From the earliest days of life on earth, natural selection has favored organisms possessing traits that made them more likely to survive and produce offspring than their peers in a given environment. Over the long term, these selection pressures have been responsible for the diversification of life on earth.
In primordial life’s earliest eras, organisms, which were relatively simple compared to today’s complex life forms, may have exported their genetic information out into the environment as an early form of asexual reproduction. As life became more sophisticated and diversified, the strategy of asexual reproduction maintained some advantages in spite of its simplicity. Asexual reproduction involves less time and does not require that energy be expended in finding a mate. The risk of disease transmission is minimized through self-reproduction. Its minimal requirements mean that higher reproduction rates can be achieved by asexual reproduction.
Yet, today, sexual reproduction has come to dominate the Animal Kingdom. This is because, despite these downsides, sexual reproduction confers fitness benefits of its own. Offspring produced through sexual reproduction have the advantage of receiving copies of genetic information from two sources. The offspring can acquire a genetic resistance to a certain disease or inherit a beneficial phenotypic trait even though it is only carried by one of the parents. The genetic diversity created by the recombination of genetic information from two parents raises the odds of a beneficial trait being created. By contrast, asexual reproduction essentially creates copies of an organism. Change can occur only when there is a mistake in one of these replicas. Sexual reproduction also provides some protection from recessive genetic diseases. One parent may carry a harmful allele of a protein, but the other parent encodes for the normal version of the protein. As a result, the fitness of their offspring will most likely not be affected if the harmful protein is not dominant.
Bacterial conjugation is the ‘next best thing’ in comparison to full sexual reproduction.”
What is true of the Animal Kingdom is not necessarily true of the Bacterial Kingdom. Most species of bacteria self-reproduce. However, more than 60 species of bacteria have been observed to been able to transform exogenous DNA (that is, DNA that comes from outside) and integrate it into their genome (Lorenz and Wackernagel 1994, Johnsborg et al. 2007). This process, known as bacterial conjugation, consists of the transmission of bacterial genetic material in the form of a plasmid between two bacterial cells. This can happen either through direct contact between the two cells or through the formation of a tube-like connection linking the two bacteria through which genetic material flows. The existence of this phenomenon was discovered in 1946 by Joshua Lederberg and Edward Tatum. In order to take up DNA, bacteria have to enter a natural competence state usually induced by stress that allows them to transport negatively charged DNA across its membrane.
Both sex and DNA transformation have fitness costs and benefits. On one hand, there is a physiological cost of maintaining a system that takes up DNA. An organism only using sexual or asexual reproduction does not have to carry genes that encode for components that take up exogenous DNA and they do not have to allocate resources to build this cellular machinery. Also, exogenous DNA that has many deleterious mutations poses a threat to the fitness of competent bacteria. For example, healthy bacteria may take up a piece of DNA that encodes mutated proteins. There is a chance that these proteins could interfere with cellular processes of the bacteria. (Redfield et al. 1997). However, there are most likely mechanisms to control the degradation rate of DNA in the extracellular environment like nuclease secretion if a population of bacteria detects harmful DNA.
However, DNA transformation may improve the fitness of the organism if the genetic effects are beneficial. In fact, the evolution of bacterial “sex” is only a further confirmation of the advantageous nature of sexual reproduction. Bacterial conjugation is the “next best thing” in comparison to full sexual reproduction, and its prevalence in multiple species is an indicator of the fitness benefits it confers.
- Lorenz MG, W Wackernagel. “Bacterial gene transfer by genetic information in the environment.” Microbiological Reviews 58.3 (1994): 563-602. Web.
- Johnsborg, Ola, Vegard Eldholm, and Leiv Sigve Ha˚varstein “Natural genetic transformation: prevalence, mechanisms and function.” Research in Microbiology 158 (2007): 767-778. Web.
Redfield R.J, M. R. Schrag, and A.M. Dean “The Evolution of Bacterial Transformation: Sex with Poor Relations.” Genetics 146.1 (1997): 27-38. Web.
Joshua Elkington is a Brevia staff writer. He can be reached at firstname.lastname@example.org.
Photo by NIAID, “Mycobacterium tuberculosis Bacteria, the Cause of TB” via Flickr, Creative Commons Attribution.
Photo by Mike Jones [CC-BY-SA-2.5], “Bacterial Conjugation” via Wikimedia Commons, Creative Commons Attribution.