Karyotype Evolution in Birds: From Conventional Staining to Chromosome Painting

Abstract

In the last few decades, there have been great efforts to reconstruct the phylogeny of Neoaves based mainly on DNA sequencing. Despite the importance of karyotype data in phylogenetic studies, especially with the advent of fluorescence in situ hybridization (FISH) techniques using different types of probes, the use of chromosomal data to clarify phylogenetic proposals is still minimal. Additionally, comparative chromosome painting in birds is restricted to a few orders, while in mammals, for example, virtually all orders have already been analyzed using this method. Most reports are based on comparisons using Gallus gallus probes, and only a small number of species have been analyzed with more informative sets of probes, such as those from Leucopternis albicollis and Gyps fulvus, which show ancestral macrochromosomes rearranged in alternative patterns. Despite this, it is appropriate to review the available cytogenetic information and possible phylogenetic conclusions. In this report, the authors gather both classical and molecular cytogenetic data and describe some interesting and unique characteristics of karyotype evolution in birds.

Keywords: avian cytotaxonomy; avian genome; classical and molecular cytogenetics; sex chromosomes.

Figure 1

Chromosomal diversity in birds: (A) the most typical formulae, with 2n close to 80, such as in Vanellus chilensis (2n = 78); (B) an extreme high diploid number, such as Ramphastos tucanus (2n = 112), an atypical low diploid numbers: (C) Myiopsitta monachus (2n = 48); and an example of bird of prey (D) Spizaetus tyrannus (2n = 68). Scale bar: 5µm.

Figure 2

Distribution of 18/28S rDNA (red signals, CY3) in Buteogallus meridionallis (Accipitriformes), in the short arm of a medium pair of macrochromosomes. Chromosomes counterstained with DAPI (blue). rDNA: ribosomic DNA. Scale bar: 5 µm.

Figure 3

Refined putative avian ancestral karyotype, based on the homology with Leucopternis albicollis. GGA: Gallus gallus; LAL: Leucopternis albicollis; PAK: Putative Avian Ancestral Karyotype.

Figure 4

Result of comparative chromosome painting using probe, corresponding to LAL26, labeled in red (CY3), on metaphases of Rhea americana. These probes hybridize on the same position as in Gallus gallus, confirming that the organization of Ratites and Gallus are similar and might correspond to the ancestral organization found in PAK. Scale bar: 5µm.

Figure 5

Result of comparative chromosome painting using corresponding to LAL18, labeled in red (CY3), on metaphases of Pulsatrix perspicillata. These probes hybridize on the same position as in G. gallus, confirming that despite the reorganization of owl’s chromosomes, they retained the ancestral organization found in PAK. Scale bar: 5 µm.

Figure 6

Chromosomal rearrangements based on PAK plotted in a current avian phylogeny (Jarvis et al.) [4]. Rearrangements are represented by fissions (red) and fusions (blue). Orders in red represent those without chromosomal data up to now, while the blue ones represent groups currently without chromosomal synapomorphies.