Background

In the process of eukaryotic sexual reproduction, meiosis is considered a key pathway for reducing chromosome numbers by half and ensuring genetic diversity. Crossover, where homologous chromosomes pair and exchange genetic material during gamete formation, is the driving force that recombines parental traits to pass them to offspring. The chiasma, a physical connection structure observed under microscopes, has served as a mechanical anchor that guides spindle fibers to precisely separate homologous chromosomes toward opposite poles during prophase I of meiosis.

It has been common knowledge that the loss of this physical link leads to chromosome non-disjunction, resulting in fatal infertility. While cases of achiasmy have been reported in male Drosophila or female silkworms, these were considered special exceptions limited to heterogametic sexes. Until now, no organism on Earth has been known to reproduce stably while excluding homologous chromosome crossover in both males and females. The question of whether male achiasmy could actually exist in the history of biological evolution has been considered an unsolved puzzle in genetics.

Key Discovery

The research team investigated the reproductive mechanism of Rhynchospora tenuis, a vascular plant in the Cyperaceae family. Rhynchospora tenuis features a minimal chromosome count among angiosperms, n = 2 (2n = 4), and is characterized by a holocentric chromosome structure where spindle fibers attach across the entire chromosome. The researchers succeeded in visualizing the entire process of meiosis in pollen mother cells and megaspore mother cells by combining chromosome-level genome assembly with molecular cytogenetic Fluorescence In Situ Hybridization (FISH).

Surprisingly, crossover between homologous chromosomes was not observed not only in the microsporogenesis process of males but also in the megasporogenesis process of females. This revealed that absolute achiasmy, where crossover is permanently absent across the entire genome, occurs in both sexes. The research team employed single-gamete sequencing to fully decode the genotypes of approximately 300 individual pollen grains and megaspore tissues. The analysis confirmed that the frequency of chromosome recombination due to crossover across all analyzed germ cells was 0%.

Homologous chromosomes aligned side by side and moved to opposite poles without defects, even without the aid of chiasmata. The analysis that the broad spindle attachment area of holocentric chromosomes and cytoskeletal control compensate for the absence of chiasmata gains credibility. This represents a unique reproductive mode that transmits the parental haploid chromosomal composition to the next generation intact.

Significance and Outlook

This discovery fundamentally overturns the basic hypothesis in genetics textbooks that meiotic crossover is essential for the accurate separation of chromosomes. Nature has proven that stable chromosome distribution is possible even without physical connections between homologous chromosomes. Furthermore, it delivers a fresh shock to the evolutionary biological dogma that a lack of crossover leads to the depletion of genetic diversity and subsequent elimination in the struggle for survival. Rhynchospora tenuis has adopted an exquisite survival strategy: preserving proven superior gene combinations by blocking outcrossing, while avoiding the risks of inbreeding through self-pollination.

It is suggested that the unique structural conditions of having only two chromosomes and holocentricity may have promoted achiasmic adaptation. A major task for the future is to determine whether similar achiasmic separation mechanisms could potentially operate in many other species with higher chromosome numbers. This is expected to provide clues for studying meiotic errors and unexplained infertility in humans and other mammals from a new perspective.