High levels of realized dispersal and phenotypic plasticity in the polymorphic squat lobster Munida gregaria (Decapoda Anomura Munididae) in Patagonia and New Zealand - a multi-marker genetic and morphological approach

Abstract

Successful species delimitation is of fundamental importance in understanding the process of speciation. The tools of molecular genetics provide enormous potential for clarifying the nature of species boundaries, which has challenged the generalization that marine species are often associated with high dispersal and mild genetic differentiation over large spatial scales by unravelling hidden diversity in the form of unrecognized cryptic species. The occurrence of two or more sharply distinct morphotypes, even if currently considered a single nominal species, can either be a case of previously overlooked genetic differentiation or it may be caused by the expression of different morphotypes from essentially the same genetic background. The squat lobster Munida gregaria (Decapoda Munididae) is a shallow-water species that is widely distributed mainly on the continental shelf off southeastern New Zealand and Patagonia. Its global disjunct distributions are separated by more than 7000 kma expanse of deep Pacific water, which is known as a major marine biogeographical barrier, the East Pacific Barrier (EPB). Under this currently accepted species (i.e., M. gregaria sensu lato) there are two ecotypes, gregaria sensu stricto and subrugosa that are characterized sharp differences in morphology, ecology and behaviour. Therefore, this species provides an excellent case study to test whether present marine major barriers and heterogeneous environmental forces would have resulted in genetic differentiation and further speciation. The superordinate aim of this study was to reconstruct the evolutionary history of the South American and New Zealand squat lobster M. gregaria. The central aspects of the thesis were investigations of i) the relationship between phenotype and genotype ii) interaction of emerging geographical barriers and oceanographic influence in shaping genetic structure, biogeography of M. gregaria. In order to fulfil this task a comprehensive analytical framework based on the combination of nuclear microsatellites and mitochondrial COI genetic markers, as well as range-wide sampling, were applied to address these major tasks. To avoid potential pitfalls of using a single marker especially the mitochondrial DNA barcoding gene, such as introgression, incomplete lineage sorting and insufficient evolutionary rates to recover recent divergence in potential cryptic speciation, a set of fast-evolving nuclear microsatellite were newly developed for M. gregaria. These microsatellites together with conventional DNA barcoding sensu stricto (mitochondrial COI) were firstly applied on South American samples to test whether the different ecotypes arise from differentiated genotypes. Both data sets yielded a congruent result of an undifferentiated genetic background corresponding to either ecotypes or geographic units across the entire South American distribution. The gregaria s. str. and subrugosa ecotypes are not underpinned by cryptic genotypes, instead the discrete phenotypic differentiation is a representation of environment-driven phenotypic plasticity or developmental variation. Morphometric analyses were performed in order to see if continental-scale sampling would cover unsampled intermediary morphotypes and then blur the boundaries of the two ecotypes. Results from PCA and using discriminant functions demonstrated the morphology of the two forms remaining discrete in the context of sampling from large-scale geographic locations as well as different ontogenetic composition. Moreover, the inclusion of different life stages of M.gregaria demonstrates that a part of the morphometric variance is adequately explained by an ontogenetic transition between moults. Population genetic analyses based on global sampling of M. gregaria detected a genetic structure across the Pacific between New Zealand and South America. But many shared alleles, as well as testing in a Bayesian framework, confirmed gene flow at different timescales across the open water barrier spanning more than 7000km. The dispersal of M. gregaria between New Zealand and South America was possibly facilitated by the ACC. This study demonstrated that the EPB is not a strict marine biogeographical barrier to gene flow. This case study demonstrates a striking case in contrast with the mounting number of cryptic speciation resulted from geographic separation or disruptive environmental selection. The combination of nuclear and mitochondrial genetic markers are highly recommended in order to improve the power of molecular data to test phylogenetic and population genetic hypotheses, especially in the event of homogeneous genetic background that is hard to falsify with easily obtainable but error-prone mitochondrial data alone.