Press release
EMBARGO: April 20th, 12:00 CEST/ 19:00 Japan

A small RNA system is involved in homology recognition between meiotic
chromosomes
Meiosis is a specialized mode of cell division that produces haploid gametes. During meiosis,
chromosomes with homologous partners undergo synaptonemal complex (SC)-mediatedpairing, while the remaining unpaired regions such as most parts of the sex chromosomes inmales are heterochromatinized through unpaired silencing.
A study, publishing this week in The EMBO Journal, introduces that a small RNA systemregulates meiotic pairing of homologous chromosomes in the nematode Caenorhabditis elegans.
Homolog pairing initiation depends on species-specific mechanisms involving interactions
between pericentric heterochromatin, centromeres, telomeres, and/or pairing center sequences.
Subsequently, the SC establishes the pairing of other regions of the homologous chromosomes.
In plants and mammals, DNA double-strand breaks (DSBs) are important or partly importantfor synapse formation between homologous chromosomes. Mysteriously, in C. elegans and fruit
fly, DSBs are not necessary for synapse formation, implying the presence of an unclarified
mechanism for recognizing homologous chromosomes during SC formation.
RNA interference (RNAi) is a form of RNA-mediated sequence-specific regulation of gene
expression in eukaryotes. Small RNAs and Argonaute proteins play key roles in the mechanismof RNAi. Researchers at National Institute of Genetics, Tokyo Women’s Medical University,and University of Tsukuba, in Japan, found new relationships between SC formation on paired
chromosomes, unpaired chromosome heterochromatinization, and an Argonaute–small RNA
pathway (endogenous RNAi). The C. elegans Argonaute proteins, CSR-1 and its paralog
interacting with small RNAs, are required for SC formation between chromosome pairs withaccurate homology and are also involved in the condensation of unpaired chromosomes. The
SC is composed of layers of strings, with two lateral elements sandwiching a central region.
CSR-1 in nuclei and meiotic cohesin, constituting the SC lateral elements, were associated with1

non-simple DNA repeats expressing small RNAs and long non-coding RNAs (lncRNAs), andalso associated weakly with coding genes. This study proposes a zipper model, in which theaccurate homolog pairing is promoted via the inter-chromosomal recognition of chromosomeassociated lncRNAs by the Argonaute-mediated small RNA system. The meiotic difference
between paired or unpaired chromosomes is also described.
Article on The EMBO Journal website
https://www.embopress.org/doi/10.15252/embj.2020105002
Reference
Tabara H, Mitani S, Mochizuki M, Kohara Y, Nagata K (2023) A small RNA system ensuresaccurate homologous pairing and unpaired silencing of meiotic chromosomes. EMBO J, in press.
DOI: 10.15252/embj.2020105002.
Image Link: https://mega.nz/folder/9A5EDKDL#TNOzhCjj2UnHoAP4432VnA
Caption: Immunofluorescence images of SYP-1 (a component of the SC central region) on
meiotic chromosomes. The string-like superstructure of the SC was often branched in the csr-1mutant.
Author Contact:
Hiroaki Tabara, PhD

Media Contact:
Public Affairs Office, National Institute of Genetics
https://www.nig.ac.jp/nig/
Bureau of Public Relations, University of Tsukuba
https://www.tsukuba.ac.jp/en/2