Cytogenomic analysis in Seriemas (Cariamidae): Insights into an atypical avian karyotype

Contrasting with most bird species that present an ancestral-like karyotype (with 2n = 80), the only extant Cariamidae birds, the Red-legged (Cariama cristata) and Black-legged (Chunga burmeisteri) Seriemas, have high 2n and atypically large Z chromosomes. This study combined cytogenetic, bioinformatic, and genomic analyses to examine the distinctive characteristics of an unusual bird karyotype, with a focus on repetitive elements and sex chromosomes. Whole-genome alignments and chromosomal painting with a Z-chromosome-specific probe were also performed against the emu (a species with an ancestral-like karyotype). The satellitomes of C. cristata and C. burmeisteri were composed of only four and 6 long satDNAs, respectively. These satDNAs showed similarity with other repetitive sequences, mostly transposable elements, and were mapped in the pericentromeric regions of several chromosome pairs. CcrSat02-1104 mostly covered the Z and W sex chromosomes, besides being spread throughout additional chromosomes. Interstitial telomeric sites were not detected, even in the Z chromosome, and none of the 16 microsatellites tested showed positive signals on the C. cristata chromosomes. The genome alignments showed that the karyotype evolution that occurred in C. cristata may have involved significant chromosomal reshuffling, particularly fissions. Notwithstanding certain internal inversions, the Z chromosome retained homology with that of the emu. However, repetitive sequences also accumulated on the Z chromosome, contributing to its enlargement relative to the pattern observed in ancestral avian groups.