Phylogeny of Horse Chromosome 5q in the Genus <i>Equus</i> and Centromere Repositioning

Abstract

Horses, asses and zebras belong to the genus Equus and are the only extant species of the family Equidae in the order Perissodactyla. In a previous work we demonstrated that a key factor in the rapid karyotypic evolution of this genus was evolutionary centromere repositioning, that is, the shift of the centromeric function to a new position without alteration of the order of markers along the chromosome. In search of previously undiscovered evolutionarily new centromeres, we traced the phylogeny of horse chromosome 5, analyzing the order of BAC markers, derived from a horse genomic library, in 7 Equus species (E. caballus, E. hemionus onager, E. kiang, E. asinus, E. grevyi, E. burchelli and E. zebra hartmannae). This analysis showed that repositioned centromeres are present in E. asinus (domestic donkey, EAS) chromosome 16 and in E. burchelli (Burchell’s zebra, EBU) chromosome 17, confirming that centromere repositioning is a strikingly frequent phenomenon in this genus. The observation that the neocentromeres in EAS16 and EBU17 are in the same chromosomal position suggests that they may derive from the same event and therefore, E. asinus and E. burchelli may be more closely related than previously proposed; alternatively, 2 centromere repositioning events, involving the same chromosomal region, may have occurred independently in different lineages, pointing to the possible existence of hot spots for neocentromere formation. Our comparative analysis also showed that, while E. caballus chromosome 5 seems to represent the ancestral configuration, centric fission followed by independent fusion events gave rise to 3 different submetacentric chromosomes in other Equus lineages.

Uncoupling of Satellite DNA and Centromeric Function in the Genus Equus

“…Cross-species chromosome painting has confirmed the great karyotypic variability of this genus [19]. In addition, we have shown that at least nine centromere repositioning events took place during the evolution of this genus, six of which occurred in E. asinus (donkey) [12],[20] and one of which occurred in horse chromosome 11 (ECA 11). These results demonstrate that the phenomenon of centromere repositioning played a key role in the rapid karyotypic evolution of the equids and point to these species as an ideal model system for the analysis of neocentromere formation and centromere evolution.…”

Discovery and Comparative Analysis of a Novel Satellite, EC137, in Horses and Other Equids

“…It is worth noting that the centromere of horse chromosome 11 lacks any satellite DNA, as it was demonstrated in our previous horse genome sequencing work [Wade et al, 2009]. The absence of FISH-detectable 37cen and 2PI signals from the centromere of several domestic donkey, Grevy's zebra and Burchelli's zebra chromosomes [Piras et al, 2009[Piras et al, , 2010 raises the question whether satellite DNA, belonging to other families, might be present at such centromeres. To investigate this possibility, we performed FISH analyses on the chromosomes of the 4 species, using their total genomic DNA as probe [Piras et al, 2010].…”

“…Still, it must be taken into account that a very small amount of the 37cen satellite, under the resolution level of FISH analysis, may be present. Moreover, we want to stress that in the horse (ECA11cen), and presumably in the domestic donkey and in the zebras, satellite-free centromeres exist [Wade et al, 2009;Piras et al, 2009Piras et al, , 2010; therefore, it cannot be ruled out that key single copy sequences could play a role in centromere function also in some satellite-positive centromeres.…”

Equine clinical genomics: A clinician's primer

“…Other sites have been identified on ECA 1 and ECA 16 that appear to be fragile and potentially predisposed to rearrangement (Lear et al 2008). Further investigation revealed repositioned centromeres in the donkey and one species of zebra, further defining the evolutionary relationship between species within this genus (Piras et al 2009; Piras. et al 2010).…”

“…Diploid chromosome numbers within the genus range from 32 to 66 (Piras et al 2009) in the mountain zebra and Przewalski’s horse respectively; the domestic horse has a diploid number of 64. An “evolutionary new centromere” was identified on equine chromosome 11 (ECA 11) in a region highly conserved across mammalian species, with the horse being the only mammal known to possess a centromere in this location (Wade et al 2009).…”