# Comparative Chromosomal Analysis of the Z Chromosome in South American Bird Species Shows a High Rate of Intrachromosomal Rearrangements

## TL;DR

This study compares Z chromosomes in South American birds and finds frequent rearrangements, offering insights into avian genomic evolution.

## Contribution

The study reveals high rates of intrachromosomal rearrangements in avian Z chromosomes using interspecies FISH with BAC probes.

## Key findings

- Most bird species showed Z chromosome rearrangements differing from chicken and zebra finch.
- Nannopterum brasilianum and Jacana jacana shared a BAC order with chicken but had centromere repositioning.
- Z chromosomes in most species were conserved in size and homologous to snake Chromosome 2 and W regions.

## Abstract

Background: Intrachromosomal rearrangements in birds play a subtle but important role in shaping genomic evolution, phenotypic diversity and speciation. However, the avian sex chromosome system (homogametic ZZ males; heterogametic ZW females) remains relatively understudied, and evolutionary rearrangements of the Z chromosome have not been mapped in most species. To address this, we employed universally hybridizing avian Z chromosome probes to metaphases of 11 avian species from South America. Methods: Chromosome preparations were obtained from fibroblast cell cultures of 11 birds representing nine different orders; four bacterial artificial chromosome (BAC) probes were used in our interspecies fluorescence in situ hybridization (FISH) experiments. We identified chromosomal rearrangements in the species investigated, tracing the evolution of the Z chromosome in these species through comparison with reptiles from Southeast Asia (three snake species used as an outgroup), along with two reference species: chicken (Galliformes) and zebra finch (Passeriformes). Results: We observed high rates of intrachromosomal rearrangements in the avian Z chromosome, with most species showing different patterns from chicken and zebra finch. Nannopterum brasilianum (Suliformes) and Jacana jacana (Charadriiformes) showed the same BAC order as chicken, but centromere repositioning was evident. Apart from Piciformes, all other species exhibited a conserved Z chromosome size. The corresponding Z chromosome sequences were homologous to regions of the long arms of Chromosome 2 and W in snakes but not on the Z chromosomes. Conclusions: Comparative analysis of the Z chromosome across avian orders provides important insights into the dynamics of avian sex chromosomes and the evolution of sex chromosome systems in general.