Atypical Presence of Interstitial Telomeric Sequences in Thamnophilus Species (Passeriformes: Thamnophilidae).

Introduction: Thamnophilidae (typical antbirds) are a diverse family of insectivorous passerine birds restricted to neotropical forests, encompassing 237 species, of which only five have been studied cytogenetically.

Methods: To investigate the chromosomal evolution of this group, we applied classical and molecular cytogenetic techniques, including conventional staining, C-banding, and fluorescence in situ hybridization (FISH) with probes for repetitive telomeric sequences (TTAGGG)5 and 18S rDNA, in two representative species: Thamnophilus caerulescens and Thamnophilus ruficapillus.

Results: The karyotypes of T. caerulescens and T. ruficapillus comprise 80 and 82 chromosomes, respectively. In addition to a possible fission in T. ruficapillus, morphological differences suggest the occurrence of pericentric inversions in the chromosomes of this species. The patterns of constitutive heterochromatin differed between the species: both showed centromeric markings and heterochromatin on the W chromosome, but T. ruficapillus also exhibited interstitial markings on seven chromosomal pairs. Both species presented interstitial telomeric sequences (ITSs) in the first seven pairs, which corresponded to constitutive heterochromatin in T. ruficapillus. The 18S rDNA probe hybridized to a single pair of microchromosomes in T. caerulescens and two pairs in T. ruficapillus.

Conclusion: This study revealed novel patterns of constitutive heterochromatin in T. ruficapillus and ITSs in both species, which have not been previously observed in Passeriformes. The correspondence between constitutive heterochromatin and ITSs in T. ruficapillus suggests that these sequences are composed of repetitive DNA highly similar to telomeric sequences and/or are remnants of pericentric inversions, whereas in T. caerulescens, other mechanisms seem to be involved. The differences in observed patterns highlight distinct chromosomal evolution between these species, emphasizing the diversity within the family Thamnophilidae and the genus Thamnophilus, in contrast to the conserved patterns typically observed in the class Aves.