Up­dat­ing Hor­i­zon­tal Gene Trans­fer

by Me­chas and Roberto

We re­cently pre­pared a lec­ture that in­cluded a brief dis­cus­sion of bac­te­r­ial evo­lu­tion. Of course, we cov­ered hor­i­zon­tal gene trans­fer (HGT), which we con­sider a key pil­lar in the evo­lu­tion­ary process. We du­ti­fully be­gan our pre­sen­ta­tion by list­ing, per­force: trans­for­ma­tion, con­ju­ga­tion, and trans­duc­tion. In do­ing so we were fol­low­ing the path likely fol­lowed by most teach­ers of mi­cro­bi­ol­ogy. This way, the knowl­edge of HGT mech­a­nisms it­self has been dili­gently trans­ferred from one gen­er­a­tion of mi­cro­bi­ol­o­gists to the next. But this time, as we com­pleted the lec­ture, we both had the same thought: our list needed up­dat­ing!

Fig. 1. Overview of mech­a­nisms of hor­i­zon­tal gene trans­fer. In ad­di­tion to the three clas­sic mech­a­nisms of DNA up­take, trans­for­ma­tion, trans­duc­tion and con­ju­ga­tion, DNA (in­clud­ing plas­mids) can also travel among cells via so-called non-canon­i­cal mech­a­nisms, in­volv­ing mem­brane vesi­cles, tiny pilus-like struc­tures called nan­otubes and phage-like gene trans­fer agents (GTAs) (blue dot­ted box, up­per part). The bot­tom part (red dot­ted box) de­tails mech­a­nisms of in­te­gra­tion. For the full leg­end, see source.

Sci­ence is con­tin­u­ally evolv­ing, ex­pand­ing as we ex­plore novel ecosys­tems and or­gan­isms other than those well-known lab­o­ra­tory model bac­te­ria, such as E. coli and Sal­mo­nella. Among the many ex­cit­ing de­vel­op­ments of re­cent decades are the dis­cov­er­ies of ad­di­tional mech­a­nisms for trans­fer­ring DNA among bac­te­r­ial cells. We found the re­cent re­view by Arnold, Huang and Han­age, "Hor­i­zon­tal gene trans­fer and adap­tive evo­lu­tion in bac­te­ria," to be an ex­cel­lent read on HGT and its evo­lu­tion­ary im­pli­ca­tions. We re­pro­duce their first fig­ure here as it does jus­tice to the more re­cently dis­cov­ered mech­a­nisms of HGT. When we see these new path­ways of HGT, we feel cer­tain that the time is ripe to re­vise our long-held prac­tice of pre­sent­ing only "the clas­sic three."

We now know that ex­tra­cel­lu­lar vesi­cles (EVs) can de­liver DNA to re­cip­i­ent cells and thus me­di­ate HGT. While this phe­nom­e­non re­mains some­what un­der­ap­pre­ci­ated, this oc­curs in di­verse en­vi­ron­ments, from guts to sea­wa­ter. As we re­cently posted in STC, these vesi­cles are not only abun­dant in the ocean but they also pro­mote gene trans­fer in the ma­rine bac­terium Prochloro­coc­cus. In dis­cussing this fourth mech­a­nism of HGT, Soler and Fortere re­cently pro­posed to name this process "vesiduc­tion." But there are yet other novel HGT path­ways. DNA trans­fer can be me­di­ated by pilus-like "nan­otubes" and phage-like "gene trans­fer agents" or GTAs. Rest as­sured; we will be cov­er­ing those in fu­ture STC posts. In the mean­time, if any of our read­ers still teach only the clas­sic three, we hope that like us, you'll up­date your list!

 

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