Osiejuk, T. S. | Search bioRxiv

bioRxiv2025

Germline-restricted chromosome of songbirds has different centromere compared to regular chromosomes

Centromeres are an important part of chromosomes which direct chromosome segregation during cell division. Their modifications can therefore explain the unusual mitotic and meiotic behaviour of certain chromosomes, such as the germline-restricted chromosome (GRC) of songbirds. This chromosome is eliminated from somatic cells during early embryogenesis and later also from male germ cells during spermatogenesis. Although the mechanism of elimination is not yet known, it is possible that it involves a modification of the centromeric sequence on the GRC, resulting in problems with the attachment of this chromosome to the mitotic or meiotic spindle and its lagging during anaphase, which eventually leads to its elimination from the nucleus. However, the repetitive nature and rapid evolution of centromeres make their identification and comparative analysis across species and chromosomes challenging. Here, we used a combination of cytogenetic and genomic approaches to identify the centromeric sequence of two closely related songbird species, the common nightingale (Luscinia megarhynchos) and the thrush nightingale (L. luscinia). We found a 436-bp satellite repeat present in the centromeric regions of all regular chromosomes, making it a strong candidate for the centromeric repeat. This centromeric repeat was highly similar between the two nightingale species. Interestingly, hybridization of the probe to this satellite repeat on meiotic spreads suggested that this repeat is missing on the GRC. Our results indicate that the change of the centromeric sequence may underlie the unusual inheritance and programmed DNA elimination of the GRC in songbirds.

genomics↗

bioRxiv2024

Inbreeding depression depends on the body size and environmental conditions in a threatened songbird, the aquatic warbler Acrocephalus paludicola

While inbreeding is known to affect individual fitness and thus population extinction risk, studies have under-represented non-model species of conservation concern, and rarely sought conditionality of inbreeding depression. Here, using SNPs identified with RAD-seq, we determined inbreeding depression in a threatened bird, the aquatic warbler Acrocephalus paludicola, and whether its magnitude depends on phenotypic and environmental factors. We found that the inbreeding coefficient (F) of adults with small tarsi was negatively associated with the seasonal breeding success (in males) and clutch size (in females), with the respective decrease in fitness in the most inbred relative to the least inbred individuals of ~89% and ~12%. In contrast, in adult males, for the average tarsus, wing and mass, support was low for F to be related to the long-term return rate to breeding grounds. For mean phenotypic covariates and male density, we also found low evidence that F is associated with the annual breeding success. Likewise, there was little support that mother F is related to egg hatch success and nestling survival, and - for average phenotypic traits, rainfall, temperature and nest density, and accounting for breeding peak - to clutch and fledged brood sizes. For nestlings, animal models showed that F is more negatively related to tarsus under higher temperatures and its effect varies by study year. However, for average brood size, temperature, rainfall and prey abundance, and when controlling for nestling sex, breeding peak and mother F, evidence for nestling F and tarsus association was weak. We conclude that (1) inbreeding depression on fitness components is stronger in smaller-bodied individuals, (2) considering interaction with phenotypic and environmental variables enables more accurate estimation of inbreeding depression, and (3) the inbreeding depression estimates will inform extinction risk analysis and conservation actions for the aquatic warbler.

evolutionary biology↗