Gloster Canary Inbreeding
Birds in Focus...

This article is intended for a general audience of Gloster Canary breeders, rather than a technical audience. However, some scientific jargon is unavoidable, so if any of the terms are unfamiliar, please see the Glossary. If you have suggestions for other terms that should be in the glossary, please e-mail me.

Gloster Canaries have known genetic defects that also plague some other breeds of canaries. This is largely a result of the breeders incorrect selection against deleterious traits that are present in modern day examples. When Glosters were developed, with amalgamations of other breeds of canaries they underwent intensive artificial selection and inbreeding, as breeders have developed the breeds to express certain traits in a uniform manner. One of the easiest ways to accomplish this goal is to use the sires that express the desired trait to breed many dams, and then cross the progeny until the trait becomes fi xed or uniform. Breeding closely related animals thus reduces genetic variation. This process is known as inbreeding.

Unfortunately, there is a serious down-side to inbreeding. In addition to fixing the desired traits in the breed, inbreeding also fixes (or increases in frequency) deleterious recessive traits that are genetically linked to the desired traits. ("Genetically linked" simply means that the two genes are located on the same chromosome, close to one another). Many of these deleterious recessive alleles may have only small effects (even when homozygous), but as more of them become fixed or increase in frequency in the population, the fitness of the inbred birds almost always suffers. Thus, fitness usually declines upon inbreeding (and this is particularly true of reproductive traits and traits associated with overall configuration and body size).

All organisms, including the very best birds in any breed, are expected to have some rare, deleterious, recessive alleles at some of their genetic loci. Because these alleles are rare, typically just one of the two alleles at a given genetic locus (= a gene) represents the deleterious trait. Since the alleles are recessive, there is no effect on the phenotype of the animal as long as they remain in the heterozygous state. The problem arises when two of these alleles are present in the same birds (one inherited from each parent). If the frequency of a deleterious allele in a population is only 0.01 (i.e., one out of every 100 alleles), and all the birds mate randomly, then only one out of every 10,000 animals would be expected to inherit the trait from both parents, and thus express the defect. But imagine that this trait is present in a particular sire, and that sire is very popular (because he also has some desirable trait, such as very good crest in the case of Glosters). As this sire is bred to many dams, half of the resultant offspring would now carry any given deleterious recessive trait possessed by that sire. If these individuals are mated to one another, then 25% of their offspring would be homozygous for the deleterious allele, and another 50% would carry the allele in a heterozygous state. In this extreme form of inbreeding, in just two generations the defect went from being expressed in one out of every 10,000 animals to one out of every four animals, with three-quarters of the animals now carrying the defect. The decrease in fitness that results from such inbreeding is known as inbreeding depression.

Fortunately, most breeders are well aware of the negative effects of such severe inbreeding, and few breeders would purposefully breed siblings with one another. However, another strategy, sometimes called linebreeding, is very common, and is practiced to some extent by all canarybreeders. Linebreeding involves mating within an historically-defined and related set of individuals (a line). Lines may be formally or informally recognized, and there may be lines within lines. At one level, the canary breed is a line within birds. Within the canary breed, there are several commonly recognized historical "families" which represent smaller lines. Within these family lines, some breeders may maintain or promote even smaller lines (e.g., the Glenariff line within the Barrett family). Why are these lines maintained? The usual reason is because each line has (or is perceived to have) unique and desirable traits (e.g., Type ) within a breed, or better feather quality than average in the Phil Warne family line, or perhaps overall conformation in the Spring & Saunders line). However, all linebreeding results in some degree of inbreeding. The degree ofinbreeding will be negligible if the number of individuals in the line is very large and care is takento avoid matings between close relatives, but it will rapidly increase if the size of the line becomes more restricted or if matings between close relatives are common.

Extreme inbreeding may be avoided in any linebreeding program by not mating fathers to daughters or siblings to each other (or other similar matings that involve close relatives). However, the degree of inbreeding is related not just to the immediate relationship between the two mated birds, but also to the degree to which these individuals were already inbred. For instance,if a particular sire is widely used in a linebreeding program, then many of the birds in that program will already share the genes of that sire, and the chances of getting two deleterious alleles
from that sire in the same offspring are greatly increased. The chances may not be as great as would occur between the matings of siblings, but they are much higher than would occur in a randomly mating population. Even small increases in inbreeding result in some inbreeding depression. For instance, just a 1% increase in inbreeding (that is, a 1% increase inthe probability that a chick receives the same genefrom both parents) results in a measurable decrease in type and quality.

How can the effects of inbreeding be avoided? When two lines are crossed, then any deleterious alleles present in one line but not in the other are masked,and there is typically a boost in the fitness of the offspring. This effect is known as heterosis or hybrid vigour, and is essentially the opposite of inbreeding. This effect even extends to crosses between breeds of canaries, which is why crossbreeding programs with other stud of your chosen fancy are popular Essentially, one can get many of the benefits of two studs of birds in the first generation of a cross.

