Genetic structure and speciation in planktonic copepods: Global phylogeography of the Calanus helgolandicus clade.

摘要

Taxonomies affording accurate species identification are not always possible for marine plankton because of their small size and the absence of divergent morphological characters in closely related taxa. Additionally, uncertainty surrounds the geographical scale and the ecological factors that create divergence in planktonic organisms, eventually leading to reproductive isolation between species. The oceanic planktonic calanoid copepod genus Calanus is an ideal group of organisms to study speciation in the marine realm and employ taxonomic genomics. Calanus species play a prominent role in the carbon cycle in the sea and constitute a major link in pelagic food webs between the phytoplankton-microzooplankton assemblages and larger zooplanktivores. Species identification of Calanus is based on subtle morphological differences, and the taxonomic validity of some taxa remains unclear. The objective of my research is to characterize the genetic structure and degree of divergence, using molecular methods, of all taxa in the most diverse Calanus lineage, the helgolandicus clade. I examined patterns of genetic divergence and genetic structuring on multiple geographic scales, among species with varying levels of morphological divergence, to gain insight into the process of speciation in marine zooplankton. The first chapter of my thesis is a phylogenetic analysis of the helgolandicus clade on a global scale. A general guideline for the level of genetic differentiation of a species within the helgolandicus clade was developed, and the putative species and subspecies were molecularly defined. The second chapter focused on levels of genetic differentiation within a species and the genetic structuring across a species' range. The third chapter examined genetic diversity in one Calanus species across temporal and small spatial scales.