## 1 Introduction  
Domestic donkeys are one of the most common species in agricultural production and transportation especially in the small-scale peasant economic model [1]. They were used to pull or bear heavy cargos in agricultural activities. As agricultural machines become the major source of power in the current society, donkeys have been utilized for the purposes of food production, such as meat, milk and skin. There are 24 local donkey breeds in China, and the Dezhou donkey is one of the most typical large donkey breeds (body height greater than 130 cm), which is also an important donkey genetic resource in China. Population genomics analysis indicate that donkeys were domesticated in Africa, which is consistent with archeological research results [2; 3]. In 2020, the stock of donkey in China is 2.32 million. In Shandong province of China, the output value of the whole donkey industry in 2020 was about 16 billion US dollars. In mammals, the vertebrate spinal column consists of five parts: the cervical, thoracic, lumber, sacral, and caudal [4; 5]. The number of cervical vertebrae are nearly constant at seven [6]; increase in the sacral and caudal vertebrae number could not effectively improve the meat production and skin yield. However, one more thoracic verteb means one more spine in vivo axis, and the increase in thoracic or lumbar vertebrae cannot only increase the body length, but also provide greater accommodation space for organs. Body length is an important factor affecting livestock slaughter performance [7], as a longer carcass means longer loins and more meat production, a larger body size means a larger body surface area and more skin yield. A variation in vertebrae number is found in a few domestic animals such as pigs [8; 9; 10], sheep [11; 12; 13], and cattle [14]. In European commercial pig breeds, a 14% variation in body length can be attributed to vertebrae number variation, for each additional vertebrae, there was an increase in length of about 15 mm [15]. In Asia Kazakh sheep, the carcass length of sheep possessing 20 thoracolumbar vertebrae increased 2.22-2.93 cm compared with that of sheep possessing 19 thoracolumbar vertebrae [13]. Few studies reported vertebrae number variation in equine animals. [PERSON] reported variation in the lumbo-sacral spine of the Przevalski and domestic horse [16]. [PERSON] reported variation in the number of lumbar vertebrae in the Arabian horse, mule and donkey ([PERSON] et al., 1975). [PERSON] and [PERSON] reported variation in thoracic and lumbar vertebrae number in 12 donkeys, the range of thoracic vertebrae number was 17-19 and the rage of lumbar vertebrae number was 5-6 [17]. However, the relationship between carcass weight and vertebrae number variation in Chinese donkeys remained unexplored. Multi-vertebrae trait is complex and regulated by a lot of genes and signal pathways. Several genes have been identified to regulate development of the vertebrae in animals. NR6A1 is an important candidate gene affecting the development of the thoracolumbar vertebrae. There have been many studies on _NR6A1_ and vertebrae number in livestock. [PERSON] et al. mapped the genetic causes of different vertebrae between commercial pigs (Large White, Duroc) and most domestic pigs to the _NR6A1_ gene [18; 19; 20; 21]. [PERSON] et al. detected allele frequencies of the _NR6A1_ causative mutation (c.575T > C) in 519 pigs from three Western and seven Chinese breeds, and found allele T for increased vertebral number was fixed in Western breeds, but scare in most of the Chinese native breeds [21]. The association analysis revealed that chr1:299291323C > T, located in the second intron of _NR6A1_, were significantly associated with the number of vertebrae in the Tongcheng \(\times\) Large White crossbreed population [22]. These findings suggest that the _NR6A1_ gene is considered a strong candidate for affecting the number of vertebrae in donkeys. Through the previous study of our research group, several SNVs were detected in NR6A1 [2].