Yes, bacteria have sex too!
Escherichia coli is a single-celled bacterium that will serve as an example. The entire organism is one cell.
In the case of E. coli, individuals can be sexually male or female, as it is common to anthropomorphize. In order for a male and female pair to mate, they must be in physical contact, and when they do so, the male transfers DNA into the female cell. If the DNA is retained within the female cell, either whole or in part, instead of being lost by degradation, then the female’s genetic makeup has changed in one fell swoop.
The transfer of genetic information (DNA) from one pre-existing cell to another is called horizontal gene transfer. The changed cell can then go on to divide or reproduce asexually, transferring its genetic information to its offspring (vertical gene transfer). Since generation after generation acquires the genetic changes that might have occurred at each prior step, including the initial sex change event described in the previous paragraph, vertical gene transfer is more simply called heredity.
In E. coli, genetically male cells can act as donors of genetic information, and genetically female cells can act as recipients. The scientific term for a female E. coli cell is F- because it lacks, or is minus, the fertility genes, the genetic information that is required for a cell to act as a male donor. There are different types of male E. coli cells, depending on how the fertility genes are present in the cell (if readers really want to know, ask). But in the simplest case, the male contains a circular piece of DNA, the Fertility plasmid, in addition to its sole chromosome, and is considered F+.
Female E. coli cells cannot transfer genetic information to other female cells by this mechanism.
When a male F+ cell mates with a female cell, the male transfers the Fertility plasmid to the female cell. You guessed it. This is a form of sex change, in which the female is converted into a male. Only now can that former female express those fertility genes so that (s)he can mate with another female.
Usually, male E. coli cells can only mate with female cells. Part of a male E. coli cell’s mating apparatus is a cell surface appendage called an F pilus that allows it to contact the female cell. If a pair of cells each have an F pilus, they obstruct the ability of either male cell to contact another. But you can ‘trick” a male E. coli cell to mate with another male E. coli cell by shaking off its F pilus by extreme agitation (this is usually accomplished with a vortex machine). In this case, the cell that has lost it’s pilus is still genetically male, but it can accept genetic information from another male.
Another word for microbial sex is conjugation. All of this is perfectly acceptable in the microbial world. There are even examples of bacteria that can conjugate with yeast and other bacteria that can conjugate with plants.
Genetic change, however it happens, is a prerequisite of evolution.
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