Chromosomal diversification and evolutionary trajectories in Neotropical spiny rats: the first karyotype of <i>Echimys chrysurus</i> Zimmermann, 1780 and a comparative synthesis across Echimyidae

Echimyidae, the largest family of Hystricognathi rodents, comprises 28 genera and 103 species across South and Central America and shows significant karyotypic variation, with diploid numbers (2n) from 14 to 118 and autosomal fundamental numbers (FNa) from 14 to 168. However, eight genera still lack karyotype descriptions. This study describes the karyotype of Echimys chrysurus from Amazonian Brazil (Paragominas and Santa Bárbara do Pará), using G-banding, C-banding, and FISH with telomeric and 18S rDNA probes. The species has a 2n = 80/FNa = 134 karyotype. The autosomal set consists of 28 meta/submetacentric pairs (1-28) and 11 acrocentric pairs (29-39); the X chromosome is a large acrocentric and the Y chromosome is a small acrocentric. Constitutive heterochromatin is centromeric in all autosomes and the X. Telomeric probe signals are distal, and the 18S rDNA probe shows a single interstitial signal. Review of 118 published karyotypes (66 species, 20 genera) integrated with multilocus phylogeny reveals bidirectional chromosomal evolution from an inferred ancestral karyotype (2n ≈ 60), through multiple lineage-specific Robertsonian translocations, centric fissions, and pericentric inversions. Arboreal Echimyini have higher 2n values, while several terrestrial or semifossorial taxa display marked reductions; these changes align with Miocene-Pliocene dispersals, Andean uplift, and habitat transitions to canopy, open areas, and flooded forests, suggesting rapid chromosomal change and speciation driven by ecological and geographic factors.