Timmers, T. | Search bioRxiv

bioRxiv2026

Sex without crossovers mimics clonal reproduction in the holocentric plant Rhynchospora tenuis

Meiotic recombination ensures accurate chromosome segregation and promotes genetic diversity by generating crossovers between homologous chromosomes1. While essential in most sexually reproducing organisms, recombination is variably regulated and can be absent in some lineages, a condition known as achiasmy2. However, obligate achiasmy in both sexes of a sexual species has not been previously documented. Here, we investigate the beak-sedge Rhynchospora tenuis, a holocentric plant with the lowest known chromosome number among flowering plants (n = 2) and inverted meiosis3. Using chromosome-scale genome assemblies from nine accessions, molecular cytogenetics, immunocytochemistry, high-throughput single-gamete sequencing and whole-genome sequencing of controlled crosses, we show that R. tenuis undergoes obligate, genome-wide achiasmy in both male and female meiosis. Despite normal early meiotic axis formation, synapsis fails, crossovers are not detected cytologically or genetically, and univalents persist at metaphase I. Extensive haplotype-specific accumulation of transposable elements (TEs) generates segregation distortion (e.g. meiotic drive), favouring the transmission of larger, TE-rich chromosomes. Remarkably, sexual reproduction is retained with fertilisation producing viable seeds only when translocation-compatible gametes meet, indicating strong post-meiotic selection that eliminates incompatible homozygous combinations. As a result, all surviving offspring are genetically identical to the maternal genotype, effectively restoring heterozygosity each generation and mimicking clonal reproduction. We propose that the combined effects of recombination loss, low chromosome number, holocentricity, inverted meiosis, and selective transmission of longer chromosomes enable faithful segregation and clonal-like inheritance despite sexual reproduction. These findings challenge the boundary between sex and clonality, revealing a unique evolutionary strategy linking genome architecture, recombination loss, and transmission bias.

evolutionary biology↗

bioRxiv2023

A simplified disease resistance assay using YFP-expressing Potato Virus X in N. benthamiana reveals a cell death-independent immune function of RBA1

R (resistance) proteins, such as intracellular NLRs (nucleotide-binding leucine-rich repeat receptors), are integral components of the plant innate immune system (van Wersch et al., 2020). Host responses following R protein activation include the generation of reactive oxygen species, sustained increases in cytosolic Ca2+, transcriptional reprogramming and, typically, rapid host cell death at sites of pathogen infection, which together ultimately lead to pathogen growth restriction (Wang et al., 2023). To assess the activity of R proteins, agroinfiltration-mediated transient gene expression assays have been widely used in Nicotiana species (e.g., N. benthamiana). In these transient assays, host cell death is often chosen as an indicator of R protein activity from the host responses mentioned above, in part because of the ease of experimentation. However, the extent to which host cell death is a proxy for disease resistance signaling has long been debated, as host cell death and pathogen growth restriction can be uncoupled in several cases (Bendahmane et al., 1999; Coll et al., 2010; Heidrich et al., 2011, Maekawa et al., 2023). To assess the disease resistance activity of R proteins, bacterial growth assays have been employed in combination with transient R gene expression in N. benthamiana (Sun et al., 2021). Bacterial growth assays, however, require multiple experimental procedures, including agroinfiltration, pathogen infection and bacterial counts, which hinders high-throughput studies of R gene-mediated disease resistance. Here, we report a simple plate reader-based assay to assess R gene-mediated disease resistance activity against PVX (Potato virus X) that expresses YFP (PVX-YFP). Unlike bacterial pathogens, PVX proliferation in N. benthamiana is not restricted by the intrinsic activity of the EDS1 signaling pathway as previously shown by virus-induced NbEDS1 gene silencing (Peart et al., 2002) and as we consistently show in this study using a Nbeds1 gene knockout mutant. This feature would increase the sensitivity of the assay, allowing it to capture a weak-to-moderate disease resistance activity of R proteins, as the contribution of basal immunity to PVX via the NbEDS1 pathway is negligible. Using this assay, we show that a non-cell death-inducing mutant of the R protein of RBA1 (Response to HopBA1), which lacks 2',3'-cAMP/cGMP synthetase activity but retains NADase activity, confers PVX resistance in an EDS1 signaling pathway-dependent manner.

plant biology↗