Sex Without Crossovers: Plant Reveals a Surprising Route Through Meiosis

In the textbook version of sexual reproduction, meiosis is inseparable from genetic recombination. Homologous chromosomes pair, exchange segments at structures called crossovers, and only then segregate into eggs or sperm, shuffling genes with every generation. But new research on the sedge Rhynchospora tenuis is forcing biologists to reconsider how rigid that link really is. The plant, it turns out, completes meiosis and produces viable offspring with almost no crossovers at all, a strategy that functionally mimics clonal reproduction while retaining the outward machinery of sex. The findings, published in Nature, illuminate a hidden flexibility in one of biology’s most conserved processes and carry implications for how recombination evolves, how asexual lineages persist, and how genome architecture shapes fertility.

Rhynchospora tenuis belongs to the Cyperaceae, a plant family famous for an unusual chromosomal arrangement known as holocentry. Unlike the monocentric chromosomes of most animals and plants, which carry a single localized centromere that anchors spindle fibers during cell division, holocentric chromosomes have kinetochore activity distributed along nearly their entire length. This means spindle microtubules can attach at many points, and fragments of broken chromosomes can still be inherited rather than lost. Holocentry is generally viewed as an evolutionary escape hatch, granting lineages that possess it tolerance against chromosome breakage and fragmentation. The new study suggests it may do something even more radical: allow the complete bypass of crossovers during meiosis without sacrificing chromosome transmission.

To understand why this is so unexpected, it helps to recall what crossovers normally accomplish. During the prolonged prophase of the first meiotic division, homologous chromosomes recognize each other, synapse along their lengths, and exchange arms. These physical links, called chiasmata, are not merely a source of genetic novelty; they are load-bearing structures. They hold homologous pairs together until the cell is ready to divide, orienting the pairs so that one copy of each chromosome goes to each pole. Experimental systems in which crossovers are eliminated, through mutations in recombination enzymes such as Spo11 or its downstream repair factors, typically suffer catastrophic consequences: univalents scatter on the spindle, chromosomes segregate randomly, gametes become aneuploid, and fertility collapses. The dogma has been that at least one crossover per homologous pair, tightly positioned and regulated, is essential for the reductional division that defines meiosis.

Rhynchospora tenuis breaks this rule. Using a combination of cytogenetic imaging, immunolocalization of meiotic proteins, and genomic analyses, the researchers documented meiotic divisions in which homologous chromosomes fail to form chiasmata yet still segregate with remarkable fidelity. Instead of relying on crossover-generated chiasmata to hold pairs together, the plant appears to exploit its holocentric architecture directly. Bipolar attachment of spindle fibers to holocentric homologs can achieve the same result that a chiasma achieves in monocentric species: the stable bi-orientation of homologous chromosomes on the division spindle and their coordinated movement to opposite poles. In effect, the chromosome itself becomes the segregation unit, and the crossover becomes dispensable.

The consequences for genetic inheritance are profound. When crossovers shuffle maternal and paternal segments along each chromosome, the alleles a gamete receives are a mosaic of both parents. When crossovers are absent, each homolog is transmitted intact, so gametes receive whole parental chromosomes unchanged. Across the entire genome, this means the genetic output of meiosis resembles what clonal, asexual reproduction would produce: offspring inherit combinations of alleles that have never been broken up by recombination. Yet the organism still goes through the full choreography of sexual reproduction, pairing homologs, executing two divisions, fusing gametes at fertilization. Sex without recombination, a combination long considered unstable or paradoxical, is realized as a functioning reproductive strategy in this sedge.

This discovery speaks directly to one of evolutionary biology’s oldest puzzles: why sex exists at all. The maintenance of recombination is usually justified by its long-term benefits, purging deleterious mutations, combining beneficial ones, and generating the variation on which selection acts. But recombination also has short-term costs, breaking up favorable allele combinations and exposing genomes to selfish genetic elements. Theoretical work has long predicted that systems suppressing recombination could enjoy transient advantages, yet most such systems, from asexual lineages to inversion heterozygotes, pay the price of meiotic dysfunction. Rhynchospora tenuis demonstrates a route around that price: recombination can be abandoned not by abandoning meiosis, but by rewiring the mechanics of chromosome segregation so that the crossover is no longer structurally necessary. Evolution, in other words, can decouple the genetic function of recombination from the mechanical demands of chromosome segregation.

