This article is a review of the existing research examining hybrid vigor in dogs.
Review articles summarize the research published to date on a particular subject and often comment on the validity or problems associated with specific studies.
Nicholas et. al. reviewed 13 studies that examined hybrid vigor in dogs and suggest that changing how data is collected is imperative to determining if hybrid vigor has any real benefit in dog breeding.
Hybrid vigor is defined as the extent of improved performance of an individual trait in the first cross (F1) generation over the parent generation. This is usually expressed as percent improvement. For example, in poultry, an F1 generation might exhibit a 3-9% increase in egg production over the parent populations.
Evidence of useful hybrid vigor in a range of other species suggests the existence of useful hybrid vigor in dogs. Hybrid vigor is most obvious in corn, where the F1 generation can result in twice as much yield per hectare as the parent strains. The benefits of hybrid vigor in livestock (poultry, pigs, sheep, beef cattle, and dairy cattle) are much lower than that seen in corn, but still significant. The level of inbreeding in dogs is similar to that of livestock, so the predicted levels of hybrid vigor are expected to be similar.
Studies of hybrid vigor in livestock have helped to define four main principles of hybrid vigor:
- The lower the heritability of the trait, the greater the hybrid vigor. This is relevant since fitness traits in dogs, such as longevity and reproduction, have low heritability and therefore could be influenced by hybrid vigor.
- The greater the genetic diversity between the parents, the greater the hybrid vigor of the offspring.
- Breeding of the F1 generation to itself dissipates the hybrid vigor by half.
- The more inbred the individual parents, the greater the hybrid vigor of the F1 offspring.
The authors of this paper reviewed thirteen published studies of hybrid vigor in dogs. In all but three studies, they point out a major shortcoming in the collection of the data, i.e. a failure to identify the specific makeup of the mixed breed populations that are being compared to purebred populations. In these studies, a dog classified as a mixed breed could be anything from a F1 cross to a limitless number of combinations.
This shortcoming is significant because, as stated in the principles above, the expected levels of hybrid vigor are directly related to the degree of inbreeding of each parent and the genetic diversity between the parents.
It is impossible to record the change in performance of a specific trait in the offspring if we don’t know the parent populations for comparison. This is especially relevant since there is significant variability in the expression of traits between different purebred populations.
Of the thirteen studies reviewed, eight looked at hybrid vigor in relation to several common disorders, including epilepsy, ophthalmic disorders, hip dysplasia, etc. Three studies examined longevity and two studies looked at guide dog populations.
In all but three of these studies the findings compared traits between purebred populations and crossbred populations but did not identify the specific makeup of the crossbreeds. Due to this limitation, the authors consider the results of these studies to indicate apparent, but not actual, hybrid vigor.
Only three published studies provide data relevant to actual hybrid vigor. These studies compared populations of identified mixes to their purebred parental populations.
One such study compared the probability of graduating from guide dog training. Graduation rates were 54% for Golden Retrievers, 51% for Labrador Retrievers, and 59% for Lab x Golden crosses. This gives an actual hybrid vigor of 12.4% in the crosses over the purebred dogs.
Another study conducted a comprehensive genetic analysis of 2127 6-week-old puppies. 119 Golden retrievers, 704 Labrador retrievers, 950 Golden x Lab mixes, 39 Golden x Golden/Lab mixes, and 315 Lab x Golden/Lab mixes. The results show hybrid vigor ranging from +2.5% to -6% on the 11 components tested. The overall result showed no hybrid vigor for the traits tested.
The last study showed a decrease (negative hybrid vigor) in vigor for the trait examined. A study of ophthalmic disorders showed a 4.6% increase in the prevalence of multifocal retina dysplasia in Labradoodles over Labradors and Poodles.
The authors conclude that based on evidence of hybrid vigor in livestock, hybrid vigor should be evident in dogs. They point out that all but three of the studies on the subject demonstrate what they call apparent hybrid vigor, but do not shed much light on actual hybrid vigor in dogs. This is due to inadequate collection of the data, specifically the identification of the parentage of the mixed breed dogs.
Data on actual hybrid vigor could be collected by knowing the specific makeup of the crosses (such as an F1 or F2 population) and comparing them to the parental populations.
The ability to utilize hybrid vigor in dogs has yet to be fully explored, but has the potential to improve the health, longevity, and optimum behavior characteristics in future breeding of dogs.