Population Genetic Structure at the Northern Edge of the Distribution of Alexandrium catenella in the Patagonian Fjords and Its Expansion Along the Open Pacific Ocean Coast

Javier Paredes,Bianca Olivares,Daniel Varela,Adrián Villarroel,Camila Martínez,Karen Correa,Andrea Zúñiga

doi:10.3389/fmars.2018.00532

Abstract

In southern Chile, Alexandrium catenella is the main species generating HABs and over time it has expanded its range since it was first recorded in the Magallanes region in 1972. In 2016 a severe bloom of an Alexandrium species occurred, which was notable for its intensity and geographical extent, extending into new areas to the north of the Patagonian fjords including areas along the open Pacific Ocean coast. Given the exceptional nature of this event, we verified the taxonomic classification of the species that generated the bloom and evaluated the influence of the range expansion process on its genetic structure and population diversity. This was achieved by isolating clones collected in 2014 from cyst beds located at the northern limit of its then known distribution, along with clones isolated from the water samples taken during the 2016 bloom. These clones were characterized genetically with LSU rDNA and AFLPs molecular markers. Phylogenetic analyses showed that all clones were aggregated in the Group I of the A. tamarense species complex, which confirmed that A. catenella was the species responsible for the 2016 bloom. High genetic diversity was observed within populations though there were no significant differences between populations. Furthermore, genetic structure showed an isolation by distance relationship among populations, and several analyses consistently indicated a high divergence among the population groups derived from both cysts and vegetative cells. Despite this study not finding the patterns expected for species range expansion (i.e. diversity gradient and/or high population divergence), the genetic diversity and structure indicated that these were influenced by the geographic distance effect and the physical restrictions to gene flow, along with the demographic processes that occurred during the distinct phases of the life cycle of A. catenella.

Highlights

Around the world, factors such as climate change and eutrophication of coastal waters have favored the recurrence and severity of harmful algal blooms (HABs), increasing the probability that harmful species will expand their ranges to new areas (Wells et al, 2015)
AFLP population characterizations have permitted the examination of the range expansion process of species such as Gonyostomum semen and Alexandrium ostenfeldii, which show no evidence of diversity gradients, but do show levels of population divergence and isolation by distance (Tahvanainen et al, 2012; Sassenhagen et al, 2015)
The phylogenetic reconstruction based on rDNA large subunit (LSU) sequences indicated that all the clones analyzed from the populations associated with the 2016 bloom, as well as those isolated from the northern limit of distribution, were aggregated in the Group I of the A. tamarense species complex, verifying that the species being studied was A. catenella (Lilly et al, 2007; John et al, 2014; Willem, 2017)

Summary

Introduction

Factors such as climate change and eutrophication of coastal waters have favored the recurrence and severity of harmful algal blooms (HABs), increasing the probability that harmful species will expand their ranges to new areas (Wells et al, 2015). AFLP population characterizations have permitted the examination of the range expansion process of species such as Gonyostomum semen and Alexandrium ostenfeldii, which show no evidence of diversity gradients, but do show levels of population divergence and isolation by distance (Tahvanainen et al, 2012; Sassenhagen et al, 2015). In the case of A. tamarense species complex, the population genetics characterization using microsatellites and AFLPs, have revealed that the population divergence could have been generated by demographic changes that have occurred over time, in the populations of cysts in the sediments, and/or in the population of vegetative cells in the water column, and as a result of reduced gene flow. The potential migration of vegetative cells could be limited by oceanographic barriers (Casabianca et al, 2012) or restricted by geographic distance (Nagai et al, 2007), generating high levels of differentiation between populations

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