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Recurrent recombination and insertion–deletion events shape the genome-wide evolutionary history of the Coronaviridae family

This study reveals that frequent recombination and insertion–deletion events across the Coronaviridae family have significantly shaped its genomic organization, diversification, and phylogeny, while simultaneously complicating efforts to forecast the emergence of novel pathogenic variants.

Original authors: Luis Daniel González-Vázquez, David Ferreiro, Filipe Pereira, Miguel Arenas

Published 2026-07-21

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CC BY 4.0

Original authors: Luis Daniel González-Vázquez, David Ferreiro, Filipe Pereira, Miguel Arenas

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ⚕️ This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer

Imagine the world of viruses as a massive, chaotic library where the books are constantly being rewritten. In this library, the "Coronaviridae" family is a special section of books made of a wobbly, flexible material called RNA. Unlike our own DNA, which is like a sturdy hardcover that rarely changes, these viral books are prone to typos, missing pages, and wild remixes. Scientists have long known that these viruses evolve quickly, but they've mostly focused on simple "typos"—single letters changing in the code. However, there are two other, more dramatic ways these books get edited: recombination and indels. Think of recombination as a librarian taking two different books, cutting them in half, and gluing the front of one to the back of the other to create a brand-new story. Indels (short for insertions and deletions) are like ripping out whole chapters or pasting in new, random paragraphs. Understanding these messy editing habits is crucial because they are the reason these viruses can jump from animals to humans, dodge our immune systems, and keep changing the rules of the game.

Now, a team of researchers decided to take a deep dive into this chaotic library to see just how much these "gluing" and "ripping" events have shaped the entire Coronaviridae family. They didn't just look at one virus; they examined the whole family, from the ones that infect bats and birds to the famous ones that affect humans like SARS-CoV-2. Their investigation revealed that the history of these viruses isn't just a straight line of simple changes; it's a tangled web of constant remixing and structural overhaul.

The study found that recombination is a superstar in this family. It happens all the time, not just in one corner of the library, but across the entire genome. The researchers discovered that these viruses frequently swap large chunks of their genetic code, sometimes even mixing genes between different genera (like swapping a chapter from a bat virus into a human virus). This isn't just a rare accident; it's a major driver of how these viruses diversify. The data showed that these swapping events are so frequent that they create "mosaics"—viruses that are part one thing and part another. While the researchers noted that some of these swaps happen between very distant relatives (which is hard to prove), the evidence suggests that these viruses are constantly borrowing and trading genetic material to survive and adapt.

But the real surprise was the sheer volume of indels. The team found that these viruses are constantly losing and gaining entire sections of their genetic code. It's as if the viruses are playing a game of "cut and paste" where they frequently delete whole chapters (genes) that they don't need anymore, or occasionally grab new ones. The study suggests that, over time, there is a strong tendency for these viral genomes to get smaller and more compact by deleting non-essential parts. For example, they found that different groups of coronaviruses have lost specific genes independently, like a family of birds losing a gene that a family of mammals kept. These deletions aren't just random noise; they act like unique fingerprints that help scientists trace the family tree. In fact, the researchers found that if you only looked at these "missing pages" (the deletions), you could still figure out how the different virus groups are related, proving that these structural changes are just as important as the simple letter-by-letter changes.

The researchers also looked at the famous SARS-CoV-2 to see how it fit into this bigger picture. They found that while SARS-CoV-2 does experience these changes, the entire Coronaviridae family experiences them at a much higher rate. This means that predicting the future of these viruses is incredibly difficult. Because they are constantly rewriting their own stories through recombination and deleting or adding massive chunks of code, trying to forecast exactly what they will look like next is like trying to predict the plot of a book that changes its own chapters every time you blink. The study concludes that to truly understand how these viruses evolve and spread, we can't just watch for small typos; we have to watch the whole library for the big, structural remixes that shape their destiny.

Problem
The evolutionary history of the Coronaviridae family is traditionally inferred based on nucleotide substitution rates, often overlooking the contributions of genetic recombination and insertion–deletion (indel) events. While recombination is known to occur in specific lineages (e.g., SARS-CoV, MERS-CoV, SARS-CoV-2) and indels are recognized for their role in viral pathogenesis (such as the ORF8 deletion in SARS-CoV), their systematic impact across the entire family remains incompletely characterized. Existing taxonomic classifications and phylogenetic reconstructions frequently fail to account for these mechanisms, which can theoretically distort evolutionary relationships and obscure the true history of viral diversification. Furthermore, the extent to which these processes drive the gain and loss of genomic fragments, thereby shaping genomic organization and molecular epidemiology, is not fully understood.

