Aborting 87% of its seeds lets a Brazilian sedge hand its genome on unchanged

Crossovers do two jobs in an ordinary meiosis. They mix the genomes a plant received from its two parents, and they hold each pair of homologous chromosomes together long enough for the cell to pull one of each pair into a gamete. Take them away and fertility usually suffers ^[3]. *Rhynchospora tenuis* has a set of features that make the second job easier to do without them: centromere activity spread along the whole length of each chromosome ^[5], a meiosis run in reverse order, separating the duplicated copies of a chromosome before separating the matching partners ^[6], and the lowest chromosome number known in any flowering plant ^[4]. The authors put the connection as a possibility, writing that this architecture may allow reproduction to continue even without crossovers ^[7].

Two filters produce the clone. The first acts before fertilization: chromosomes that have grown larger by accumulating transposable elements are preferentially passed on through both pollen and egg cells ^[12], and exchanges of chromosome end segments have created structural differences that limit which combinations can contribute at all ^[13]. The second acts after fertilization, and it is blunt. If 87% of seeds abort, 13% survive ^[14][17], so the plant sets roughly eight seeds for every one that lives ^[18].

The crossover result comes with a denominator: zero events across more than 10,000 pollen nuclei ^[9]. For zero observations in n trials the standard 95% upper bound is 3/n, which puts the per-nucleus crossover rate under about 3 in 10,000, or 0.03% ^[19]. A single-cell assay can bound what it detects; it cannot prove literal zero. Earlier cytology had already pointed to a male meiosis without crossovers; the female side was the open question ^[11].

"Beyond the evolutionary question, controlling recombination is a long-standing goal in plant breeding: fixing a high-performing hybrid genotype so that it is passed on unchanged through seeds is exactly what R. tenuis achieves naturally, and understanding how it does may inform future breeding strategies," Marques said ^[15].

The goal is real, and so is the demonstration: this plant sets seed with no crossovers in either sex ^[10]. It gets there by discarding about seven seeds for every one that lives ^[18]. In a grain or oilseed crop the seed is the harvest, so a breeder would need the fixation without the discard. In this sedge the two come together.

The work is by Meng Zhang and colleagues in Nature ^[16]. It documents what one sedge does. What happens when crossovers are removed from a plant built like a cereal is still untested.