From Micro to Macro: Avian Chromosome Evolution is Dominated by Natural Selection.

Area of Science:

• Evolutionary biology
• Genomics
• Comparative genomics

Background:

• Avian karyotypes exhibit substantial variation in chromosome number, challenging previous assumptions of conservation.
• The evolutionary mechanisms driving this chromosomal diversity in birds are not well understood.

Purpose of the Study:

• To investigate the evolutionary drivers of chromosome number variation across bird species.
• To estimate rates of chromosome evolution, including fusions and fissions, for total, micro-, and macrochromosomes.
• To assess the influence of other evolving traits and population size on karyotype evolution.

Main Methods:

• Fitted probabilistic models of chromosome number evolution to avian genomic data.
• Simultaneously analyzed rates of chromosome fusion and fission.
• Correlated rearrangement rates with migratory behavior and population size.

Main Results:

• Observed a higher rate of chromosome fusion than fission across avian lineages.
• Identified Passeriformes, particularly migratory species, as major contributors to the fusion bias.
• Found a strong correlation between microchromosome and macrochromosome rearrangement rates, suggesting genome-wide processes.
• Demonstrated that larger population sizes correlate with increased rates of both fusion and fission, indicating a role for positive selection.

Conclusions:

• Evolutionary dynamics of avian karyotypes are characterized by a bias towards chromosome fusions.
• Genome-wide mechanisms and positive selection in large populations are key drivers of karyotype divergence in birds.
• Beneficial mutations likely play a significant role in shaping chromosome number evolution in certain avian clades.