Abstract: Although cytogenetics studies in cuckoos (Aves, Cuculiformes) have demonstrated an
interesting karyotype variation, such as variations in the chromosome morphology and diploid
number, their chromosome organization and evolution, and relation with other birds are poorly
understood. Hence, we combined conventional and molecular cytogenetic approaches to investigate
chromosome homologies between chicken and the smooth-billed ani (Crotophaga ani). Our results
demonstrate extensive chromosome reorganization in C. ani, with interchromosomal rearrangements
involving macro and microchromosomes. Intrachromosomal rearrangements were observed in some
macrochromosomes, including the Z chromosome. The most evolutionary notable finding was a
Robertsonian translocation between the microchromosome 17 and the Z chromosome, a rare event in
birds. Additionally, the simple short repeats (SSRs) tested here were preferentially accumulated in the
microchromosomes and in the Z and W chromosomes, showing no relationship with the constitutive
heterochromatin regions, except in the W chromosome. Taken together, our results suggest that
the avian sex chromosome is more complex than previously postulated and revealed the role of
microchromosomes in the avian sex chromosome evolution, especially cuckoos.
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