Global patterns of genetic diversity and geographical distribution in the marine protist morphospecies Oxyrrhis marina

Species’ diversity and the related processes that drive it are fundamental to understanding patterns of biodiversity. There is a wealth of literature that quantifies the ranges of “large” organisms, and hence our understanding of the processes that determine macrospecies’ distributions; by contrast, this is obviously a more challenging field for microorganisms, of which protists seem to be least studied. Despite this, there are two long debated views regarding protist distribution, proposing that protists display either ubiquity or moderate endemicity. Oxyrrhis marina is an ecologically important marine protist that is used widely as a model organism, and from the pattern of genetic divergence, likely consists of two species. However, patterns of diversity and the distribution of different species (or genetic lineages) are unknown on a global scale. In this thesis I use Oxyrrhis as a model protist to quantify the amount of genetic diversity that may exist between regions at a global scale and to determine whether there are any patterns between genetic diversity and geography. First, I assess current levels of diversity within the O. marina morphospecies. Morphological and molecular literature on O. marina was reviewed and the genus subsequently split into two species: Oxyrrhis marina and Oxyrrhis maritima. Second, genetic divergence between global samples of the Oxyrrhis genus was quantified using multiple gene phylogenies to determine levels of diversity and global patterns of distribution. The three genes COI (mitochondrial), 5.8S ITS and α-tubulin (nuclear) defined the two distinct lineages (O. marina and O. maritima); moreover, 5.8S ITS and α-tubulin uncovered further genetic diversity in strains that were predominantly from East Asian waters. The divergence between these strains and both O. marina and O. maritima is such that they may represent a new species, but further morphological and phylogenetic characterisation is required to support this. The lineages displayed contrasting patterns of distribution and abundance, one being broadly distributed and abundant and the other being geographically restricted and rare in comparison, seemingly supporting both sides of the protist distribution debate. These patterns were not exclusive (i.e. they overlapped) and require further sampling to draw more precise conclusions about the processes that led to their present distributions. This thesis has uncovered high levels of genetic diversity and contrasting distribution patterns displayed in a single genus; it is therefore clearly unrealistic to make generalisations about “protist biogeography” as they display a wide range of responses and distributions