An Exceptionally Complex Chromosome Rearrangement in the Great Tit <i>(Parus major)</i>: Genetic Composition, Meiotic Behavior and Population Frequency

Chromosomal inversions and copy-number variants (CNVs) drive genomic and phenotypic diversification in birds by reshaping recombination, gene expression, and genome architecture. Here, we report a complex structural polymorphism on great tit (<i>Parus major</i>) chromosome 1A that occurs in the Siberian population with a 19% heterozygote frequency. Using cytogenetic and genomic approaches, we show that this rearrangement combines a ~55 Mb paracentric inversion in the long arm with a dramatic (>30 Mb) expansion of the short arm driven by extensive amplification of multiple genomic loci. These include a region homologous to the poorly characterized <i>FAM118A</i> gene, whose paralog <i>FAM118B</i> has been recently shown to play a pivotal role in innate immune activation. This region is missing from the current reference genome assembly while present in ~20 copies on wild-type 1A chromosome and nearly twentyfold amplified in the rearranged variant. It contains a nested 630 bp tandem repeat, encompassing the entire exon 3, which has burst to a total of ~50,000 copies in the rearranged chromosome. While functional analyses are required to uncover the biological effects of the genomic features linked to this rearrangement, our results offer a unique framework for studying how complex structural polymorphisms drive genome innovation and adaptive diversity.