The research, led by Penghui Song, Lijuan Ma, and Dong Yan and published in PLOS Genetics, set out to answer a deceptively simple question: what does Mettl16 actually do in Drosophila melanogaster? In mammals, METTL16 is essential for embryonic development, and its loss is lethal, largely because it modifies a specific structural RNA involved in splicing and helps regulate the synthesis of S-adenosylmethionine, the universal methyl donor of the cell. Because flies lack some of the mammalian regulatory features, the expectation was that Mettl16 might behave similarly, or perhaps be dispensable altogether. The team generated flies carrying loss-of-function mutations in the Mettl16 gene and watched what happened across the entire life cycle. The result was unexpected: unlike in mammals, Mettl16 mutant flies survived to adulthood. They were alive, but far from well, displaying a constellation of developmental and behavioral abnormalities that hinted at a deep, if selective, importance of this enzyme.

The most dramatic phenotype was sterility. Both male and female mutants were completely infertile, and microscopic examination of their gonads revealed severe defects in gametogenesis, the intricate choreography by which germ cells divide, specialize, and mature into sperm and eggs. This was a crucial clue. Fertility is one of the most sensitive readouts of RNA metabolism, because germ cells undergo some of the most dramatic transcriptional and developmental transitions in any tissue. The fact that Mettl16 loss brought the entire reproductive program to a grinding halt suggested that the enzyme supports specific, non-redundant steps in the formation of gametes, rather than serving as a general housekeeping factor that cells could easily compensate for.

To pinpoint where things went wrong, the researchers traced germ cell development stage by stage. Remarkably, Mettl16 mutant germ cells were able to navigate the early milestones without obvious catastrophe: they completed the mitotic divisions that expand the germline population and progressed through meiosis, the specialized reductional division that generates haploid cells. The breakdown came later, during spermiogenesis, the transformation of round spermatids into the sleek, elongated, motile sperm that define male fertility. In the mutants, spermatid elongation faltered and individualization, the process by which each spermatid is sculpted, compacted, and encased in its own membrane sheath, failed to proceed properly. This stage-specific failure is telling, because spermatid elongation and individualization demand massive, precisely timed remodeling of the cytoskeleton, of chromatin, and of RNA populations inherited from earlier stages. Mettl16 therefore appears to be required for the late maturation program of sperm rather than for the fundamental mechanics of cell division.

Where does the enzyme live inside the cell, and what does it do when it gets there? The team tagged Mettl16 with green fluorescent protein and found that the fusion protein localized predominantly to the nucleus, consistent with a role in nuclear RNA processing rather than in the cytoplasmic control of translation. To test whether Mettl16 loss broadly crippled protein synthesis, the researchers examined wing disc epithelial cells, a classic Drosophila model tissue, and found that global protein production was not impaired in the absence of the enzyme. This observation carries real weight. If Mettl16 were a major regulator of mRNA translation, its removal should have left a visible signature on the protein output of the cell. Instead, the data pointed toward a narrower, more targeted function, in which only a specific set of RNA substrates depends on Mettl16 activity.

That selectivity was confirmed at the level of the transcriptome-wide methylome. Using MeRIP-Seq, a technique that immunoprecipitates methylated RNA fragments and maps them back to the genome, the researchers compared the m6A landscape of normal flies with that of Mettl16 mutants. The comparison produced a striking contrast. When Mettl3 is lost, the effect on m6A is broad, sweeping away the modification from a large fraction of transcripts across the transcriptome. Mettl16 loss, by comparison, altered m6A on only a small subset of RNAs. This result reframes the two enzymes as complementary writers with distinct substrate preferences: Mettl3 acts as the bulk methyltransferase decorating thousands of messages, while Mettl16 behaves as a specialist, placing marks on a curated list of targets whose identity appears central to the phenotypes observed in the mutant flies.

Among those targets, one stood out with particular clarity: U6 snRNA, the small nuclear RNA that forms the catalytic heart of the spliceosome, the molecular machine that removes intervening sequences from pre-messenger RNAs. The team demonstrated that Mettl16 physically interacts with U6 snRNA and is required for its m6A modification. This finding connects the fly enzyme to one of the most conserved functions described for METTL16 in other organisms, the methylation of U6, and it immediately suggested a mechanism by which the enzyme could exert widespread downstream effects. Splicing is the process that stitches together the coding segments of genes, and alternative splicing allows a single gene to produce multiple protein variants. If Mettl16 fails to methylate U6, the spliceosome’s precision could be subtly compromised, reshuffling the splicing patterns of many genes even though only a handful of transcripts lose their m6A marks directly.

