Molecular phylogeny, phylogeography and population genetics of the red seaweed genus Asparagopsis

Andreakis, Nikolaos (2006). Molecular phylogeny, phylogeography and population genetics of the red seaweed genus Asparagopsis. PhD thesis The Open University.



The red seaweed genus Asparagopsis Montagne (Bonnemaisoniales) was studied with respect to its taxonomy, phylogeny, phylogeography and population genetics. The representatives of this genus, A. armata Harvey and A. taxiformis (Delile) Trevisan, are notorious invaders. Both species occur worldwide and show disjunct distribution patterns. Such patterns may result from recent jump-dispersal or from fragmentation of once panglobally distributed species. First, a phylogeographic approach was deployed in order to delineate the taxonomic units in local scale and to assess if European populations of each of the species originated from a single introduction or multiple cryptic ones. Results showed that the two species recognized A. armata and A. taxiformis are also genetically distinct. Asparagopsis armata was found to consist of a single species worldwide, whereas A. taxiformis constituted three and probably four morphologically cryptic but genetically distinct lineages. At times, lineages were encountered in sympatry and two of them were detected in the Mediterranean Sea.

In order to confirm distinction between lineages and to assess invasive potential and colonization mechanisms of the species along the western Italian coast, eight nuclear micro satellite markers were identified against the invasive lineage 2 of A. taxiformis. The markers cross-hybridised only with lineages I and 2. Moreover, it was demonstrated that carpogonia present on many female thalli can affect microsatellite reading patterns because of external (male) allelic contribution. Even after removal of the carpogonia, gametophyte thalli exhibited multiple allelic patterns, which is indicative for polyploidy. The markers were then used to assess genetic structure and diversity within and among Mediterranean populations of A. taxiformis lineages 1 and 2. Analyses based on statistics developed for polyploid species showed that the lineage l-population (HAW) was distinct from Mediterranean lineage 2 populations. A geographically distant Californian lineage 2· population was genetically distinct from the Mediterranean ones as well. The Mediterranean lineage 2-samples showed panmixia. High genotypic diversity, high gene flow, and low differentiation encountered amongst these populations probably are due to a recent invasion of this lineage into the basin.

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