Question

the bacteria in figure 1.8 reproduce asexually, yet they are evolving. What is their source of genetic variation?

Answer 1

Bacteria reproduce asexually by budding. This mode of reproduction should not introduce genetic variation as the DNA is only inherited from one parent. However, random mutations may occur in the organism due to exposure to certain chemicals etc, which can be inherited by subsequent generations.

Bacteria also undergo horizontal gene transfer mechanism. These HGT include conjugation, transduction and transformation. Genes may be added into the bacteria when bacteria acquire them from related or unrelated species. Conjugation is transfer of DNA, usually plasmids from one organism to other via the F plasmid. The F plasmid is a conjugation plasmid that codes for proteins forming the conjugation F pilus. The bacteria that have the F plasmid are F+ cells while those lacking F plasmid are F- cells. The F+ cell will contact the F- cell at the site of F pilus. A single copy of F plasmid is then transferred into F- cells, where a complimentary copy is synthesized. Thus, all F- cells will become F+ cells.

In transformation, the bacteria can take up DNA from the extracellular environment. This DNA is present from lysed or dead cells. The cells bind to the DNA at their cell envelopes, transport it across the cell membrane into the cytoplasm. The dsDNA is converted to a single strand to protect against nucleases. This ssDNA will integrate in host chromosome and is then converted to dsDNA. This foreign DNA can then be transmitted to subsequent generations.

Transduction is phage mediated transfer of foreign DNA or genes into bacteria. It can be generalized or specialized. In generalized transduction, any phage DNA can be transferred when phages containing bacterial DNA infect and transfer the DNA to that host. The phages contain bacterial chromosomal DNA due to accidental packing of chromosomal DNA in phage head in the previous host cell. In specialized transduction, there is excision of lysogenic phage DNA imprecisely. These lysogenic phages now carry bacterial DNA along with phage DNA and will transfer the bacterial DNA when it infects another host.

The genetic variation is a result of transfer of foreign DNA in to the bacteria cells and will result in evolution due to inheritance in subsequent generation. HGT is responsible for generation of antibiotic resistant strains of bacteria. All genes that are transferred will be retained if they provide selective advantage to the bacteria in subsequent generation by natural selection.