That prediction held up. The researchers found that Mettl16 mutants display widespread alterations in alternative splicing, meaning that the way numerous pre-messenger RNAs are assembled into mature messages changes throughout the animal. This splicing dysregulation provides a plausible mechanistic bridge between the enzyme’s narrow biochemical footprint and its pleiotropic developmental consequences. Genes required for spermatid elongation, for oogenesis, and for normal behavior could each be mis-spliced in the mutant, producing protein isoforms that are truncated, unstable, or functionally altered. The picture that emerges is one of indirect amplification: a single, targeted RNA modification on a core splicing component ripples outward into broad changes in gene expression, which in turn manifest as the sterile, behaviorally abnormal flies observed in the study.

The evolutionary implications of the work are equally compelling. Comparing flies with mammals reveals a remarkable divergence in biological logic. In mammals, METTL16 is indispensable, its loss lethal during embryogenesis, whereas in flies the enzyme is dispensable for viability but essential for reproduction and normal development. This kind of comparative data helps researchers understand how the epitranscriptome has been rewired across hundreds of millions of years of evolution, with the same chemical modification and the same family of writer enzymes being deployed in different tissues and at different developmental checkpoints depending on the organism. It also cautions against assuming that findings from one model system translate directly to another, a lesson with practical consequences for biomedical research that relies on m6A biology as a therapeutic target.

For the growing community of scientists studying RNA modifications, the Drosophila study adds an important piece to the puzzle of how m6A writers divide their labor. Mettl3 emerges as the generalist, methylating a broad swath of the transcriptome, while Mettl16 operates as a specialist whose most clearly defined substrate is the U6 snRNA at the heart of the splicing machinery. The pleiotropic phenotypes of Mettl16 mutants, from failed spermatid individualization to behavioral defects, most likely stem from the combination of direct m6A deposition on specific target transcripts and the indirect consequences of U6-dependent splicing regulation. As epitranscriptomics moves from cataloging modifications toward understanding their causal roles in development and disease, this fly work demonstrates that even a small subset of methylated RNAs, when left unmodified, can be enough to derail some of the most elaborate programs an animal ever runs, including the construction of the next generation itself.

Subject of Research: The role of the m6A methyltransferase Mettl16 in Drosophila development, fertility, and RNA splicing regulation

Article Title: Role of the m 6 A methyltransferase Mettl16 in Drosophila development

Article References: Song, P., Ma, L., & Yan, D. (2026). Role of the m6A methyltransferase Mettl16 in Drosophila development. PLOS Genetics, 22(9), e1012317. https://doi.org/10.1371/journal.pgen.1012317

Image Credits: AI Generated

DOI: 10.1371/journal.pgen.1012317

Keywords: m6A, Mettl16, Drosophila, epitranscriptomics, RNA methylation, U6 snRNA, alternative splicing, gametogenesis, spermatid individualization, PLOS Genetics, methyltransferase, developmental biology

The m6A mark is deposited on RNA by a conserved writer complex. In plants, this complex comprises the methyltransferases MTA and MTB together with FIP37, VIRILIZER-like proteins, and HAKAI, counterparts of the METTL3-METTL14-WTAP machinery familiar from animal cells. The modification preferentially lands on adenosines within the DRACH sequence motif, clustering near stop codons and in 3-prime untranslated regions. Demethylase enzymes known as erasers, including ALKBH9B and ALKBH10B in plants, can strip the mark away, while YTH-domain reader proteins bind methylated transcripts and determine their fate, whether stabilization, degradation, translation, or sequestration. Because the mark is reversible and acts post-transcriptionally, it provides a rapid regulatory layer that operates without altering the underlying genetic sequence. Disrupting core writers such as MTA or FIP37 is lethal to plant embryos, underscoring how deeply this system is woven into normal development, from meristem maintenance to flowering time and cold-stress adaptation.

