Plant Defies Inheritance: Sex Without Shuffle

An unusual combination of chromosome organisation and selective inheritance enables a Brazilian sedge to maintain its genetic identity across generations

Sexual reproduction usually reshuffles genetic information, creating offspring that differ from their parents. But one small plant takes an unexpected route. In a new study, now published in Nature, an international team led by André Marques at the Max Planck Institute for Plant Breeding Research in Cologne has shown that the Brazilian beak-sedge Rhynchospora tenuis reproduces sexually - producing pollen, fertilising its egg cells and sets seeds - but has completely lost the reshuffling of DNA between the chromosomes from each parent. The offspring retained the same genetic make-up as their mothers, effectively mimicking clonal reproduction.

Most sexually reproducing organisms rely on meiosis, a specialised form of cell division that produces reproductive cells, during which homologous chromosomes inherited from each parent exchange DNA segments. These crossovers generate genetic diversity and ensure accurate chromosome segregation; without them, fertility can be compromised.

Earlier cytological studies suggested that male meiosis in R. tenuis proceeds without crossovers. However, it remained unclear whether genetic recombination-normally generating new combinations of parental genes in the offspring-is absent in both sexes and, if so, how the plant still ensures proper chromosome segregation and maintains fertility.

Rhynchospora tenuis appears to have overcome this problem. Among the flowering plants, R. tenuis has the lowest known chromosome number. Its chromosomes also have an unusual organisation - the centromere activity spreads across its chromosome entire length, known as holocentric chromosomes. In addition, the plant performs meiosis in a reversed order, an alternative cell-division track, separating duplicated chromosome copies before separating their matching partners.

Together, these features of unusual genome architecture may allow reproduction to continue even without crossovers.

Investigating the reproductive puzzle

The researchers combined detailed genome assemblies from nine plants collected in Brazil with chromosome imaging, sequencing of individual pollen nuclei and controlled crossing experiments. Across more than 10,000 pollen nuclei, the team detected no crossovers and obtained also no evidence of chromosome exchanges through the female reproductive pathway.

Using independent approaches of cytological, genomic, single-cell and developmental analysis the findings provide evidence for a complete absence of crossovers in both male and female meiosis, a combination documented for the first time in an otherwise sexually reproducing species.

A selective route to the next generation

Strikingly, the team discovered that inheritance is strongly biased.

Some chromosomes have grown larger through the accumulation of transposable elements, stretches of DNA that can copy themselves and insert elsewhere in the genome, so called 'jumping genes'. These larger chromosomes were preferentially passed on through both pollen and egg cells. Meanwhile, exchanges of chromosome end segments have created structural differences that restrict which combinations can successfully contribute to the next generation.

Further analyses revealed strong selection after fertilisation: around 87 % of seeds abort, and only those offspring that reconstitute the parental heterozygous chromosome combination - retaining two different versions of the genome - survive, resulting in a clone-like outcome. Because certain chromosome versions are systematically favoured during the formation of sex cells, and only seeds carrying the right chromosome combination survive, every surviving offspring ends up genetically identical to its mother: sex that produces clones.

The discovery expands our understanding of how flexible plant reproduction can be and redraws the textbook boundary between sexual and clonal reproduction.