Reassortment

Reassortment is responsible for some of the largest genetic changes in the history of the influenza virus. In the 1957 "Asian flu" and 1968 "Hong Kong flu" pandemics, influenza virus strains were caused by a reassortment between an avian virus and a human virus. In addition, the H1N1 virus responsible for the 2009 swine flu pandemic has an unusual mix of genetic sequences from swine, avian and human influenza.

Reassortment is defined as the exchange of intact genes within the entire segment that takes place during coinfection. Reassortment can lead to drastic changes within a short period of time. The phenomenon has been observed in Bunyaviridae, reoviruses, arenaviruses and orthomyxoviruses. For example, genetic replacement of influenza A viruses can occur when genome segments of different strains rearrange in coinfected cells. (Taylor KY et al. 2023). For example, influenza A virus (IAV) is an RNA virus with a segmented genome, consisting of eight different RNA segments. These characteristics of the virus enable rapid evolution of IAV under selective pressure due to mutations caused by defective replication and the exchange of gene segments within a coinfected cell, known as reassortment.

When a single host (a human, chicken or other animal) is infected with two different strains of influenza virus, it is possible that new composite virus particles will emerge from segments whose origins are mixed, with some coming from one strain and some from another. The new reassortant strain will have characteristics of both parental lines. Both mutation and reassortment lead to genetic diversity, but limitations and influence their impact on viral evolution. The phenomenon of segmental mismatch involves both RNA- and protein-based incompatibilities between co-infecting viruses and leads to the production of progeny viruses with fitness defects (White MC et al.2018).

1. Chen KY et al. (2023) High-throughput droplet-based analysis of influenza A virus genetic reassortment by single-virus RNA sequencing. Proc Natl Acad Sci U S A. 120(6):e2211098120.
2. Gong X et al. (2021) Reassortment network of influenza A virus. Front Microbiol 12:793500.
3. Taylor KY et al. (2023) Influenza A virus reassortment is strain dependent. PLoS Pathog 19:e1011155.
4. White MC et al.(2018) Implications of segment mismatch for influenza A virus evolution. J Gen Virol 99:3-16.