Whether plant viral RNAs actually carry m6A was unresolved for decades, but since 2017 a series of studies has delivered converging proof. The first biochemical evidence came from Alfalfa mosaic virus, whose RNAs were enriched by immunoprecipitation with anti-m6A antibodies. Subsequent work mapped high-confidence methylation peaks on Wheat yellow mosaic virus, Plum pox virus, Potato virus Y, and Pepino mosaic virus, the latter detected even inside purified viral particles. More recently, antibody-independent nanopore direct RNA sequencing has confirmed m6A on Cucumber mosaic virus genomes, with sites overlapping those found by antibody-based methods, and liquid chromatography coupled to tandem mass spectrometry has verified the modification chemically. In Sugarcane mosaic virus, a potyvirus of maize, researchers pinpointed a single modified adenosine at position 6556, deposited by the maize writer ZmMTA, and showed that a synonymous mutation at that site significantly enhanced infection. Bamboo mosaic virus yielded 122 candidate sites by direct RNA sequencing. Together these techniques leave little doubt that m6A is an authentic feature of diverse plant viral RNAs.

How cytoplasmic plant viruses gain access to a methyltransferase complex that normally resides in the nucleus has become one of the most intriguing mechanistic questions in the field. Unlike nuclear-replicating animal viruses such as influenza or HIV, most plant viruses replicate exclusively in the cytoplasm, seemingly far from the writer machinery. The answer, it turns out, is that viruses actively drag the machinery to themselves. In wheat, the WYMV NIb protein physically interacts with the writer TaMTB and triggers its translocation to cytoplasmic replication sites, where the enzyme methylates viral RNA at a defined position. In Arabidopsis infected with Cucumber mosaic virus, the viral coat protein binds the core writer MTB directly and promotes nuclear-to-cytoplasmic relocation of the entire complex. During Sugarcane mosaic virus infection, ZmMTA co-localizes with viral genomic RNA in cytoplasmic aggregates, though the exact relocation mechanism remains to be worked out. Notably, not every virus succeeds in this recruitment; recent antibody-independent analyses of Chikungunya and Dengue viruses found no detectable m6A, a reminder that epitranscriptomic engagement is selective rather than universal.

Once deposited, the mark can cut in either direction. In several systems m6A behaves as a classic antiviral flag. Hypermethylated Cucumber mosaic virus RNAs are recognized by the reader protein ECT8, which directs them into decay pathways, likely through processing bodies. In Pepino mosaic virus infection of Nicotiana benthamiana, overexpressing the writers MTA or HAKAI boosts viral RNA methylation and restricts infection, while the methylated transcripts are bound by the YTH-domain readers NbECT2A, 2B, and 2C, which recruit nonsense-mediated decay factors such as UPF3 and SMG7 to destroy them. Silencing NbECT2B or the NMD components increases viral RNA stability and susceptibility. In maize, the reader ZmECT23 recognizes methylated Sugarcane mosaic virus RNA and recruits the CCR4-NOT deadenylation complex to destabilize it, and in Nicotiana benthamiana the writer NbMTA targets Potato virus Y genomes for degradation.

Yet some viruses have turned the same mark to their advantage. When the WYMV NIb protein lures TaMTB to replication complexes, the resulting site-specific methylation stabilizes the viral RNA and prevents its degradation, and mutating the methylation site reduces viral stability and pathogenicity. A parallel strategy appears in the plant rhabdovirus Barley yellow striate mosaic virus, whose P6 messenger RNA is hypermethylated; loss of the mark reduces P6 stability and infectivity. Intriguingly, barley fights back with its own eraser, HvALKBH1B, which binds P6 mRNA into cytoplasmic condensates through liquid-liquid phase separation, a process requiring the protein’s intrinsically disordered region, thereby stripping the viral advantage. The lesson is that m6A is not inherently antiviral or proviral; it is a regulatory currency that whichever side controls it can spend.