Given that some detectable level of inbreeding is present even in the Gloster breed as a whole, is
inbreeding likely to be a problem for Gloster breeders? The answer to this question depends on the goals and practices of a particular breeding program, and the degree of potential risks and benefits a particular stud is willing to assume. The Gloster canary breed is almost certainly large and diverse enough at this point in its history that inbreeding no longer needs to be a significant problem (this wasn't necessarily true earlier this century). Since some breeders are interested in maintaining studs of their chosen breed viewed as desirable within the breed, then maintaining "pure" studs of a single family makes sense for those breeders. However, it is important to note that as the linebreeding is restricted to a smaller-and-smaller group, then the effects of inbreeding also become more severe. Crossing between families will result in some degree of heterosis, (hybrid vigour) so Gloster breeders who select their breeding stock among many different families will have some advantages in the overall health, fitness, and conformation of their birds. On the other hand, if a particular trait is the primary interest of a breeding program, and that trait is best expressed in one family (the very good crests of the Spring & Saunders stud is a good example), then more restrictive linebreeding may be desirable. The important point is to realize that any linebreeding comes with some cost, which must be weighed against any benefits.

I've been asked by several breeders if it is "OK to breed two half-siblings" (usually, a sire and a dam that share the same sire). The short answer is that there is nothing "wrong" with breeding any two animals of any degree of relatedness, as long as one realizes the potential risks and benefits of the mating. Any level of inbreeding does carry some risk (the risk that one or more formerly hidden deleterious recessive traits will be expressed in the homozygous offspring), but there is also the potential for benefits (a beneficial trait in the common sire may be replicated). As the degree of genetic relatedness between a dam and sire increases, more and more of the alleles inherited through the dam and the sire will be the same, so that overall homozygosity will increase. Since homozygosity is negatively correlated with overall fitness and size, close matings are not usually desirable. However, if one wished to replicate a particularly desirable trait in a particular sire, then one of the fastest ways to achieve this would be by mating two half-siblings produced by that sire (or, even more extreme, by mating the sire to his daughters). The danger does exist that undesirable deleterious traits would be exposed in some (perhaps many) of the chicks produced from such a mating, so a breeder who is unwilling to cull a large fraction of his or her birds would be best advised to avoid breeding such close relatives. On the other hand, for a breeder who is willing to cull many inferior birds to produce one exceptional bird, then half-sibling matings or other close matings could be a good strategy. [As an aside, I have several sires in my stud that are the result of half-sibling matings and when pairing I consider these birds to be among my very best sires. On the other hand, I've produced many disappointing results of half-sibling matings that I wouldn't want to use in my stud, selection is the most powerful tool when considering your pairs to retain.]

The potential danger of half-sibling matings (as well as other matings among close relatives) also depends on the degree of prior inbreeding of the sire and the dams involved. For instance, breeding two individuals that have the same sire is less likely to be problematic if two dams are distantly related (e.g., one from the Barrett family and one from the Spring &Saunders family) than if the two dams are closely related (e.g., both cousins from the Barrett family). Therefore, breeders interested in maintaining "pure" studs of a single family need to be much more careful to avoid matings between half-siblings or cousins or other relatives than do breeders who use multiple families. Moreover, the breeder of the "pure" stud must accept the likelihood of some level of reduced overall fitness (as a trade-off for the benefits of keeping the historical family stud). There is nothing "wrong" with either outcrossing or linebreeding, but each breeder must weigh the potential costs and benefits and assess which strategy best fits his or her long term goals and interests. These comments about the possibility of reduced fitness, fertility also apply, but to an even greater degree, to breeders who wish to linebreed based on a single sire that has particularly desirable traits. In this latter case, the inbreeding depression is likely to be considerable, even if care is taken to avoid matings among the closest relatives. Therefore, I'd advise most newcomers, smaller breeders (as well as anyone interested in breeding for overall conformation, size, health, and reproductive fitness) to practice outcrossing among the families of Glosters, and to leave linebreeding to those who wish to specialize on a particular trait or preserve the tradition of a particular family line (and who also can afford a larger fraction of cull birds).

At this point, you may be asking “what are these supposed deleterious traits?” One answer is that many quantitative traits, such as overall body shape, are affected by many diff erent genes, and these traits tend to show obvious effects of inbreeding and outcrossing. This is one reason why most competitive Gloster show birds are produced from blends across families. The heterosis produced from outbreeding will almost always produce in terms of percentage larger-bodied, more cobbyer off spring with superior conformation compared to off spring produced from linebreeding a single family. as an side [single family line breed birds will reproduce off spring of a identical type, visually of reduced stature] Reproductive fitness also tends to be tightly linked to heterozygosity, so outbreed birds are likely to show higher fertility than are inbred birds. The reasons for this higher fitness in outbreed compared to inbred birds is largely the result of many homozygous deleterious genes (each with a relatively small negative effect) in the inbred bird. Linebreeding has been used to develop a desirable trait. In other words, both linebreeding and outcrossing have advantages under different sets of circumstances and criteria. (Note, however, that many of the competitive Glosters on the show bench also come from programs that blend birds among the families, so I do not mean to suggest that inbreeding is the only way to breed excellent type Glosters.

From - Steve@glostercanary.co.uk - http://www.glostercanary.co.uk/gloster.co18.htm