A Bird’s-Eye View of Chromosomic Evolution in the Class Aves

Abstract

Birds (Aves) are the most speciose of terrestrial vertebrates, displaying Class-specific characteristics yet incredible external phenotypic diversity. Critical to agriculture and as model organisms, birds have adapted to many habitats. The only extant examples of dinosaurs, birds emerged ~150 mya and >10% are currently threatened with extinction. This review is a comprehensive overview of avian genome (“chromosomic”) organization research based mostly on chromosome painting and BAC-based studies. We discuss traditional and contemporary tools for reliably generating chromosome-level assemblies and analyzing multiple species at a higher resolution and wider phylogenetic distance than previously possible. These results permit more detailed investigations into inter- and intrachromosomal rearrangements, providing unique insights into evolution and speciation mechanisms. The ‘signature’ avian karyotype likely arose ~250 mya and remained largely unchanged in most groups including extinct dinosaurs. Exceptions include Psittaciformes, Falconiformes, Caprimulgiformes, Cuculiformes, Suliformes, occasional Passeriformes, Ciconiiformes, and Pelecaniformes. The reasons for this remarkable conservation may be the greater diploid chromosome number generating variation (the driver of natural selection) through a greater possible combination of gametes and/or an increase in recombination rate. A deeper understanding of avian genomic structure permits the exploration of fundamental biological questions pertaining to the role of evolutionary breakpoint regions and homologous synteny blocks.

Satellitome Analysis in the Southern Lapwing (Vanellus chilensis) Genome: Implications for SatDNA Evolution in Charadriiform Birds

“…The Z and W chromosomes of all birds are descended from the same ancestral autosomal pair, most likely between 60 and 100 million years ago [74]. Most bird species have a W chromosome that is frequently small and extremely heterochromatic [2,4,[75][76][77][78][79][80]. However, an overview of bird's sex chromosome evolution refuted the theory that W chromosomes gradually shrank during evolution, indicating their high plasticity [81,82].…”

“…The extensive chromosomal variation found in birds makes them an excellent group for investigating evolutionary mechanisms responsible for karyotype reorganization and stability [1][2][3][4]. For these kinds of investigations, birds from Charadriiformes constitute an outstanding model.…”

The rate of W chromosome degeneration across multiple avian neo-sex chromosomes

“…The sex chromosomes of birds, which formed > 100 million years ago (Mya), were previously thought to be extremely stable and share the same gene content across the entire clade 16 , 17 . Recently, however, this notion has been challenged with neo-sex chromosomes being found in several lineages 18 , including in parrots, cuckoos, crested ibis, and songbirds 19 – 24 .…”