Temporal changes in population structure of a marine planktonic diatom.

Sylvie V M Tesson,Marina Montresor,Wiebe H C F Kooistra,Gabriele Procaccini,Dee A Carter

doi:10.1371/journal.pone.0114984

Abstract

A prevailing question in phytoplankton research addresses changes of genetic diversity in the face of huge population sizes and apparently unlimited dispersal capabilities. We investigated population genetic structure of the pennate planktonic marine diatom Pseudo-nitzschia multistriata at the LTER station MareChiara in the Gulf of Naples (Italy) over four consecutive years and explored possible changes over seasons and from year to year. A total of 525 strains were genotyped using seven microsatellite markers, for a genotypic diversity of 75.05%, comparable to that found in other Pseudo-nitzschia species. Evidence from Bayesian clustering analysis (BA) identified two genetically distinct clusters, here interpreted as populations, and several strains that could not be assigned with ≥90% probability to either population, here interpreted as putative hybrids. Principal Component Analysis (PCA) recovered these two clusters in distinct clouds with most of the putative hybrids located in-between. Relative proportions of the two populations and the putative hybrids remained similar within years, but changed radically between 2008 and 2009 and between 2010 and 2011, when the 2008-population apparently became the dominant one again. Strains from the two populations are inter-fertile, and so is their offspring. Inclusion of genotypes of parental strains and their offspring shows that the majority of the latter could not be assigned to any of the two parental populations. Therefore, field strains classified by BA as the putative hybrids could be biological hybrids. We hypothesize that P. multistriata population dynamics in the Gulf of Naples follows a meta-population-like model, including establishment of populations by cell inocula at the beginning of each growth season and remixing and dispersal governed by moving and mildly turbulent water masses.

Highlights

Marine planktonic organisms can grow extremely fast
A total of 525 strains out of the 735 successfully grown in culture were successfully genotyped from 22 of the net-samples in which the species was present (157 strains in samples 1–7 in 2008; 193 in samples 8–15 in 2009; 162 in samples 16–21 in 2010, and 13 in sample 22 in 2011; Table 1). Twelve of these strains belonged to the frustule size class $55 mm, in which sexual reproduction cannot be induced, whereas the remainder belonged to the 55–30 mm size-interval, i.e. the size window in which sex can be induced (Fig. 3)
Results of the Bayesian analysis and the Factorial Correspondence Analysis (FCA) of the microsatellite genotypes obtained from the Pseudo-nitzschia multistriata strains uncovered two genetically distinct populations in the Gulf of Naples

Summary

Introduction

Marine planktonic organisms can grow extremely fast Such fast growth, sustained by plentiful resources and temporally relaxed predation pressure, can lead to episodic, rapid and vast increases in their population sizes. Unicellular phytoplankton species usually show high genotypic diversity and in cases where genetically distinct populations are observed, they are often correlated with hydrographic or geographic features [1, 4,5,6,7,8,9,10]. Episodic sexual reproduction in a population composed of large numbers of clones generates huge numbers of F1 cells with distinct genotypes, each of which in its turn can form a clone [11]. The likelihood of sampling multiple individuals belonging to the same clone in a large phytoplankton population is very small, given the sample sizes normally deployed in population genetic studies [12]

Methods

Results

Conclusion