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

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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.