Genetic and phenotypic lineages in Neogastropod molluscs : a journey through time and morphospace : a thesis presented in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Zoology, Massey University, Manawatū, New Zealand
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2017
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Massey University
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Extant and fossil snail shells provide important insight into the morphological evolution of species allowing assessment of evolutionary mode within lineages. Recognising biologically distinct groups is problematic when only shell characters are available on which to base decisions, as is the case with fossil material. Therefore, the concordance of evolutionary lineages inferred by different methods need to be confirmed before evolutionary patterns and models can be tested. Recent advances in techniques and technology enabling more detailed investigations into both morphologic and genetic variation is helping to recognise taxonomic groupings that reflect evolutionary relationships. Genetic tools may provide the most accurate description of evolutionary relationships among taxa and morphologic tools can be used to investigate evolutionary patterns extending into the fossil record. These two methods have the ability to both complement and challenge one another. In this thesis I have generated genetic data using high throughput DNA sequencing and amplicon sequences. This provides a range of data from whole mitochondrial genomes through to nuclear single nucleotide polymorphisms to help clarify genetic clusters and build phylogenies. The morphologic data was generated using two dimensional landmark morphometric tools allowing empirical comparison of complex shell shapes without the confounding effects of size. By separating the variation among snail shells into uncorrelated variables (principal components) model-based Bayesian assignment analyses provide an unbiased tool to cluster specimens without priori hypotheses. This combination of new methods to both describe shape variation and determine the most efficient model to group specimens greatly improves studies of natural phenotypic variation and identification of evolutionary lineages. I have used these approaches to examine morphologic and genetic variation in three genera of Neogastropod molluscs from New Zealand to clarify species boundaries and test evolutionary theory. Variation in snail shell shape has provided evolutionary biologists with excellent material for the study of local adaptation to local environments. However, treating shell shape variation as evidence of isolated lineages (species) may have led to taxonomic inflation within some gastropod lineages. Here I compare shell shape variation and genetic structure of rocky shore whelks, Buccinulum, from the coast of New Zealand. The species B. vittatum is split into two subspecies, B. vittatum vittatum and B. vittatum littorinoides, and is interposed geographically by the species B. colensoi. The current taxonomy based primarily on shell morphology, is in conflict with results obtained from genetic data from both amplicon sequencing and next generation sequencing. I found that B. colensoi and B. vittatum littorinoides form a genetic cluster distinct from B. vittatum. I conclude that the shell shape variation associated with B. colensoi would be better viewed as an ecotype of B. v. littorinoides, and the northern taxa as a separate genotypic cluster. As the ecotype B. colensoi is not genetically isolated from B. v. littorinoides, this suggests selection is stronger than gene flow. Punctuated equilibrium encompasses both morphological stasis and rapid morphological change. The period of rapid morphological change is linked to the process of lineages splitting to increase taxon diversity (speciation). In order to go some way towards testing the theory of punctuated equilibrium a number of elements within a taxon should be resolved: (1) the biological reality of fossil “species” which can be assessed via examination of traits in living relatives (2) the elimination of biological invasion that might be mistaken for speciation, which can be achieved if monophyly of the group is established. (3) a period of morphological stasis separate from speciation events. The olive shells (Amalda) are a speciose genus of Neogastropod mollusc with an extensive distribution. In New Zealand they have an excellent fossil record and have been presented as support for biologically distinct extant and fossil lineages, and morphological stasis in the fossil record, two prerequisites for punctuated equilibrium. I tested the hypothesis of New Zealand monophyly of Amalda using long DNA sequences from both mitochondrial (whole mitogenome) and nuclear (45S rDNA) markers from 14 Amalda species collected from New Zealand and around the world. Genomic data was generated with high throughput sequencing and whole mitogenome and 45S alignments were assembled from short reads. The reconstructed phylogenies from both the mitochondrial and nuclear DNA sequences support the hypothesis that the New Zealand Amalda are a monophyletic group and the extant species evolved in New Zealand waters in isolation. Within the olive shells (Amalda) in New Zealand there is strong concordance between the accepted taxonomy in the extant taxa and genetic data generated with amplicon and next generation sequencing. The recognised species show distinct genetic clustering associated with morphologic clustering based on two dimensional morphometrics. There is some overlap of morphologic clusters as is to be expected within such a morphologically homogeneous genus. There is a comprehensive fossil record for Amalda in New Zealand providing the means to test fundamental evolutionary theories and recent improvements in dating fossil horizons allows incorporation of better data. By using a morphometric data set including fossil representatives of three extant species I show that the species in the fossil record represent lineages with the modern fauna at the tips enabling the testing of evolutionary models. Three candidate evolutionary explanations were represented as statistical models, which were then evaluated using likelihood-based inference. Mixed models showing both stasis and unbiased random walk were best fit for phenotypic traits in all three species. Having shown that stasis in the fossil record is temporally separated from speciation events in distinct lineages some of the prerequisite conditions for the evolutionary theory of punctuated equilibrium have been met.
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Neogastropoda, Evolution, Genetics, Species, New Zealand, Research Subject Categories::NATURAL SCIENCES::Biology::Organism biology::Systematics and phylogenetics