Mutation is the Main Driving Force in the Diversification of the Vibrio splendidus Clade

Abstract

The Vibrio splendidus clade is the biggest in Vibrionales composed of 11 described species (25). Diversification of these species may have occurred 260 million years ago. The main driving forces of speciation in this clade have never been studied. Population biological parameters (population base recombination rate (ρ), population base mutation rate (θ), and index of association (Ia)) were determined among 16 strains of 9 defined species in the Splendidus cluster. A comparison of individual gene phylogeny indicated significant incongruence in tree topology, which suggests the occurrence of recombination between species. Homologous recombination between species was detected at four loci. However, the mutation rate θ was higher than the recombination rate ρ, suggesting that mutation is the main driving force in the diversification of V. splendidus-related species.Key words: diversification, population biology, vibrios, Vibrio splendidus, multilocus sequence analysis Vibrio splendidus was originally described by Beijerinck in 1900 as a luminous marine bacterium (1,2,22). The taxonomic status of this species was confirmed through DNA-DNA hybridization by Reichelt et al. (22). V. splendidusrelated strains isolated from marine environments remained in focus due to the psychrophilic nature of these vibrios in the global warming era (27,34,35). A role for V. splendidusrelated strains in the mortality of mollusks and fish was also reported (6, 9, 13). V. splendidus and related species are widespread in marine environments (14,30,32,34,35). Recently, two genetically distinctive populations of V. splendidus-related strains from North Atlantic coastal water have been distinguished. Bacteria of these two populations differed in range of temperature for growth and had less than 1% of the 16S rRNA gene sequence heterogeneity (30,32).Because V. splendidus-related species share niches in the environment, including the gut and tissue of bivalves, it would be reasonable to assume that horizontal gene transfer and recombination are important forces leading to diversification in this group. The multilocus sequence analysis (MLSA) is a reliable tool for both inferring the taxonomic backbone of species groups and estimating the relative contributions of mutation and recombination (31,33,36). Our recent MLSA clearly separated the V. splendidus clade from all other recognized species of vibrios (25). However, V. splendidus-related species are genetically and phenotypically similar (29). Overall, the species have more than 30% reciprocal DNA-DNA similarity, 90.6-96.5% nucleotide sequence similarity of MLSA concatenated genes and 96.5-99.8% average amino acid identity at gene loci, but a rather broad GC content. The high genomic similarity might suggest that these species readily exchange DNA at conserved loci. To test this hypothesis, we analyzed the relative contribution of mutation and recombination to the diversification using an extended set of strains belonging to the Splendidus group by means of MLSA. Materials and Methods Bacteria...

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