The use of endogenous retroviruses as markers to describe the genetic relationships among some local Swedish sheep breeds

Sammanfattning: The present modern sheep is believed to have evolved from its wild ancestors 2.5 million years ago. Furthermore archeological evidence suggest that sheep has been domesticated by man since 8500-9000 years BC for their beneficial products to man including; milk, meat, skin and wool. Through centuries, sheep have been spread from the center of domestication to different parts of the world where they have adapted to environmental factors in such areas giving rise to what is known today as local or indigenous breeds. Indigenous breeds are generally characterized by the presence of horns in both rams and ewes, dark and coarse fleece and moulting coat. Such local breeds in Sweden include; Dala Fur sheep, Gestrike sheep, Helsinge sheep, Klövsjö sheep, Svärdsjö sheep, Åsen sheep, Roslagen sheep, Värmland sheep, Gotland sheep and Gute sheep. Though there have been some studies on some of these breeds, information on genetic distinctiveness between them remains scarce. The sheep genome contains more than 30 polymorphic endogenous proviruses (enJSRV) that are related to the Jaagsiekte retrovirus (JSRV), a Beta retrovirus known for causing Ovine Pulmonary Adenocarcinoma. These proviruses are remains of exogenous retroviruses that infected germ cells of ancestors or great grandparents and have been passed on in a Mendelian manner to the present day sheep. Some of these endogenous proviruses have been found to have different functions in sheep species including the protection of hosts from infection by related defective exogenous viruses and also aid in conception and implantation during reproduction. Recently these polymorphic loci have been used to study the domestication trend of sheep and genetic distinctiveness between some sheep breeds. The aim of the current study was to use 6 of the polymorphic proviruses to define genetic difference between five local Swedish breeds. The presence or absence of each of these proviruses in the studied individuals was tested using polymerase chain reaction (PCR) and agarose gel-electrophoresis on genomic DNA of 66 individuals. Results were analyzed based on the frequencies of the individual loci in the tested samples, retrotype or provirus combination frequencies and a principal component analysis was performed based on the provirus frequencies to visualize genetic distances between studied breeds. The study revealed a close genetic relationship sheep between Gute, Swedish fine wool sheep and Roslag. Furthermore we also found a strong genetic closeness between Värmland and Klövsjö sheep breeds. Although all the local Swedish breeds tested were found to have genomes of ancestral or primitive makeup, Värmland and Klövsjö were more primitive than others. Further comparison of each breed with Texel showed that Swedish fine wool sheep was more close to the Texel breed than any other breed in the study. However, the number of samples used and their distribution among herds was small. Hence, more samples are needed for a larger study to allow for better conclusions on the genetic diversity or relatedness between these breeds and other Swedish breeds not included in our study.