Evolution of Karyotypes in Chameleons

A study examines the evolution of karyotypes in chameleons, focusing on chromosome number, morphology, and sex determination systems. Chameleons exhibit high variability in their karyotypes compared to other squamates, with differences in chromosome structure and interstitial telomeric signals (ITS). However, the location of ribosomal DNA (rDNA) remains relatively conserved across species.

Researchers conducted cytogenetic analyses on 16 chameleon species from 8 genera, covering a broad phylogenetic spectrum. Their findings suggest that the ancestral chromosome number for chameleons is 2n = 36, similar to other members of the Iguania clade, including agamids and iguanas. Over time, chameleons have shown a tendency toward chromosome number reduction, although exceptions exist. For instance, Rieppeleon brevicaudatus was found to have 2n = 62 chromosomes, an unusually high count among squamates, likely resulting from chromosomal fission events.

The study also investigated sex chromosome differentiation in chameleons. While Furcifer species exhibit highly differentiated ZZ/ZW sex chromosomes, other chameleon species showed little to no sexual differences in their karyotypes. Differential staining and comparative genomic hybridization failed to reveal distinct sex-linked chromosomal markers in most species, suggesting that sex chromosomes in chameleons are poorly differentiatedcompared to other reptiles.

These findings contribute to the broader understanding of chromosomal evolution in squamates. The study highlights the complexity of karyotype changes, including chromosome fusions, fissions, and structural modifications that have shaped chameleon genomes over time. The presence of interstitial telomeric sequences in certain species suggests that chromosomal rearrangements have played a significant role in their evolutionary history.

The research also raises questions about the mechanisms driving karyotype evolution in chameleons. The observed chromosomal diversity may be linked to adaptive pressures, such as habitat specialization or reproductive strategies. Additionally, the lack of strong sex chromosome differentiation in most species suggests that sex determination mechanisms in chameleons may be more flexible than previously thought.

Overall, this study provides valuable insights into the genetic and evolutionary dynamics of chameleons, emphasizing the importance of cytogenetic research in understanding reptilian biodiversity. Future studies should explore functional aspects of chromosome variation, including its impact on development, reproduction, and adaptation in different ecological contexts.

Original publication: 

Rovatsos, M., Altmanová, M., Johnson Pokorná, M., Velenský, P., Sánchez Baca, A., & Kratochvíl, L. (2017). Evolution of Karyotypes in Chameleons. Genes, 8(12), 382. https://doi.org/10.3390/genes8120382.