Complete mitogenomes from Kurdistani sheep: abundant centromeric nuclear copies representing diverse ancestors

Abstract

The geographical centre of domestication and species diversity for sheep (Ovis aries) lies around the Kurdistan region of Northern Iraq, within the ‘Fertile Crescent’. From whole genome sequence reads, we assembled the mitochondrial genomes (mtDNA or mitogenome) of five animals of the two main Kurdistani sheep breeds Hamdani and Karadi and found they fitted into known sheep haplogroups (or matrilineages), with some SNPs. Haplotyping 31 animals showed presence of the main Asian (hpgA) and European (hpgB) haplogroups, as well as the rarer Anatolian haplogroup hpgC. From the sequence reads, near-complete genomes of mitochondria from wild sheep species (or subspecies), and even many sequences similar to goat (Capra) mitochondria, could be extracted. Analysis suggested that these polymorphic reads were nuclear mitochondrial DNA segments (numts). In situ hybridization with seven regions of mitochondria chosen from across the whole genome showed strong hybridization to the centromeric regions of all autosomal sheep chromosomes, but not the Y. Centromeres of the three submetacentric pairs and the X chromosomes showed fewer copies of numts, with varying abundance of different mitochondrial regions. Some mitochondrial-nuclear transfer presumably occurred before species divergence within the genus, and there has been further introgression of sheep mitochondrial sequences more recently. This high abundance of nuclear mitochondrial sequences is not reflected in the whole nuclear genome assemblies, and the accumulation near major satellite sequences at centromeres was unexpected. Mitochondrial variants including SNPs, numts and heteroplasmy must be rigorously validated to interpret correctly mitochondrial phylogenies and SNPs.