Abstract

With companion animals, there is a large diversity in breeding practices, which may impact genetic variability and health of selected breeds. Based on a survey, we illustrate the specificities of scent hound dog breeders regarding either their breeding goals or their mating practices. Matings between close relatives are also investigated in different cat breeds. Such matings are more or less frequent according to breeds and countries, but seem to occur more rarely in the last few years. Deleterious impact of inbreeding on fitness traits is also investigated for four dog breeds. In most cases, litter size and longevity are found significantly reduced for individuals with large inbreeding levels. These results illustrate how some breeding practices, such as mating between close relatives, may have an impact on fitness and welfare in cat and dog breeds.

Introduction

Selection in companion animals such as dogs or cats differs from other domesticated species, as they are generally not raised by production or profits. A large majority of breeders are occasional or hobby breeders, and it seems to exist a large diversity of breeding practices according to those breeders (Leroy et al. 2007). Recently, several reports have pointed out the potential deleterious impact that may have on some of those selection practices regarding the welfare of purebred dogs (Nicholas 2011), in relation for instance to traits selection which, when taken to extremes, are deleterious to health (Collins et al. 2011). Inbreeding has also been shown to have deleterious impact on traits related to reproduction and occurrences of some diseases (Urfer 2009; Mäki et al. 2001).

We propose here to show, on a few examples in the dog and cat, the diversity of breeding practices between breeds, between countries, and over time. Impact of inbreeding depression on litter size and longevity will also be illustrated based on some preliminary results on dog breeds.

Breeding Practices According to Breeds, Results From a Dog Breeders Survey

In dog species, where breeds show a particularly large morphological diversity, in relation, among others, to their different uses (pets, hunting, herding...), one may expect to find also a large diversity of breeding goals according to breeds. To investigate if some differences could also be found in relation to selection tools, management methods, and reproduction tools, a survey composed of 55 questions was carried out in 2007 among 985 French dog breeders (Leroy et al. 2007). Two main explanatory variables were used to analyse the results, namely the number of litters produced and the FCI group of the main breed raised.

Table 1. Average rank of breeding goals declared by dog breeders according to the FCI breed group, the lower number being the best goal. (There was no significant effect of number of litters produced on the answers.)

NS = non-significant; *** p < 0.001
(From Leroy et al. 2007)

One of the main results of the study was related to the specificities of scent hound breeders, considering either breeding objectives or mating practices. Indeed, if "morphology" was in general considered as the first or the second breeding goal (see Table 1), "working abilities" were more important for scent hounds (FCI group 6) and pointing dog groups (FCI group 7) breeders. However, "health" as a selection goal appears completely secondary for scent hound breeders, and only 27% of them indicated it as a breeding objective versus 56% for pointing dog breeders and 71% for overall breeders. It is difficult to interpret in what extent this result is related to a low number of health problems in scent hounds, which infers that health does not appear as a problem for breeders, or to the fact that scent hound breeders, who often raise their dogs in packs, paying less attention to the health of their dogs, relative to breeders raising dogs more "individually." Scent hound breeders show other specificities in relation to mating practices. When paying for a mating made by a sire that does not belong to them, the main modality used is monetary payment, used by 85% of overall breeders, while in 6th group breeders, only 29% use this modality. Indeed, 85% scent hound breeders prefer to give a puppy of the litter instead, versus 28% on average. Scent hound breeders also indicate using less artificial insemination (AI) than other breeders: 85% of those breeders indicate they never used AI, versus 58% on average. Finally, according to the French Kennel Club, breeders of the 6th group are also the only ones to regularly register dogs with unknown origin, as in 2012, those registrations represented 5% of the total registrations within the group, versus 0.1% for the other breeds. All those differences show how some breeds may have their own specificities regarding breeding practices. Those specificities may eventually be linked with specific patterns concerning within breed genetic structure and variability, which can have consequences for health and welfare of those breeds.

Inbreeding Practices According to Breeds, Countries and Time: Examples in Cat Breeds

Inbreeding practices correspond to intentional mating of related individuals, such as when breeders attempt to fix or maintain specific traits from a common ancestor. This constitutes a controversial practice, due to the eventual impact that inbreeding may have on the fitness of litters produced (inbreeding depression). As a consequence, mating between close relatives (full or half-sibs for instance) has been banned in several countries, such as the UK a few years ago. It is therefore particularly interesting to investigate differences that may exist according to these practices. In France, a recent study on 8 cat breeds and groups of breeds (Leroy et al. 2013a) have shown for instance that the % of individuals inbred when considering 2 generations (i.e., individuals which are the products of mating between sibs or direct parents) ranged from 2.7% (Main Coon) to 8.4% (Persian/Exotic Shorthair), illustrating the differences according to breeds.

