We are proud of the diversity of our flock of RBST-registered British Soay Sheep. At its maximum, our collection of 100 adults included 74 breeding ewes who were the offspring of almost 60 different mothers and over 40 different fathers. Although our operation is now much smaller, and we no longer set up six breeding groups each fall, we have left the following discussion of our former large-scale formal rotational conservation breeding system intact. We hope it will be of help to other Soay breeders who also are committed to preserving the diversity of Soay Sheep.
Each spring's lambs serve two purposes from the standpoint of conservation breeding. Most of the lambs will be dispersed as starter flocks to people interested in participating in the overall conservation of this ancient and unique breed. We set up our breeding groups each year according to a plan, described below, that ensures that each year's lamb crop contains a wide range of genetic combinations and, importantly, that there will also be a lot of variation between years as well. This is the first of the two purposes.
Of course we need to hold back some lambs each year – young rams to serve as flock sires two to five years later, and ewe lambs to replace aging ewes. Since we no longer bring new animals into our flock, the ewe lambs and ram lambs we keep are the only way we have to perpetuate the flock's genetic makeup. So we want to be very deliberate in what we are doing. This, then, is the second purpose.
There are two elements to our overall conservation breeding strategy. The first involves flock size, simply to keep a large number of sheep. The second is to maintain our flock as three distinct bloodlines through a system of rotational breeding. Let's consider these in sequence.
Genetic theory teaches us that, within reason, larger populations are less likely to lose genetic diversity through chance alone. The idea is that the number of breeding animals should be large relative to the amount of genetic diversity to be preserved. The amount of diversity can be thought of as the number of distinct alternative forms of genes, or alleles. In other words, we want to have enough breeding animals so that every allele is carried by several sheep at least. This minimizes the chances that a particular allele will be lost forever simply because no sheep that carried it happened to pass it on.
Conservation breeding isn't really about conserving sheep but rather sheep genes. The sheep themselves are just containers of genes, and their role is to transmit those genes to new containers, their offspring. Suppose that British soay as a group carry 12 alternative forms, or alleles, of some particular gene. Assume we are lucky enough that our flock as a group carries all 12 of these alternatives. The point of conservation breeding is to ensure that all 12 will be passed on the next generation. Because an animal transmits only half of the genes it carries to each offspring, it is best if each allele is carried by multiple animals. This increases the chances that every allele will be passed on to at least one lamb.
In setting up our breeding system, we were strongly influenced by the suggestions to be found in the Chapter 9 of the ALBC Conservation Breeding Handbook. Like the ALBC system, our approach involved dividing our flock into three cohorts, or “bloodlines.” But whereas the ALBC plan uses a single ram each year, we used three rams for several years and now we typically use at least six. We do this to reduce the overall level of inbreeding in our flock. It turns out that the use of multiple rams has a very large effect in minimizing inbreeding.
The basic idea of rotational breeding is to balance inbreeding and outbreeding. We want to be able to cross our ewes to relatively unrelated rams. But in order to do this we have to make sure that we have relatively unrelated pairings available. That is why we maintain the three bloodlines. But to maintain a bloodline requires line crosses within that bloodline, and those involve relatively high levels of inbreeding. But that's a necessary evil in order to make outcrosses, which have lower levels of inbreeding, possible.
If you sense something of a Catch-22 here, your instincts are good. It is in fact a balancing act. We alternate between line crosses within a bloodline (higher inbreeding) and out crosses between bloodlines (lower inbreeding). Two years out of three, ewes are out crossed to rams of a different bloodline. But every third year is a line cross year, and ewes are bred to a ram of their own bloodline.
This one-in-three balance keeps the overall level of inbreeding at a manageable level, while maintaining a degree of genetic separation between the groups. It insures that two years out of three, ewes can be crossed with a relatively unrelated ram.
|Year||A ewes||B ewes||C ewes|
|07/08||B ram||A ram||C ram|
|08/09||A ram||C ram||B ram|
|09/10||C ram||B ram||A ram|
|10/11||B ram||A ram||C ram|
|11/12||A ram||C ram||B ram|
|12/13||C ram||B ram||A ram|
|13/14||B ram||A ram||C ram|
|14/15||A ram||C ram||B ram|
As you can see, the current cycle (2014-15) is a linecross year for the A bloodline, so the A ewes are bred to a A ram. The B ewes are outcrossed with a C ram, and the C ewes are outcrossed to a B ram. Several A ram lambs will be retained in the flock to serve as flocksires – two each year – beginning in 2016 when they will be 18 months old. Rams from the two outcross groups (BxC and CxB) are sold and not used as flocksires in our flock. Any ewe lambs retained as replacement breeders are assigned to the same bloodline as their mother.