The study of sex is a sexy topic in biology. Sexual intercourse in animals has been studied meticulously, especially in humans (Arnow et al., 2002; Drago, 1994; Thornhill and Gangestad, 1994), and the origin of sex is a hotly debated topic in behavioural ecology (Hurst and Peck, 1996). In biology, sex is generally defined as the act of transferring sperm from one individual to another such that it may fertilize the eggs of the other individual. The majority of animals reproduce at least occasionally via sexual reproduction (Hurst and Peck, 1996). That being said, it should be noted that in the domain of life, not all reproductive behaviour is sexual in nature. For example, viruses replicate by parasitizing living cells, manipulating their enzymatic machinery to synthesize copies of their genome and surrounding molecular armour. This process seems like it’s as far as it could be from the concept of animalistic sex, but in another sense of the word, viruses have sex frequently and the consequences can be dire.
Sex can be framed on a molecular level just like it can be framed on an organismal level. To wit, sex can be defined as the mixture of genetic material from two individuals leading to the formation of a new individual, also known as recombination. By this definition, viruses can have sex when one host cell is co-infected by two or more viruses at once (Walling et al., 1995). Co-infection leads to recombination when pieces of the genome of two or more virions (individual viruses) become packaged into a single new virion (Walling et al., 1995). This new individual’s genome will be a composite of the genomes that combined to form it instead of simply being a copy of its parent’s genetic code. This can even occur when two virions that belong to genetically distinct strains co-infect a cell, leading to the formation of a drastically different daughter virion that combines the traits of its parents. While the idea of viruses having sex may seem innocuous and quaint, its effects on human health can be devastating.
In 2002, an outbreak of Severe Acute Respiratory Syndrome Coronavirus (SARS-CoV) occurred, killing over 700 people and causing international panic (Marshall, 2004). This led virologists (those that study viruses) to ask where this new human pathogen came from and why it arose when it did (Marshall, 2004). Once the zoonotic source of SARS-CoV had been traced to the live animal markets of the Guangdong region of China, researchers made an amazing discovery about its genome: it was the result of recombination between as many as 6 strains of innocuous coronaviruses (Zhang, Yap, and Danchin, 2005)! In other words, the genome of this major human pathogen is a conglomerate of genetic material from several strains of relatively non-harmful viruses. Through viral recombination, new human pathogens can be produced from non-harmful viral strains.
The above is not the only example of increased virulence due to viral sex. On the contrary, genetic recombination and reassortment (another kind of genetic mixing) have been implicated in some of the most harmful human epidemics to date. Much of the variability in the H1N1 Influenza strain has been attributed to genetic mixing of different strains, leading to unanticipated shifts in the strains traits (Young and Palese, 1979). Additionally, it is thought that the contributions of recombination to the genetic variability of Human Immunodeficiency Virus (HIV) has led to its hallmark ability to develop rapid drug resistance rapidly in humans (Burke, 1997). So, the next time you here about viruses having sex, don’t scoff, because the next viral epidemic may just be as sexy as the last.
References:
Arnow, B., Desmond, J., Banner, L., Glover, G., Solomon, A., Polan, M., Lue, T. and Atlas, S., 2002. Brain activation and sexual arousal in healthy, heterosexual males. Brain, 125(5), pp.1014-1023.
Beards, G., 2008. Phage. Wikimedia Commons. [Accessed February 29th, 2018].
Burke, D., 1997. Recombination in HIV: An Important Viral Evolutionary Strategy. Emerging Infectious Diseases, 3(3), pp.253-259.
Drago, F., 1984. Prolactin and sexual behavior: A review. Neuroscience & Biobehavioral Reviews, 8(4), pp.433-439.
Hurst, L. and Peck, J., 1996. Recent advances in understanding of the evolution and maintenance of sex. Trends in Ecology & Evolution, 11(2), pp.46-52.
Marshall, E., 2004. Caution Urged on SARS Vaccines. Science, 303(5660), pp.944-946.
Ripp, S., 2009. Bacteriophage-Based Pathogen Detection. Whole Cell Sensing System, 2(1), pp.65-83.
Thornhill, R. and Gangestad, S., 1994. Human Fluctuating Asymmetry and Sexual Behavior. Psychological Science, 5(5), pp.297-302.
Young, J. and Palese, P., 1979. Evolution of human influenza A viruses in nature: recombination contributes to genetic variation of H1N1 strains. Proceedings of the National Academy of Sciences, 76(12), pp.6547-6551.
Zhang, X., Yap, Y. and Danchin, A., 2004. Testing the hypothesis of a recombinant origin of the SARS-associated coronavirus. Archives of Virology, 150(1), pp.1-20.