Maintenance of clonal diversity during a spring bloom of the centric diatom Ditylum brightwellii

Maintenance of genetic diversity in eukaryotic microbes reflects a synergism between reproductive mode (asexual vs. sexual) and environmental conditions. We determined clonal diversity in field samples of the planktonic marine diatom, Ditylum brightwellii, during a bloom, when cell number increased by seven-fold because of rapid asexual division. The genotypes at three microsatellite loci were determined for 607 individual cell lines isolated during the 11 days of sampling. Genetic diversity remained high during the bloom and 87% of the cells sampled each day were genetically distinct. Sixty-nine clonal lineages were sampled two or more times during the bloom, and two clones were sampled seven times. Based on the frequency of resampled clonal lineages, capture-recapture statistics were used to determine that at least 2400 genetically distinct clonal lineages comprised the bloom population. No significant differences in microsatellite allele frequencies were observed among daily samples indicating that the bloom was comprised of a single population. No sexual stages were observed, although linkage equilibrium at two loci, high levels of allelic and genotypic diversity, and heterozygote deficiencies were all indicative of past sexual reproduction events. At the height of the bloom, a windstorm diluted cell numbers by 51% and coincided with a change in the frequency distribution of some resampled lineages. The extensive clonal diversity generated through past sexual reproduction events coupled with frequent environmental changes appear to prevent individual clonal lineages from becoming numerically dominant, maintaining genetic diversity and the adaptive potential of the population.     

Minimal increase in genetic diversity enhances predation resistance

The importance of species diversity to emergent, ecological properties of communities is increasingly appreciated, but the importance of within‐species genetic diversity for analogous emergent properties of populations is only just becoming apparent. Here, the properties and effects of genetic variation on predation resistance in populations were assessed and the molecular mechanism underlying these emergent effects was investigated. Using biofilms of the ubiquitous bacterium Serratia marcescens, we tested the importance of genetic diversity in defending biofilms against protozoan grazing, a main source of mortality for bacteria in all natural ecosystems. S. marcescens biofilms established from wild‐type cells produce heritable, stable variants, which when experimentally combined, persist as a diverse assemblage and are significantly more resistant to grazing than either wild type or variant biofilms grown in monoculture. This diversity effect is biofilm‐specific, a result of either facilitation or resource partitioning among variants, with equivalent experiments using planktonic cultures and grazers resulting in dominance by a single resistant strain. The variants studied are all the result of single nucleotide polymorphisms in one regulatory gene suggesting that the benefits of genetic diversity in clonal biofilms can occur through remarkably minimal genetic change. The findings presented here provide a new insight on the integration of genetics and population ecology, in which diversity arising through minimal changes in genotype can have major ecological implications for natural populations.     

Hidden diversity in the freshwater planktonic diatom Asterionella formosa

Many freshwater and marine algal species are described as having cosmopolitan distributions. Whether these widely distributed morphologically similar algae also share a similar gene pool remains often unclear. In the context of island biogeography theory, stronger spatial isolation deemed typical of freshwater lakes should restrict gene flow and lead to higher genetic differentiation among lakes. Using nine microsatellite loci, we investigate the genetic diversity of a widely distributed freshwater planktonic diatom, Asterionella formosa, across different lakes in Switzerland and the Netherlands. We applied a hierarchical spatial sampling design to determine the geographical scale at which populations are structured. A subset of the isolates was additionally analysed using amplified fragment length polymorphism (AFLP) markers. Our results revealed complex and unexpected population structure in A. formosa with evidence for both restricted and moderate to high gene flow at the same time. Different genetic markers (microsatellites and AFLPs) analysed with a variety of multivariate methods consistently revealed that genetic differentiation within lakes was much stronger than among lakes, indicating the presence of cryptic species within A. formosa. We conclude that the hidden diversity found in this study is expected to have implications for the further use of A. formosa in biogeographical, conservation and ecological studies. Further research using species‐level phylogenetic markers is necessary to place the observed differentiation in an evolutionary context of speciation.     

Rapid increase in southern elephant seal genetic diversity after a founder event

Genetic diversity provides the raw material for populations to respond to changing environmental conditions. The evolution of diversity within populations is based on the accumulation of mutations and their retention or loss through selection and genetic drift, while migration can also introduce new variation. However, the extent to which population growth and sustained large population size can lead to rapid and significant increases in diversity has not been widely investigated. Here, we assess this empirically by applying approximate Bayesian computation to a novel ancient DNA dataset that spans the life of a southern elephant seal (Mirounga leonina) population, from initial founding approximately 7000 years ago to eventual extinction within the past millennium. We find that rapid population growth and sustained large population size can explain substantial increases in population genetic diversity over a period of several hundred generations, subsequently lost when the population went to extinction. Results suggest that the impact of diversity introduced through migration was relatively minor. We thus demonstrate, by examining genetic diversity across the life of a population, that environmental change could generate the raw material for adaptive evolution over a very short evolutionary time scale through rapid establishment of a large, stable population.     

Prevalence of chytrid parasitism among diatom populations in the lower Columbia River (2009–2013)

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