The study also reframes the significance of holocentricity. Chromosome biologists have catalogued holocentric lineages across the tree of life, including plants in the Cyperaceae and Juncaceae, several groups of insects such as butterflies and moths, and various microscopic eukaryotes. In these lineages, recombination patterns are often unusual, with reduced or relocalized crossovers, and some species exhibit inverted meiosis, in which the first division segregates sister chromatids rather than homologs and the second division is reductional. The new findings suggest that these quirks are not isolated oddities but expressions of a general principle: when kinetochore activity is distributed along the chromosome, the cell gains mechanistic freedom to reorganize the meiotic program. In Rhynchospora tenuis, that freedom has been cashed in for the elimination of crossovers altogether.

The researchers back these interpretations with a suite of technical observations. Cytological markers of recombination, including foci of the DNA repair protein RAD51 and the crossover-associated protein MLH1, are drastically reduced or absent during prophase I, confirming that the molecular machinery of crossing over is not engaged even though homologs still pair. Chromosome spreads reveal synapsed homologs that separate cleanly at anaphase I despite lacking chiasmata. Fluorescent in situ hybridization with chromosome-specific probes shows that entire parental chromosomes, rather than recombined mosaics, are transmitted through meiosis. And the resulting offspring are viable and fertile, demonstrating that this recombination-free sexual cycle is not a developmental accident but a stable, heritable reproductive mode. Together, these lines of evidence rule out the possibility that crossovers occur below the detection threshold in a few sites; the plant genuinely reproduces sexually without them.

For broader biology, the implications stretch in several directions. Crop breeders, who rely on recombination to assemble favorable alleles, have long been interested in manipulating crossover rates, and understanding how a genome can function without crossovers clarifies which parts of the meiotic machinery are truly indispensable. Evolutionary biologists studying asexual lineages, many of which face long-term extinction because they cannot purge deleterious mutations, now have a living model of a lineage that occupies a middle ground: genetically clonal in its output, yet cytologically and ecologically sexual. And researchers probing the evolution of holocentric chromosomes gain a concrete example of how this architecture can unlock meiotic innovations that would be lethal in monocentric genomes. Rhynchospora tenuis, a modest sedge, has become a case study in how evolution can rebuild one of life’s most fundamental processes from the chromosome outward.

Open questions remain. It is not yet fully resolved how homologs achieve stable bi-orientation without chiasmata in molecular detail, which spindle-attachment geometries the holocentric kinetochore permits, or how frequently this mode of reproduction arises and is maintained across the sedge family and beyond. Nor is it clear what selective pressures favored the loss of crossovers in this species, whether escape from selfish elements, preservation of local adaptation, or simple historical contingency. But the central message is already firm: the coupling between sex, meiosis, and recombination is not a law of nature but a historical arrangement, one that at least one plant lineage has successfully renegotiated. Sex, it appears, can survive the loss of its most celebrated source of novelty, as long as the chromosomes themselves know where to go.

Subject of Research: Meiosis without crossovers and clonal-like sexual reproduction in the holocentric sedge Rhynchospora tenuis

Article Title: Sex without crossovers mimics clonal reproduction in Rhynchospora tenuis

Article References: Zhang, M., Castellani, M., Steckenborn, S., Majka, M., Tsipas, G., Nascimento, T., Neumann, U., Thangavel, G., Robledillo, L. A., Lux, T., Deberón, L., Pfordt, U., Campoy, J. A., Vijayan, A., Timmers, T., Sargheini, N., Marek, M., Sun, H., Hofstatter, P. G., … Marques, A. (2026). Sex without crossovers mimics clonal reproduction in Rhynchospora tenuis. Nature. https://doi.org/10.1038/s41586-026-11057-7

Image Credits: AI Generated

DOI: 10.1038/s41586-026-11057-7

Keywords: meiosis, crossovers, recombination, holocentric chromosomes, Rhynchospora tenuis, sedge, chiasmata, clonal reproduction, chromosome segregation, asexual reproduction, centromere, evolution of sex