English bulldogs’ shallow gene pool problems

English bulldogs are among the world’s favourite dogs, but centuries of inbreeding has cursed the pooches with poor health and a shorter life expectancy – and now, new research shows their gene pool is so small only procreation with other breeds can save them.

A trio of researchers at the University of California set about revealing just how much genetic diversity is evident in among bulldog populations, and whether there’s enough variety to curb changes that have damaged the breed.

“These changes have occurred over hundreds of years but have become particularly rapid over the last few decades,” explains lead researcher Niels Pedersen.

“Breeders are managing the little diversity that still exists in the best possible manner, but there are still many individuals sired from highly inbred parents.”

The bulldog’s squat body, drooping jowls and short muzzle make the breed especially distinctive, and the researchers say the dog’s “baby-like appearance” captures dog-owners’ hearts.

Historically, the breed is recognised as an icon of the United Kingdom, and for some, the official mascot US Marine Corps.

But this love comes with a hefty price. “The English bulldog has reached the point where popularity can no longer excuse the health problems that the average bulldog endures in its often brief lifetime,” says Pedersen.

“More people seemed to be enamoured with its appearance than concerned about its health.”

Over the past 200 years, humans have used specialised breeding to produce bulldogs with particular body shapes and behaviours to meet increasing demand. With poor health becoming an enormous issue, some believe it’s time to “breed out” the traits that are causing ill health.

Today’s bulldogs are prone to heat stroke, heart defects, hip dysplasia, cysts and respiratory problems. According to the paper, published in the journal Canine Genetics and Epidemiology by Pedersen, Ashley Pooch and Hongwei Liu, pressure is mounting on bulldog breeders to “moderate the extreme physical changes that now affect the breed and its health”.

Groups in The Netherlands have called for a ban on English bulldogs based on a belief that the breed can no longer be returned to a health.

Others argue the breed can be returned to health from within. But this proposed return to health requires an existing level of diversity among the breed’s collective gene pool.

To measure this diversity, the team analysed the DNA of 102 bulldogs from the US, Finland, Austria, Canada, Czechoslovakia, Hungary and Argentina to identify common genes.

According to the results, the bulldogs exhibited four paternal haplotypes – that is, groups of genes inherited from one common parent. One of those haplotypes was dominant in more than 93% of the dogs tested.

On the maternal side, five different matrilines were identified, three of which were found among more than 90% of the dogs.

The researchers also analysed the DNA of a group of sick bulldogs from a local vet hospital – this was to test the theory that poor health is the result of puppy farms or less responsible breeders. The genetic results were almost identical among the two populations.

The researchers argue the breed can’t be saved from within – rather, help will be needed from other breeds to offer wider genetic diversity.

“Improving health through genetic manipulations presumes that enough diversity still exists to improve the breed from within, and if not, to add diversity by outcrossing to other breeds,” say Pedersen.

“We found that little genetic ‘wiggle room’ still exists in the breed to make additional genetic changes […] Unfortunately, eliminating all the mutations may not solve the problem as this would further reduce genetic diversity,” Pedersen explains.

“We would also question whether further modifications, such as rapidly introducing new rare coat colours, making the body smaller and more compact and adding more wrinkles in the coat, could improve the bulldog’s already fragile genetic diversity.”