Report reveals a healthy genetic picture for North Ronaldsay Sheep on the Island of North Ronaldsay.

A project that was recognised as a step into the unknown when it was launched has resulted in a report that reveals that the seaweed-eating sheep of North Ronaldsay have a healthy effective population.

Preserving and maintaining the genetics of the North Ronaldsay sheep was one of the projects identified in RBST’s conservation plan. The first step involved taking nasal pharyngeal swabs from a representative number of the island’s sheep population, which was carried out by the then-Chairman of RBST, Gail Sprake, and her husband Michael. These swabs were used for the genome-wide association study that would assess the genomic structure of the North Ronaldsay sheep on the island and estimate the levels of genetic diversity and inbreeding in the studied population.

RBST Senior Conservation Adviser Tom Blunt says: “This was the first time such a study had been undertaken and there was a real concern that, with an isolated and closed population with relatively low numbers, levels of inbreeding could be high – hence the ‘step into the unknown’. However, what was revealed was that, in terms of genetic diversity, the island has a viable population of North Ronaldsay sheep.”

For the purposes of the study, swabs were collected from 176 individual animals of both genders across seven flocks from around the island. After a series of quality control processes, genotypes from 164 sheep went on to be analysed. While the analysis of the genomic diversity showed considerable historic inbreeding, it also showed that only very limited recent inbreeding has been added on.

Effective population
In fact, the study showed that the effective population size of the studied population was broadly of a similar size to the actual size of the studied population. And since the current population on the island is around 1,500, the number of sheep in the study constitutes a 10% sample which can be considered representative of the entire population.

The results of the study also reflect the fact that flocks mingle on the shore, with results collectively suggesting that genetic links have been gradually developing between flocks, with some flocks more involved in the development of these links than others. Of the seven flocks studied, genetic diversity measurements across and within the flocks were broadly similar, with only subtle differences revealed between flocks. One flock, however, was genetically the most distinct, with a higher level of inbreeding. This is believed to be a flock that is geographically more isolated, being more or less landlocked on one area of the shore. These sheep stay in that location unless storms have swept the seaweed away, when a gate is opened to allow them to feed elsewhere and then they will mingle with sheep from different flocks.

Prof Georgios Banos comments: “This study gave us insights into the genetic profile of a distinct population of sheep reared on the island. Using genomic data, we were able to determine population structure and assess levels of genetic diversity and inbreeding, with data analysis revealing that diversity remains within the population after decades of random breeding. These results can inform future strategies to maintain or even enhance diversity levels and control inbreeding in the population.”

A light touch
Management of the North Ronaldsays on the island is carried out with what Michael Scott, Clerk to the Sheep Court, describes as ‘a light touch’. In fact, it could be said that all breeding decisions are taken by the sheep themselves as matings are random, with no way of identifying the pairings.

Although all of the sheep are owned in individual flocks, when on the shore, they intermingle. Rather than grouping by flock, the sheep remain generally in their own ‘clow-gang’ (pronounced clowjung), the particular portion of the shore to which they have become attached and where they are accustomed to feeding. Michael Scott says: “It is almost as though each sheep is attached to its clow-gang by a piece of elastic – they are free to roam but will go only so far and always return to that section of the shore they are most familiar with.
Saying that, there are always one or two outliers that turn up far from the rest of their flock when the sheep on the shore are punded (corralled into stone enclosures known as “punds”) in spring and summer, but those travellers are exceptions to the rule.”

The males live on the shore year round, and for the sheep that are due to be sold for meat, this is the key to their unique flavour. Mating takes place on the shore in late November and through December and ewes are brought in to their respective crofts for lambing in the spring and stay on the grass until they are returned to the shore with their lambs in August or September.

In good shape
There is a developing market for North Ronaldsay meat at the high end of the market and when deciding which lambs are to be kept as rams the individual owners of each flock will have their own criteria but will always select a ram only from the very best of his or her lambs. Rams are therefore chosen on the basis of quality and good size although colour can also be a factor as the breed comes in a variety of colours and the sheep owners are keen to maintain that diversity. The general view among the crofters is that the flock has improved in terms of quality compared to 20 to 30 years ago, when the numbers of sheep on the shore were driven up by headage payments putting pressure on grazing. In contrast, today’s flock is seen to be very healthy because of its living conditions, a greater quantity and better quality of feed leading to stronger and healthier animals on the shore.

In an initial response to the study, Michael commented on behalf of the group of crofters involved: “We weren’t hugely surprised by the findings. It is clear to the eye that the sheep are in good shape and the study reassures us that the way we are working with them is effective. That is important because we are looking to safeguard genetics that go back some 8,000 years. Whilst no immediate concerns have been raised, this will help us make decisions in the future because we need to make sure we protect what we have here on the island.”

Final Project Report

The science behind the report
The genomic analysis of the North Ronaldsay sheep was led by Prof Georgios Banos at Scotland’s Rural College (SRUC).

• For the purposes of the project nasal swabs were collected from 176 individual sheep assigned to seven flocks on the island of North
  Ronaldsay. Nasal swabs were used to derive genome-wide genotypes for each animal.
• After rigorous quality assurance and control, a total of 164 genotypes each consisting of 40,645 Single Nucleotide Polymorphism (SNP) markers were retained for the ensuing analyses.
• A Principal Component Analysis (PCA) was first conducted on the genotypes to determine potential population structure and examine
  the genotypic profile across the seven flocks and the two animal genders. Linkage Disequilibrium (LD) levels were then assessed in the
  entire population of study and within the subpopulation structures revealed in PCA.
• Animal genotypic data were used to derive several measurements of genetic diversity including levels of genomic inbreeding.
  Results from the study revealed the presence of genetic structure in the studied North Ronaldsay sheep population. Historic inbreeding was evident but not much recent inbreeding was observed. Results collectively suggest that genetic links might have been gradually developing between flocks, but some flocks seem to have been more involved in the development of these links than others. At present, the population is not fully homogeneous and there are satisfactory levels of genetic diversity.

Thanks go to the flock owners and sheep breeders for their participation in this study and Michael and Gail Sprake who collected the nasal samples for sheep genotyping. Funding was provided by RBST, North Ronaldsay resident and Pride of Britain award winner Billy Muir, and the Scottish government RESAS programme.