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Zitting Cisticola Sex Chromosome Revealed Two Evolutionary Strata on Chromosome 4

Zitting Cisticola perched on a branch against blue sky
Research · Genetics & genomics

The sex chromosome system of zitting cisticola (Cisticola juncidis) carries a large section derived from chromosome 4, and that section records at least two stages in the chromosome’s evolutionary history. In birds, males are typically ZZ and females ZW. When material from an ordinary chromosome becomes attached to a sex chromosome, the enlarged combination is called a neo-sex chromosome. Sylvioidea already share an older arrangement involving chromosome 4A; the new study examined additional chromosome-4 material that was incorporated later in particular lineages. Published in BMC Genomics on 20 April 2026, it compared whole-genome data from zitting cisticola, grey-backed camaroptera (Camaroptera brevicaudata) and yellow-throated nicator (Nicator vireo).

In zitting cisticola, a large part of chromosome 4 had become sex-linked. The researchers identified two evolutionary strata within it. Recombination is the exchange of DNA between paired chromosome copies during the formation of eggs or sperm; when Z and W stop recombining in a region, they begin to evolve more independently. An evolutionary stratum is therefore a section where this exchange stopped at a particular stage in the past. The older stratum had stopped recombining earlier than the younger one.

The team inferred these regions by comparing male and female sequencing patterns along the chromosome. Older non-recombining regions tend to show stronger sex differences because the W copy has had more time to diverge and lose sequence, whereas younger regions retain more similarity between Z and W.

Plots showing evolutionary strata on chromosome 4 in three Sylvioidea species
Figure 2 from Brown et al. (2026): male–female genomic differences along chromosome 4 in Nicator vireo, Camaroptera brevicaudata and Cisticola juncidis. The patterns were used to map older and younger regions where Z and W no longer recombine, together with the chromosome end where recombination continues. © The Author(s) 2026, CC BY 4.0. Source.

Two evolutionary strata emerged on chromosome 4

The older stratum had the same boundary in zitting cisticola and grey-backed camaroptera. The authors interpret this shared pattern as evidence that chromosome-4 material was translocated—moved and attached—to the sex chromosomes once in an ancestor of the family Cisticolidae, before the two genera diverged. Published phylogenies place that event roughly 23–11 million years ago.

The younger non-recombining region later extended about 2.2 million DNA base pairs farther in zitting cisticola than in grey-backed camaroptera, showing that Z–W recombination continued to shut down differently after the lineages separated. Yellow-throated nicator had a different chromosome-4 attachment point and a much larger sex-linked region, supporting an independent translocation in Nicatoridae rather than the same ancestral event.

Sources and image credit

  1. Brown, T. J., Ellerstrand, S. J., Sigeman, H., Melo, M., Lundberg, M., & Hansson, B. (2026). Unique sex chromosome translocations and evolutionary strata in two Sylvioidea songbird families. BMC Genomics, 27(1), Article 401. https://doi.org/10.1186/s12864-026-12861-1

Image: Alexis Lours — wild zitting cisticola photographed on Île d'Aix, Charente-Maritime, France, 19 April 2024; Wikimedia Commons; representative species image; cropped · CC BY 4.0 · Source

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