The Chromosomal Organization of Isleria hauxwelli and Its Meaning for the Karyotypic Evolution in Thamnophilidae (Passeriformes).

The order Passeriformes, the most speciose avian order, includes over 6000 species (~60% of all birds). The Amazon Basin alone hosts > 900 bird species, with Passeriformes representing > 500 of them. The family Thamnophilidae comprises ~200 insectivorous species, accounting for 15% of Neotropical passerine richness. Within Thamnophilinae, Isleria is the least speciose genus, containing only two species. It was erected after phylogenetic analyses revealed Myrmotherula as non-monophyletic. To advance knowledge of chromosomal evolution in Passeriformes, we analyzed the karyotype of Isleria hauxwelli using molecular cytogenetics. We found a diploid number of 2n = 84 and a fundamental number (FN) of 92. The BOE-7 probe, corresponding to the ancestral fusion of GGA-9 with two microchromosomes, maintained that configuration without undergoing fission to produce IHA-5. The BOE-14 probe, typically fragmented in all cytogenomically characterized Passeriformes, corresponded to two distinct GGA segments: the R3 portion and a region of unknown homology. This is the first documentation of such chromosomal organization in passerines. The absence of these fissions suggests a possible autapomorphic chromosomal condition within Thamnophilidae. Additionally, a potential fission involving microchromosomes homeologous to probes BOE-15 and BOE-16 is suggested: while these probes mark three microchromosome pairs in GGA, they mark four pairs in I. hauxwelli-a feature not yet observed in other passerines. As most Thamnophilidae remain cytogenetically uncharacterized, systematic study of this family is essential to determine whether these rearrangements represent synapomorphies, autapomorphies, or convergent events.