Evolution Update

Evolution Update

Can STIs be beneficial?

Irma Shaboian September 17, 2015

A recently published review delves into whether sexually transmitted microbes provide advantages for its host and suggests the role that microbe diversity has in promoting promiscuous behaviour.

Natural selection proposes organisms that are better adapted to their environment are more likely to survive and pass on offspring. Extending this theory further, it is thus likely that co-operation between organisms was important in safeguarding their survival; that is, a symbiotic relationship.

While there are many types of symbiotic relationships, the underlying premise is that organisms from different species can co-exist and interact in ways that are advantageous or detrimental for either, or both organisms. This is a topic that is one branch of evolution theory that does not garner much attention but is nonetheless informative.

Pursuing this line of thinking were researchers Smith and Mueller who published a review in Trends in Ecology & Evolution . In their article, they address the common association between sexually transmitted infections (STIs) and pathogenesis by viewing it from a symbiotic perspective. Namely, they look to investigate the sexual transmission of beneficial microbes.

Generally speaking, STIs are less deadly than other pathogens that are capable of being transmitted more easily, such as air. This is because organisms transmitted sexually rely on their host to be healthy and capable of reproducing in order for the microbe to be passed on.

In their review, the researchers point to increasing evidence of sexual transmission as a practical form of transmission for beneficial symbionts. For instance, GB virus C (GBV-C) is an RNA virus that is associated with reducing the mortality rate of HIV patients. The virus accomplishes this by diminishing the ability of the HIV virus to hijack a patient’s T-cells.

The GBV-C virus is a lymphotrophic virus that is nonpathogenic and results in B- and T-cell replication; that is, no clinical illness results when infected by GBV-C. It is transmitted sexually between humans and infects many worldwide, but is usually cleared by the immune system of the person. However, if the infection persists enough, a mother can transmit GVB-C to her infant and confer advantage over HIV proliferation.

Another way that symbionts may influence the host is by influencing the physical environment of the female’s reproductive tract, inviting the possibility that the microbes may affect the survival of certain sperm. Notwithstanding the current evidence supporting beneficial STIs, there is no available evidence that can directly support whether certain acquired STIs may make a mate more preferred or more likely to mate.

The researchers note, however, that sexual selection should favour organisms that somehow demonstrate their benefits. Accordingly, by being able to physiologically alter some aspect of an organism to signal health and/or reduce the virulence of a pathogen, the reproductive interests of the host and the symbiont are aligned and thus favour transmission of the STI.

Perhaps the benefits conferred by symbionts transmitted sexually may be why promiscuity evolved as a way of acquiring symbionts. Mating on multiple occurrences would lead to greater exposure of available microbes, increasing the microbe diversity and possibility of acquired benefits. Likely this occurred on a trial and error basis, as this behaviour would also increase the risk of pathogenic infections too.

Instead, the researchers call upon other scientists to survey the diversity of sexually transmitted microbes and understand their functions as it relates to protecting and/or influencing its host in order for the host to reproduce and propagate the microbe.

Identifying the functions of nonpathogenic microbes that are sexually transmitted can further pave way for assessing whether and how cooperation between hosts and microbes promotes host survivorship, since the microbes rely on a healthy host as a vehicle of transmission. This is particularly true as hosts commonly cease reproductive behaviour when they are infected, thus preventing a microbe from being passed on further.

The goal would be to recognize the pathways through which the benefits occur and transfer them for therapeutic causes, which is currently being attempted with GBV-C and HIV.

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