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Some bacteria can undergo gene transfer by conjugation. Conjugation is a form of horizontal gene transfer, meaning from one (unrelated) bacterium to another (in contrast to vertical gene transfer, which is from predecessor bacterial cell to successor (daughter) cells). Conjugation is initiated when one bacterium has an F-plasmid (Fertility plasmid) and forms an F-pilus, connecting the F+ bacterium with an F- bacterium, initiating transfer of the F-plasmid.

Why is it that during evolution not all bacteria have acquired this F-plasmid?

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Conjugation occurs between cells of the same species too. For this to occur cell have to be close to each other. Now, if you have an isolated population of bacteria that never gets in contact with an F+ bacteria then this population would stay F-. Also not all conjugation events are successful, mechanical perturbations can disrupt the pilus through which genetic material is transferred. This old article can give you good details: THE ORIGIN OF BACTERIAL SPECIES - GENETIC RECOMBINATION AND FACTORS LIMITING IT BETWEEN BACTERIAL POPULATIONS ARNOLD by W.RAVIN

F+ shares, however, an important property of bacteriophage: after infection it behaves as though it were an addition to the bacterial genome. As an addition, it can furthermore exist in one of two forms: in one form, it is an unintegrated part of the bacterial genome, and may be replicated either more or less rapidly than the "chromosomal" or linked genes (being infectious or lost through dilution, respectively); in its other form, it is integrated in the sense that its replication is coordinated with the replication of the host's genes, as though attached to them. In this latter form, found in Hfr bacteria, the F+ factor is incapable of infecting other bacteria (while permitting conjugation with them) and also prevents superinfection of its host bacterium by a nonintegrated F+ factor.

Taken from the linked paper. So you can see that are situations where conjugation occurs yet the F- won't become F+.

Also :

What is particularly interesting about the F+ factor is the fact that there seem to be a number of different kinds. An F+ factor making a bacterium fertile with some F- cultures does not necessarily make it capable of conjugating with others (49).

So not every F+ cell can conjugate with another F- cell.

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  • $\begingroup$ Thank you for your answer. Also I was wondering if there are mechanisms that limit the amount of times conjugation and F-plasmid transfer can happen. E.g. "a bacterium can only transfer his plasmid once during his lifetime". $\endgroup$ – Wolgast Mar 29 '15 at 9:02
  • $\begingroup$ To be honest I've never thought about that question, might worth asking it. $\endgroup$ – Nandor Poka Mar 29 '15 at 9:20
  • $\begingroup$ Alright, will do. Do I just create a new question for this? (I am new to this site :-)) $\endgroup$ – Wolgast Mar 29 '15 at 9:22
  • $\begingroup$ Yes, that is the right way. $\endgroup$ – Nandor Poka Mar 29 '15 at 9:24

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