Methodology
The study analyzed a curated dataset of 65 representative whole-genome sequences spanning all four recognized genera of Coronaviridae (Alphacoronavirus, Betacoronavirus, Gammacoronavirus, and Deltacoronavirus), including human and animal-infecting viruses. The methodology employed a multi-faceted approach:

Genome Alignment and Structural Analysis: A stepwise, gene-wise alignment strategy was used to accommodate high sequence divergence, translocations, and variable accessory gene content. Genomes were segmented into functional blocks (ORF1ab, S, E, M, N, and accessory ORFs) and aligned using MAFFT. Overlapping regions were treated conservatively to preserve functional context.
Phylogenetic Inference: Maximum Likelihood (ML) trees were reconstructed using RAxML-NG with the best-fit nucleotide substitution model. Additionally, phylogenetic networks were generated using the NeighborNet method in SplitsTree to visualize reticulate evolutionary signals.
Recombination Detection: A comprehensive suite of recombination detection tests (RDP, GENECONV, BootScan, MaxChi, SisScan) implemented in RDP5 was applied to identify breakpoints. Events were considered valid if detected by four or more tests with a p-value < 0.05. Population recombination rates ( $\rho$ ) were estimated using the LDhat framework.
Indel Rate Estimation: Insertion and deletion rates were estimated using an Approximate Bayesian Computation (ABC) approach. The study evaluated two indel length distribution models (Binomial and truncated Zipf) using the simulator INDELible. Model selection and parameter estimation were validated using independent simulated test data to ensure accuracy before applying the method to real datasets (whole-genome and S-gene sequences for both the family and SARS-CoV-2).
Ancestral Reconstruction: Ancestral sequence reconstruction was performed to infer the history of indel events, distinguishing between inherited ancestral indels and novel events arising on terminal branches.

Key Contributions and Results

Prevalence of Recombination: Recombination events were found to be frequent and heterogeneously distributed across the Coronaviridae family. While most events occurred within ORF1ab and the Spike (S) gene, recombination was also detected in structural and accessory genes. The analysis revealed that recombination occurs both within and across genera, including complex events involving large genomic fragments (e.g., exchanges between shrew Alphacoronaviruses and Betacoronaviruses). The estimated mean population recombination rate was $3.67 \times 10^{-2}$ per site, with high signals concentrated in ORF1ab and S.
Indel Dynamics: Genome-wide indel rates were found to be high, with a tendency for deletions to occur more frequently than insertions, supporting a general trend toward genome size reduction. The Binomial model provided the best fit for indel length distributions across all datasets.
Phylogenetic Congruence: Phylogenetic networks based on indel data alone were largely congruent with those derived from nucleotide sequences, successfully resolving the four major genera. This suggests that indels contain sufficient phylogenetic information to reconstruct deep evolutionary divergences.
Gene Loss and Gain: The study identified significant variability in accessory gene content, driven by both recombination and indels. Key findings include:
- The HE (hemagglutinin esterase) gene is present in Embecoviruses but absent in other genera, suggesting secondary acquisition (likely via recombination with influenza C virus) rather than ancestral presence.
- The ORF8 gene, present in Gammacoronaviruses and Sarbecoviruses, has undergone multiple independent losses and splits (e.g., the 29-nt deletion in SARS-CoV and the 382-nt deletion in SARS-CoV-2).
- Deltacoronaviruses exhibit a more compact genome with fewer accessory genes compared to Alpha- and Betacoronaviruses.
Convergent Evolution: Similar indel patterns were observed in the N-terminal domain (NTD) of the S gene across different genera, suggesting convergent evolution driven by selective pressures such as immune evasion.

Significance
The paper asserts that recombination and indel events are fundamental drivers of Coronaviridae evolution, shaping genomic organization, diversification, and phylogeny. These processes are not limited to specific lineages but are widespread across the family. The findings provide a comparative framework for understanding how different forms of genomic change contribute to coronavirus diversity and offer a more comprehensive view of the evolutionary history of SARS-CoV-2 and related viruses.

Crucially, the authors claim that the complexity of modeling these evolutionary processes—specifically the interplay of recombination, gene loss, and gain—makes forecasting the future evolution of coronaviruses inherently challenging. Consequently, evolutionary surveillance and predictive strategies must move beyond simple substitution models to incorporate recombination and indel dynamics to accurately interpret viral diversification and anticipate the emergence of novel pathogenic variants.

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