Genomic and molecular evolutionary dynamics of transcriptional response regulator genes in bacterial species of the Harveyi clade of Vibrio

•The genetic repertory of transcriptional response regulators in Harveyi clade species is conformed by 16 core and 38 accessory genes.•The TRR genes by species varies from 18 to 40 and could be related to genome size and phylogenetic position.•Natural selection could shape an ancestral state of conserved genes by purifying processes and the loss of non-conserved genes by diversifying processes.•Random genetic drift could be driving gene gain of non-conserved genes. Transcriptional response regulators (TRR) are the most abundant signal transducers in prokaryotic systems that mediate intracellular changes in response to environmental signals. They are involved in a wide range of biological processes that allow bacteria to persist in particular habitats. There is strong evidence that the bacterial habitat and their lifestyle influence the size of their TRR genetic repertoire. Therefore, it would be expected that the evolution of bacterial genomes could be linked to natural selection processes. To test this hypothesis, we explored the evolutionary dynamics of TRR genes of the widely studied Harveyi clade of the genus Vibrio at the molecular and genomic levels. Our results suggest that the TRR genetic repertoire of the species belonging to the Harveyi clade is a product of genomic reduction and expansion. The gene loss and gains that drive their genomic reduction and expansion could be attributed to natural selection and random genetic drift. It seems that natural selection acts to maintain the ancestral state of core TRR genes (shared by all species) by purifying processes and could be driving the loss of some accessory (found in certain species) genes through the diversification of sequences. The neutrality observed in gene gain could be attributed to spontaneous events as horizontal gene transfer driven by stochastic events as occurs in random genetic drift.