Author(s)

Cheramie Trahan¹, Ravi S. Pandey², Utkarsh Singh², Anushka Choudhary¹, Hyuk Cho³, Rajeev K. Azad^2,4, Madhusudan Choudhary^1*

¹Department of Biological Sciences, Sam Houston State University, Huntsville, TX, USA.
²Department of Biological Sciences, University of North Texas, Denton, TX, USA.
³Department of Computer Science, Sam Houston State University, Huntsville, TX, USA.
⁴Department of Mathematics, University of North Texas, Denton, TX, USA.

Multiple chromosomes in bacteria are designated as a larger primary chromosome (CI) and smaller accessory chromosomes (CII and CIII). Although previous studies examined multiple chromosomes in several bacterial species, the evolutionary mechanisms for the origin of CIIs still remain unclear. In this study, the four following hypotheses were tested. 1) CIIs exhibit lower sequence conservation and sequence divergence compared to their corresponding CIs across species of Proteobacteria. 2) The differential sequence divergence of CI and CII depends on pathogenic and non-pathogenic lifestyles. 3) CIIs harbor a higher level of horizontal gene transfers (HGTs) than CIs. 4) Orthologs located on CIIs experience less purifying selection than their corresponding orthologs on CIs. Results reveal a higher level of sequence conservation of CIs than the sequence conservation of CIIs. There is no significant difference in HGT estimates between CIs and CIIs. A majority of orthologous genes of CIs and CIIs experience purifying selection; however, genes on CIIs were significantly less constrained than the corresponding ones on CIs. This finding is true for both pathogenic and non-pathogenic bacteria, but the selective constraints for non-pathogenic bacteria are relatively less constrained. It was concluded that the differential selective constraint is a potent driving force for the rapid evolution of CII. Therefore, gene expression analysis at the transcriptome and proteome levels may shed light on the gene regulation mechanisms that might affect the sequence divergence between CI and CII.

Multiple Chromosomes, Proteobacteria, Horizontal Gene Transfer, Selective Constraint, Pathogenic and Non-pathogenic Lifestyles

Trahan, C. , Pandey, R. , Singh, U. , Choudhary, A. , Cho, H. , Azad, R. and Choudhary, M. (2019) Multiple Chromosomes in Bacteria: Low Level of Evolutionary Constraint Drives the Rapid Genetic Divergence of Chromosome II. Advances in Microbiology, 9, 656-677. doi: 10.4236/aim.2019.97041.