Lower diversity linked to disease issues | Domestic birds originated from one wild sub-species
LINDELL BEACH, B.C. — Many people think of turkeys as a European import from when North America was first settled.
The truth is that turkeys are indigenous to southern Mexico and were first domesticated there around 800 B.C.
However, researchers from the U.S. Department of Agriculture and the Smithsonian Conservation Biology Institute were recently surprised to discover that today’s domestic turkey is vastly different genetically from its Central American ancestor.
Their research showed that turkeys have far less genetic variation than their wild counterparts and exhibit even less diversity than that found in other livestock species, such as chickens and pigs.
“This result comes from our (research) paper where we say that the turkey genome has a relatively low frequence of heterozygous SNPs (single nucleotide polymorphisms) compared to other livestock species,” said Robert Fleischer, head of the institute’s Center for Conservation and Evolutionary Genetics.
The results of the study were published in the journal BMC Genomics.
Each SNP (pronounced “snip”) represents a slight difference in a single DNA building block, or nucleotide. These slight differences are what define genetic variation, and SNPs contain the most abundant source of genetic variation within a genome.
“I am not certain why the variation is lower, but it could be because of the significant population bottlenecks that likely occurred during the domestication process and subsequent to the transport of turkeys to Europe where modern day commercial breeds apparently originated,” said Fleischer.
The modern domestic turkey descended from only one of six subspecies of wild turkey, Meleagris gallopavo, which was found in what is now the southern Mexican states of Jalisco, Guerrero and Veracruz.
The Aztecs domesticated the southern subspecies, and the Spaniards brought the tamed birds to Europe during Spain’s conquest of the Aztec empire in the 16th century.
Europeans continued to domesticate the bird and developed a variety of lines. The popular farm bird returned to North America with English settlers 100 years later.
The turkey has since become an important agricultural species and the second largest contributor to world poultry production.
Canada had 543 turkey farmers in 2011, who produced almost 159 million kilograms with a farmgate value of more than $353 million.
In the United States, production per bird doubled between 1970 and 2008, mostly because of pressure from breeders for certain economically valued traits such as weight, meat quality and egg production.
Researchers in the whole genome sequencing study used males from seven commercial lines provided by two breeding companies, three heritage varieties (Beltsville Small White, the Royal Palm and the Narragansett) and 113-year-old tissue samples from wild turkeys from southern Mexico.
The tissue samples from the wild population came from the Smithsonian’s National Museum of Natural History. They were originally collected in 1899 in Chihuahua, Mexico, and represented the progenitor of the modern domestic turkey.
“I was pleasantly surprised that the USDA sequencing lab got such good genomic sequences from the three ancient specimens,” said Fleischer.
“We do a lot of ancient DNA work in our lab, but this was the first time ancient material from our lab was shotgun sequenced to get genomic sequences, and I was very happy that it worked so well.”
Researchers found segments of chromosomes that show low genetic variation in domestic strains relative to the wild ancestral stock.
“(This is) due either to early in-breeding or selection, presumably for traits of importance during the process of domestication,” he said. “This will be future work for other collaborators.”
The transport of turkeys to Europe may have been the first step toward the lack of genetic diversity in modern turkeys.
“Only a small number of turkeys may have been taken to Europe and thus there was a limited gene pool,” he said. “Artificial selection for particular traits could have also continued to erode the genetic variability.”
The hallmark of genetic diversity in domestic livestock is a steady im-provement in breeding lines and the breed’s ability to adapt to physical changes.
“Chickens and pigs may have had greater input of additional wild lineages during their domestication because of continued association in regions where wild stock occurred, i.e., in Asia and Europe,” said Fleischer.
Selective breeding for qualities such as body size and breast muscle in a small population size from a common origin resulted in loss of genetic diversity and possibly led to compromised health.
“I do not believe this is known with certainty for turkeys, but there is evidence from studies of other organisms that lower genetic variation does often result in greater risk of infectious disease,” said Fleischer.