// NATURE NEWS — SPAZIO & SCIENZA
Sex without crossovers mimics clonal reproduction in Rhynchospora tenuis
Nature
(2026) Cite this article
Meiotic recombination ensures accurate chromosome segregation and promotes genetic diversity by generating crossovers between homologous chromosomes1. Although 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 documented. Here we investigate Rhynchospora tenuis, a flowering plant with the lowest known chromosome number and inverted meiosis3. Combining genomics with molecular experiments, 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 undetectable cytologically and genetically, and univalents persist at metaphase I. Haplotype-specific accumulation of transposable elements generates segregation distortion favouring the transmission of larger, repeat-rich chromosomes. Sexual reproduction is nevertheless retained: fertilization yields viable seeds only when translocation-compatible gametes meet, indicating strong post-meiotic selection against incompatible homozygous combinations. As a result, all surviving offspring are genetically identical, effectively maintaining heterozygosity by sexual reproduction with parental genotype restitution mimicking clonal reproduction. We propose that recombination loss, a low chromosome number, inverted meiosis and selection for compatible gamete combinations together enable faithful segregation and clonal-like inheritance despite sexual reproduction. These findings blur the boundary between sex and clonality, linking genome architecture, recombination loss and transmission bias.
Although meiotic recombination is critical for most sexually reproducing species, its frequency varies widely, often differing between sexes (heterochiasmy)4,5, and can be entirely suppressed in specific chromosomes or lineages, known as achiasmy2. In male Drosophila melanogaster, recombination and chiasmata are absent entirely, requiring alternative mechanisms such as chromatin threads or meiotic drive for correct chromosome segregation6,7,8,9,10. The absence of recombination is expected to impair chromosome segregation, reduce genetic diversity and accelerate mutation accumulation through the Meselson effect11,12,13,14. However, obligate, genome-wide bisexual achiasmy of a sexually reproducing organism has not been described previously.
Rhynchospora tenuis Link is a small perennial beak-sedge of open, seasonally wet grasslands and savannas across tropical and subtropical America. It has the lowest known chromosome number among flowering plants (n = 2). Rhynchospora tenuis is also holocentric, with kinetochore activity distributed along the entire chromosome length, and undergoes inverted meiosis, in which sister chromatids segregate in meiosis I and homologues separate only in meiosis II3,15—the reverse of the canonical order. Holocentric chromosomes tolerate structural rearrangements such as end-to-end translocations (hereafter, fusions)16, which may have facilitated the evolution of this extremely reduced chromosome number17. Previous cytological work showed that male meiosis in R. tenuis proceeds without homologous pairing or chiasmata, displaying four univalents at diakinesis and raising the possibility of achiasmy despite apparent meiotic double-stranded break formation3. Whether recombination is entirely absent in both sexes, and how accurate chromosome transmission and fertility are then maintained without crossovers, has remained unclear.
Here we combine chromosome-scale pangenomics, molecular cytogenetics and immunocytochemistry, single-gamete sequencing of pollen/seed nuclei and whole-genome sequencing of offspring from controlled crosses to characterize the recombination landscape, meiotic progression and inheritance patterns of R. tenuis. We sho