In the animal world there is a rule: the genome of germ cells is inviolable, while somatic cells can afford certain liberties. In humans, red blood cells throw out their nucleus altogether, and lymphocytes deliberately reshuffle their genes to create millions of antibody variants. But in some nematodes these liberties go further: body cells lose entire chunks of chromosomes according to a prewritten program. This phenomenon, programmed DNA elimination, has been known for more than a hundred years, but for a long time it was considered an exotic quirk of parasitic ascarids. Recent findings in free-living species such as Oscheius tipulae showed that the mechanism is more widespread than people thought. The work on Auanema rhodense stands alongside these findings: it adds to the picture a species with an extreme scale of elimination and an unusual sexual system, one in which the body genome and the germline genome differ more radically than in most known animals.
The authors reassembled the genome of A. rhodense using two tools: long PacBio HiFi reads, which allow DNA to be read in chunks of several thousand nucleotides, and Hi-C, a method that records which parts of chromosomes sit next to each other in the nucleus. The DNA was taken from cultures enriched with young adult individuals, in order to capture as many germ cells as possible. The assembly came out three times longer than the previous one: 180.9 megabases instead of 60. Then the authors looked for break sites by the truncated telomeric repeats at the ends of long reads: if a repeat begins inside a chromosome, that means a new telomere was once added there. In parallel, they watched embryos under the microscope: they stained DNA, histone marks and microtubules, and also hybridized fluorescent probes to the repeats destined to disappear.
The germline genome of A. rhodense consists of 180.9 megabases and is assembled into seven chromosomes. The body genome is only 60 megabases, broken into 14 chromosomes: each of the seven original chromosomes is cut in two. The breaks occur at 28 sites, and each one carries a palindromic motif 29 nucleotides long. It is almost identical to the motif in O. tipulae, which points to a common origin of the mechanism. Elimination begins at the 12-cell stage: in four blastomeres out of eight the DNA is cut, and micronucleus-like bodies stuffed with discarded repeats appear in the cytoplasm. By the 30-cell stage, almost all nuclei have already lost the repeats, and only two nuclei retain them: these are the future germ cells. The discarded DNA consists almost entirely of tandem repeats with a unit length of 167 to 504 nucleotides. There are few protein-coding genes in it: only 360 out of 11,943, and most of them resemble the remnants of mobile elements. Only ten discarded genes are actively expressed, and almost all of them have working copies in the somatic part of the genome.
The work shows that one and the same molecular mechanism can serve completely different evolutionary strategies. In O. tipulae, 0.6% of the genome is discarded; in A. rhodense, 61%, yet the recognition motif is the same. This means that it is not the amount discarded that defines the essence of the phenomenon, but the very existence of the program. Among neighboring nematode studies, this one stands out by its scale: in Caenorhabditis less than 2.3% of the genome is lost, in ascarids up to 90%, but there the breaks are imprecise and lack motifs. A. rhodense combines the precision of rhabditins with the scale of ascarids. In addition, the work links DNA elimination to the species' unusual sexual system: the non-Mendelian inheritance of the X chromosome may rely on unique repeats that in A. rhodense exist only on the sex chromosome.
The next step is to find the proteins that recognize the palindromic motif and cut the DNA. In O. tipulae it has already been shown that CRISPR editing of the motif cancels elimination, but the machine itself remains unknown. Comparing protein complexes in A. rhodense, O. tipulae and Caenorhabditis may reveal the conserved core of the mechanism. Another direction is to understand how the accumulation of repeats in the discarded regions is linked to the evolution of meiosis and meiotic drive. Perhaps these repeats became landing pads for proteins that help chromosomes find partners or determine which chromosome ends up in the gamete.
The results matter for the evolutionary biology of genomes, cytogenetics and research on meiotic drive. They also provide a model example of how a cell can massively and precisely rework its genome without harming the organism.
Identify the nuclease and accessory proteins responsible for the breaks; test the role of the unique X-chromosome repeats in non-Mendelian inheritance; compare the mechanism across a larger number of rhabditin species.
For developmental biology and genomics, one question remains open: how do embryo cells know which parts of the genome to discard, and why does this process spare the germ cells. The work on A. rhodense provides a precise map of the recognition sites and shows that the program switches on strictly at the 12-cell stage, but the molecular switch itself is unknown. It is also unclear how the accumulation of giant repeats in the germline is reconciled with maintaining genome stability across generations.
Auanema rhodense has three sexes: males, females and hermaphrodites. What is more, males pass on only the X chromosome to their offspring, while hermaphrodites produce only sperm carrying two X chromosomes, whereas their eggs contain no X at all.
The idea of cells deliberately throwing away part of their genetic material echoes the motif of forbidden knowledge in science fiction: in Greg Bear's novel Blood Music, for instance, cells rearrange their DNA and acquire a will of their own. Here, though, everything is under the control of evolution rather than rebellion.
Not published on arXiv. Here are its page, the PDF and the formalization: you can check it yourself, not only read our account.