1 | INTRODUCTION
Mitochondrial DNA (mtDNA) is an extranuclear genetic material with a specific structure that does not undergo recombination during generational transmission and is maternally derived (Xia et al., 2019). As a genetic marker, mtDNA is of great significance in genetic and evolutionary studies of livestock (Bruford et al., 2003; Saccone et al., 2000). mtDNA contains exons and noncoding regions − the control region (CR) − also known as the displacement-loop region (D-loop) (Wolf et al., 1999). In the mammalian mtDNA D-loop, the most variable region exists between tRNAPro and the large conserved sequence block (Nobushige et al., 1995). Additionally, mtDNA has been used to infer the wild ancestors and determine domestication centers of modern domestic animals, and to study the origins, evolutionary relationships and genetic diversity of several domestic animals (Bruford et al., 2003; Gissi et al., 2000).
The domestication of donkey is generally considered to have taken place in the tropics or subtropics of Africa (Beja-Pereira et al., 2004), though it remains unclear. Studies of donkey mitochondrial sequences have suggested that there were two highly differentiated maternal lineages (Clade I and Clade II) during donkey domestication (Equus asinus ), Clade I clustered clearly with the Nubian wild ass (E. africanus africanus ); Clade II originated from the Somali wild ass (E. africanus somaliensis ) which was probably on the verge of extinction (Beja-Pereira et al., 2004; Chen et al., 2006; Kimura et al., 2011). In recent years, an increasing number of studies on donkeys’ mtDNA have assessed the evolutionary relationship, variation and genetic diversity among Asian, European, and African breeds (Gan et al., 2011; Han et al., 2014; Lei et al., 2007; Ma et al., 2020; Ozkan Unal et al., 2020; Perez-Pardal et al., 2014).
The three groups of Chinese donkey are distinguished based on body size: large, medium, and small donkeys. Huaibei grey donkey (HGD) is a small donkeys with grey fur, and its main area of production is in Huaibei City, Anhui Province. Few reports have analyzed mtDNA D-loop polymorphisms in HGD. Therefore, in the present study we used PCR sequencing to detect partial sequence polymorphisms of the D-loop and analyzed the mtDNA diversity and evolutionary relationships of HGD to inform its management.