a crucial role in establishing the role of natural selection in maintaining morphological variation, with the genus becoming a pre- eminent model for ecological genetics, alongside the peppered moth (Cook & Saccheri, 2013; Grant et al, 1996; Majerus et al, 2000; Walton & Stevens, 2018)
The banding phenotype of Cepaea snails has typically been scored as a qualitative character, even though shells with the same number of bands may have a quite different outward appearance
Summary
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 and is established and maintained within a population (Johnson et al, 2019). There are likely other loci, or environmental factors which act during growth, that exert a multifactorial effect on the phenotype, including modifiers of bandwidth, band fusion, band colour, suppression of individual bands, and the timing of band expression (e.g., bands only on last whorl) These loci are not useful in understanding how bands are placed, because they mainly specify presence/absence, or character, rather than position. As we move toward identifying the genes involved in setting the patterns, these findings may together be used to develop a model for band placement in snail shells, set in the general context of understanding shell growth parameters
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