Population Genetics of the Aeromonas hydrophila Species Complex

Population genetics studies the genetic variability of individuals in a population based on the allele frequencies at several genes or loci and tries to explain this variability in terms of mutation, selection or genetic recombination. The statistical analysis of these frequencies allows models of evolution to be established, which will help us to understand and predict the past and present gene flow in the population (Maynard-Smith, 1991). For the most part population genetics has been designed for diploid organisms with sexual reproduction. In the words of Bruce Levin, “the genetic theory of adaptive evolution was developed by sexually reproducing eukaryotes, for sexually reproducing eukaryotes” (Levin & Bergstrom, 2000). As a consequence, before being applied to prokaryotes, population genetics needs to be adapted. In theory the haploid nature of bacteria should simplify their analysis, since dominance or over-dominance is not an issue and the genotype can usually be deduced directly from the phenotype. However, central to classical population genetics are infinite population size, random mating, and free recombination. Consequently, as expressed by Maynard-Smith, “the alleles present at one locus are independent of those at other loci. Changes in the frequency of an allele at one locus, therefore, are independent of what is happening elsewhere in the genome: each locus can be treated individually” (Maynard-Smith, 1995). It is true that the size of bacterial populations can be practically infinite but recombination occurs extremely rarely so that changes affecting one locus can lead to the modification of others. In the succinct words of Maynard-Smith, “the genome should be treated as an interrelated whole, and not as a set of independently changing genes”. The crux of the problem is knowing the exact level of recombination in bacterial populations, since “it is considerably more challenging to elaborate a theory for a population with little recombination than for one with no recombination, or a lot” (Maynard-Smith, 1995). In bacterial population genetics, sometimes we detect a degree of recombination that is too high for a pure phylogenetic approach, but too low for assessing a random interchange..