Those differences may also exist within a given breed. As an illustration, we analysed an international pedigree database for Birman breed, provided by Jerold Bell. We computed the % of individuals inbred when considering different generations.

Table 2. Percentage of individuals inbred considering 2 or 3 generations during the 1991–2010 period according to four countries

As illustrated by Table 2, when comparing different countries over the 1991–2000 period, breeders from Nordic countries seem to make such mating rather rarely compared to the UK or the USA. In these two countries, 26% and 27% of kittens born over this period of time are inbred when considering 3 generations, versus 7% and 12% in Finland and Sweden, respectively.

It appears also that such mating practices are less and less frequent (see Figure 1): from the 70s to the 2000s, the percentage of individuals inbred considering 3 generations have decreased from 44% to less than 10%. These results are probably explained by the fact that welfare is a growing concern, which is particularly taken into account in Nordic countries.

Figure 1. Evolution of % of Birman cats inbred according to different number of generations considered, over the 1970–2010 period

Inbreeding Consequences on Litter Size and Longevity: Examples in Dog

It is not easy to quantify the impact of inbreeding on breed health, since they depend on the mating system, demographic history of the breed, and the genetic mechanism involved (Ballou 1997). Here we propose to illustrate the consequences of inbreeding on prenatal and postnatal survival of purebred dogs, considering litter size and longevity, based on births and deaths declared for 4 breeds raised in France. Litters born over the 1990–2012 period as well as dogs declared as dead over 2007–2012 were considered for this (see Leroy et al. 2013b). Here dogs were divided into three inbreeding classes, considering either individuals with inbreeding coefficient lower than 6.25% (corresponding to an inbreeding equivalent to a mating between cousins), between 6.25 and 12.5% (mating between half-sibs), and 12.5% and larger.

Figure 2 shows the reduction in prolificacy and survival within dog breeds in relation to inbreeding depression. In all cases, except for longevity in West Highland White Terrier, inbreeding classes were found to have a significant impact on the traits considered (p < 0.001). For instance, in the German Shepherd Dog breed, the average litter size decreased from 5.1 for litters with low inbreeding coefficient, to 4.7 for litters with inbreeding coefficient larger than 12.5%. Similarly, Epagneul Breton dogs with inbreeding coefficient lower than 6.25% showed an average around 11.5 years, while this longevity was reduced to 10.4 years for dogs with inbreeding larger than 12.5%. These results show that mating between close relatives clearly impacts the fitness of litters produced, even if there are other factors that affect more largely the survival and the welfare of animals raised.

Figure 2. Evolution of litter size and longevity according to inbreeding coefficient Inbreeding coefficient for Bernese Mountain Dog (BMD), German Shepherd Dog (GSD), Epagneul Breton (EPB), and West Highland White Terrier (WHW). 95% standard error indicated.

Discussion

As illustrated above, breeders of companion animals show a large diversity of breeding practices, which may impact the genetic variability, as well as the health of populations and individuals selected. Inbreeding practices may have, in theory, positive effects at the population level. Indeed it is supposed to increase the exposure of recessive deleterious alleles to selection, increasing inbreeding purge and reducing the risk of dissemination of a specific defect (Leroy 2011). Yet, given the deleterious consequences that high level of inbreeding may have on traits related to fitness, namely the litter size and longevity, mating between close relatives should not be recommended in any case. In practice, it is quite difficult to avoid any level of inbreeding in a selection program, especially in breeds with small population size. However, one may be recommended to limit rapid increase of inbreeding as, in theory, slow rates of inbreeding result in more efficient selection against deleterious defects (Fu et al. 1998). At the population scale, the overuse of some reproducers should also be avoided, as it may increase the risk of dissemination of genetic disorders (Leroy, Baumung 2011). Finally, choosing reproducers unrelated, or eventually belonging to another breed, may constitute another option to introduce genetic variability within a given kennel or breed. To conclude, it has to be emphasized that the management of breed health has to be planned both at the breeder scale and at the breed club scale. This is why to avoid health problems and get rid of inherited disease, the best chance for a dog or cat breed is to have breeders and clubs fully cooperating in this common goal.

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