The phylogeographic history of Gracilaria transtasmanica in Australian and New Zealand.

Often genetic disjunctions are present in populations separated by large distances, or even across relatively short distances in some cases. Natural barrier, such as currents, sheltered bays, andextensive bodies of water can hinder gene flow, creating genetic differentiation between populations. Many studies have addressed the interaction between geographical distribution and phylogenetic relationships and have identified common patterns that are associated with events such as expansion from glacial refugia and historical land bridges following glaciations. Bioregion boundaries are defined at locations where genetic breaks are commonly observed in species and these act as guides to future studies, suggesting potential locations to sample. Sometimes observed patterns are unique, not adhering to bioregions, revealing a fundamental difference in the gene flow of the species and its populations.

This field of research is termed phylogeography and has been pivotal in combining macro and microevolutionary principles of population genetics and phylogenetics since its inception in 1987.

The field has benefited from the advent of modern molecular tools, mostly from the improvement in DNA sequencing methods, allowing large amounts of sequence data to be used to provide information about haplotype frequency and diversity. Phylogeography has helped to reveal genetic patterns that would otherwise go unnoticed, such as the presence of cryptic species that are morphologically indistinguishable in the field, requiring genetics to discern.

Gracilaria transtasmanica exists in Australia and New Zealand, yet only a single phylogeographic study has addressed the distribution of the species and exclusively in New Zealand. Using ITS2 and a new plastid marker, the present study expands on the knowledge of G. transtasmanica in New Zealand, and we incorporate Australian populations to compare the genetic diversity of the species in the two countries. The results from the ITS2 data indicate that the level of diversity in each country is comparable but with a distinct differentiation from one another, sharing no ribotypes. The plastid marker shows shared haplotypes but with little phylogeographic information beyond this. We also show that the distribution of G. transtasmanica is more continuous than previously shown, with populations all along the west coast of the North Island. We conclude that Australian and New Zealand populations have experienced isolation, that the origin of the species remains unclear, andthat the last glacial maximum is likely to have impacted the contemporary genetic patterns in each country.