The mark also shapes viral systemic movement, not merely local accumulation. Alfalfa mosaic virus depends on the host demethylase ALKBH9B removing m6A from its RNAs for efficient long-distance spread, an interaction mediated by the viral coat protein. Excessive methylation reduces phloem transport and restricts movement through the plant, so a balanced level of the mark is essential for the virus to travel. Consistent with this, infection by Plum pox virus and Potato virus Y lowers global m6A levels in Nicotiana benthamiana, and knocking down ALKBH9 homologs diminishes accumulation of both viruses. On the defensive side, the YTHDF-family readers ECT2, ECT3, and ECT5 bind Alfalfa mosaic virus RNAs in an m6A-dependent manner and act as restriction factors suppressing viral accumulation. Strikingly, this regulation is specific to ALKBH9B, since disrupting the related homologs ALKBH9A or ALKBH9C leaves infection untouched, revealing a fine-grained specificity in how plants deploy their epitranscriptomic arsenal.

Viruses, unsurprisingly, have evolved countermeasures. The 2b protein of Cucumber mosaic virus, already infamous as a suppressor of RNA silencing, competitively binds the writer components HAKAI and MTB, impairing the integrity of the writer complex in both nucleus and cytoplasm. This suppresses methylation not only on viral RNAs but across the host transcriptome; plants lacking m6A are more susceptible to a 2b-deficient mutant but not to wild-type virus, and simply expressing 2b in transgenic plants lowers global m6A levels. Pepino mosaic virus takes a different route: its RNA-dependent RNA polymerase interacts with the tomato writer SlHAKAI and the autophagy protein SlBeclin1, forming cytoplasmic granules that route SlHAKAI to autophagic destruction, thereby stripping methylation from viral RNA. In maize, the Sugarcane mosaic virus protease NIa-Pro hijacks the initiation factor ZmeIF4A3 into viral replication complexes, sterically blocking ZmMTA-mediated methylation and sparing viral RNA from ZmECT23-directed decay. Each case illustrates a mutually antagonistic arms race fought at the level of RNA chemistry.

Viral infection also rewrites the host’s own methylation landscape in ways that reshape immunity. Tobacco mosaic virus lowers m6A levels in tobacco, coinciding with upregulation of the eraser NbALKBH5, while Rice stripe virus and Rice black-streaked dwarf virus raise methylation levels in rice, enriching marks on transcripts involved in RNA silencing and hormone defense, including the antiviral genes OsAGO18 and OsSLRL1. In watermelon facing Cucumber green mottle mosaic virus, susceptible plants show early hypermethylation of stress-related transcripts, whereas resistant cultivars exhibit later hypomethylation that preserves defense gene expression. In Arabidopsis, CMV-induced hypomethylation upregulates the salicylic acid regulators NPR3 and CBP60a, and Potato virus Y triggers a transient rise in methylation at five to ten days post-inoculation that falls by day fourteen, accompanied by activation of a transcription factor that boosts NbMTA expression and accelerates viral RNA degradation. These virus-specific, time-dependent shifts integrate m6A into the broader networks of hormone signaling and stress adaptation.

The review’s authors argue that these findings position m6A as a promising lever for crop protection, whether through overexpressing writer components, modulating reader activity, or breeding viral methylation sites out of susceptible hosts. Significant questions remain, however. Recent work warns that false-positive signals can arise in both antibody-based and some antibody-independent mapping methods, making rigorous negative controls essential, and emerging single-molecule technologies are expected to sharpen the resolution of modification detection. Researchers still do not fully understand what sequence or structural features direct methylation to particular viral transcripts, how the temporal choreography of methylation unfolds during infection, or how reliably these mechanisms can be engineered into durable field resistance. What is already clear is that the epitranscriptome has moved from the margins of plant virology to its center, and that the contest over a single methyl group on adenosine may help decide the outcome of infections that devastate staple crops worldwide.

Subject of Research: The role of m6A RNA modification in plant-virus interactions

Article Title: The expanding role of m6A RNA modification in plant-virus dynamics: friend, foe, or both?

Article References: Liu, J.-H., Yu, H., & Duan, C.-G. (2026). The expanding role of m6A RNA modification in plant-virus dynamics: friend, foe, or both?. Advanced Biotechnology, 4(1), Article 7. https://doi.org/10.1007/s44307-026-00100-3

Image Credits: AI Generated

DOI: 10.1007/s44307-026-00100-3

Keywords: m6A, RNA modification, epitranscriptome, plant viruses, plant immunity, methyltransferase, YTH-domain proteins, RNA decay, viral countermeasures, crop resistance, Cucumber mosaic virus, RNA methylation