Reciprocal chromosome painting among human, aardvark, and elephant (superorder Afrotheria) reveals the likely eutherian ancestral karyotype

Abstract

The Afrotheria, a supraordinal grouping of mammals whose radiation is rooted in Africa, is strongly supported by DNA sequence data but not by their disparate anatomical features. We have used flow-sorted human, aardvark, and African elephant chromosome painting probes and applied reciprocal painting schemes to representatives of two of the Afrotherian orders, the Tubulidentata (aardvark) and Proboscidea (elephants), in an attempt to shed additional light on the evolutionary affinities of this enigmatic group of mammals. Although we have not yet found any unique cytogenetic signatures that support the monophyly of the Afrotheria, embedded within the aardvark genome we find the strongest evidence yet of a mammalian ancestral karyotype comprising 2n = 44. This karyotype includes nine chromosomes that show complete conserved synteny to those of man, six that show conservation as single chromosome arms or blocks in the human karyotype but that occur on two different chromosomes in the ancestor, and seven neighbor-joining combinations (i.e., the synteny is maintained in the majority of species of the orders studied so far, but which corresponds to two chromosomes in humans). The comparative chromosome maps presented between human and these Afrotherian species provide further insight into mammalian genome organization and comparative genomic data for the Afrotheria, one of the four major evolutionary clades postulated for the Eutheria.

Figure 1

Bivariate chromosome sorting of the aardvark, O. afer (a), and the African elephant, L. africana (b), showing chromosomal assignments to flow karyotypes of each species. Each flow peak was shown to represent a single aardvark chromosome type. In the case of the elephant, pure sorts of single chromosomes were obtained for 23 chromosomes with the two homologues of chromosome 7 and 24, each resolved into a different peak due to heterochromatic differences, and six peaks contained two chromosomes.

Figure 2

A schematic summary of the genome-wide correspondence among aardvark (Oaf, Left), human (Hsa, Center), and African elephant (Laf, Right) chromosomes as determined by cross-species reciprocal chromosome painting using the human idiogram as reference.

Figure 3

G-banded karyotype of the aardvark O. afer (2n = 20). The region of homology to the respective human chromosomes is shown to the right of each aardvark chromosome pair.

Figure 4

G-banded comparisons of homologues between the Asian (Ema) and African (Laf) elephants. The regions of homology to the respective human chromosomes are shown to the right of each elephant chromosome pair. Karyotypic differences between the two elephant species are attributable to variation in the amount and position of heterochromatin.

Figure 5

The eutherian ancestral karyotype. Chromosomes are ordered according to approximate length. Numbers to the right indicate human homologues. The position of centromeres is not shown, because this cannot be deduced from fluorescence in situ hybridization using chromosome-specific painting probes. Homology with the human Y chromosome is not shown because it was not hybridized.