The ancestral eutherian karyotype is present in Xenarthra

Abstract

Molecular studies have led recently to the proposal of a new super-ordinal arrangement of the 18 extant Eutherian orders. From the four proposed super-orders, Afrotheria and Xenarthra were considered the most basal. Chromosome-painting studies with human probes in these two mammalian groups are thus key in the quest to establish the ancestral Eutherian karyotype. Although a reasonable amount of chromosome-painting data with human probes have already been obtained for Afrotheria, no Xenarthra species has been thoroughly analyzed with this approach. We hybridized human chromosome probes to metaphases of species (Dasypus novemcinctus, Tamandua tetradactyla, and Choloepus hoffmanii) representing three of the four Xenarthra families. Our data allowed us to review the current hypotheses for the ancestral Eutherian karyotype, which range from 2n = 44 to 2n = 48. One of the species studied, the two-toed sloth C. hoffmanii (2n = 50), showed a chromosome complement strikingly similar to the proposed 2n = 48 ancestral Eutherian karyotype, strongly reinforcing it.

Figure 1. Partial Metaphases of D. novemcinctus, T. tetradactyla, and C. hoffmanii after In Situ Hybridization with Human Chromosome–Specific Probes

(A–C) D. novemcinctus, (D–F) T. tetradactyla, and (G–I) C. hoffmanii. Two probes were used in each experiment, and the hybridizations were detected with avidin–FITC (green) and antidigoxigenin–rhodamine (red). The chromosomes were counterstained with DAPI, and the human chromosome probes used are indicated.

Figure 2. G-Banded Karyotypes of Female Nine-Banded Armadillo and Female Lesser Anteater

(A) Female nine-banded armadillo D. novemcinctus (2n = 64) and (B) female lesser anteater T. tetradactyla (2n = 54). The correspondence to human chromosomes as revealed by chromosome painting is shown on the left of each chromosome. The question mark indicates questionable results. Some regions were not painted by any human probe and are probably composed of repetitive sequences (see text).

Figure 3. The Karyotype of Hoffmann's Two-Toed Sloth (2n = 50) Closely Resembles the Proposed Eutherian Ancestral Karyotype (2n = 48)

(A) G-banded karyotype of a male C. hoffmannii (2n = 50) with the corresponding human chromosomes indicated to the left of each chromosome. (B) Diagram of C. hoffmannii chromosomes. (C) The proposed ancestral Eutherian karyotype (2n = 48) [17,21]. In (B) and (C), the corresponding human chromosomes are each represented by a different color, and their numbers are indicated at the bottom of each chromosome. The chromosomes are drawn only roughly to scale. In (C), the corresponding C. hoffmannii chromosomes are indicated to the left of each ancestral chromosome. The asterisks and lines show the chromosomes that differ between the karyotypes of C. hoffmanii (CHO) and those proposed as ancestral (ANC) for Eutheria (CHO 14 and ANC 10q and 10p [black asterisks]; CHO 13 and 19 and ANC 8q [green asterisks]; and CHO 20 and 21 and ANC 7b/16p [red asterisks]).