Jackson, H
(2022)
Quantitative definition of phenotypic variation in land snail shells.
PhD thesis, University of Nottingham.
Abstract
The study of both animal colouration and patterning across the natural world has been imperative for understanding some of the key principles of biology throughout the past century, particularly with respect to evolution and genetics. Generally, colour and pattern have each been described qualitatively, often being binned into discrete groups relying on human perception of colour or pattern, rather than being considered in a biologically relevant context. ‘Binning’ traits into discrete groups has the consequence that variation within discrete morphs is often overlooked. Terrestrial gastropods, such as the selected study genera Cepaea and Auriculella, provide an ideal system for the study of polymorphisms and colour variation due to the extreme variety of morphs present across the taxa, as well as the nature of shell growth providing a complete ontogeny of an individual.
The aims of this thesis are threefold; firstly, I aimed to understand finescale variation within and between established banding pattern morphs in Cepaea, to allow inferences to be made regarding the genetic mechanisms responsible for this variation. The implementation of two quantitative methods for measuring variation in band position and width in individual shells found that individual band absence has a minor but significant effect on the position of the remaining bands, implying that the locus controlling band presence/absence acts mainly after the position of bands is determined. I establish a method which is useful for comparative studies of quantitative banding variation in snail shells, and for extraction of growth parameters and morphometrics, highlighting the importance and usefulness of gastropod shells in the understanding of how variation is established and maintained in a population.
Secondly, I aimed to understand the shell colour variation present in both Cepaea nemoralis and Cepaea hortensis by using spectrophotometry and psychophysical modelling in tandem. It was revealed that colour variation in Cepaea hortensis is continuous, with no detectable effects of geographic location with the exception of an association of the paleness of yellow shells with latitude. Differences between the colour of Cepaea hortensis and Cepaea nemoralis, both in terms of exact shade and overall colour were revealed; Cepaea hortensis are generally paler, and less pink-toned, but slightly more brown-toned. Precise shade variation of yellow individuals from genetically diverse lineages of Cepaea nemoralis were also detected. The results presented have significance in furthering the understanding of the precise nature of the colour polymorphism displayed in Cepaea spp., and the nature of the selection which acts upon it, as well as highlighting the importance of considering colour as a continuous trait, rather than binning it into discrete groups.
Thirdly, I aimed to investigate colour variation across a number of scales in the Hawaiian land snail genus Auriculella, to allow inferences about the genetic architecture responsible for the variation, and to highlight the usefulness of museum collections of gastropod shells in understanding variation in extinct or endangered species. I demonstrated that there are differences in colour within single shells of Auriculella, similar to variation displayed by other Pacific Island snails. I described significant variation between isolated populations of the same species, and determined that there is no difference in colour variation between shells on the islands of Maui and Oahu. Finally, I demonstrated that there is no difference in colour between shell chiralities, suggesting that interchiral mating is not uncommon, and that the loci responsible for colour variation and chirality are not closely linked. By describing the variation present in Auriculella in a biologically relevant context, inference regarding genetic mechanisms of variation becomes possible in a taxa of conservation concern.
By achieving these aims, and synthesising conclusions drawn from their achievement, I have highlighted the importance of accurately defining phenotypes for the purposes of evolutionary ecology and genetics. Defining phenotypes and investigating variation present within morphs has allowed inferences to be made regarding the underpinning genetic mechanisms which control variation in two gastropod genera, although the principles are applicable to other taxonomic groups. Finally, and more broadly, I have demonstrated the usefulness of gastropods as study systems, particularly where large collections of shells are available.
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