Sally Isberg has a special place in her heart for a 15-foot reptile that can tear a person limb from limb.
As the chief scientist at Australia's largest saltwater crocodile farm, the geneticist can gaze out the window at pens full of saltwater crocs that will one day be turned into designer handbags. This week, Isberg and her collaborators published a genetic linkage map of this type of crocodile, Crocodylus porosus, the first such map for a reptile and an important milestone in a genetically guided breeding program that promises to improve the handbag business.
Reaching lengths up to 20 feet in the wild, saltwater crocodiles are the largest crocodiles in the world, and they are also one of the most dangerous. Throughout northern Australia, yellow warning signs are posted at every water body alerting travelers to the presence of these long-jawed predators. In April, a 20-year-old man went missing after taking a dip in the Daly River, about 135 miles from Darwin. The crocs' legendary ferocity has also sparked a tourist industry, which is how the Darwin Crocodile Farm got its start in the 1980s.
Today, it is not their razor-sharp teeth but rather their skin—known in the leather industry as "Singapore small scale"—that has become the farm's primary source of income. The skins are used to make Hermès designer handbags that can sell for $10,000 apiece in the U.S. The company's slogan: "If you have to ask, you can't afford it." Darwin Crocodile Farm now houses 55,000 animals and aims to produce 10,000 croc skins each year.
For crocodile farmers, the most important traits in a croc are its growth rate, survival and the number of scales on its belly, which relates to skin quality. Growth rate is important because bigger crocs are worth more. "We get paid per centimeter belly width," Isberg explains, adding that "the pressure from the tanneries now is to produce larger skins because ladies want larger handbags." The faster they get there, the better. The farm gets paid about $400 for a typical croc with a 16-inch belly. Because raising a crocodile costs about 80 dollars per year, crocodiles that are ready for slaughter in about two years are ideal, but they sometimes take five years to reach that stage, which takes a bigger bite out of the farm's bottom line.
Isberg started thinking about improving crocodile breeding while studying in Christopher Moran's laboratory at the University of Sydney—a lab that has focused on untangling the genetics of furrier creatures like pigs and mice. Moran remembers Isberg's first exposure to the reptiles in the late 1990s, when she worked at a croc farm as part of her PhD program's professional experience component. "She became totally fascinated by crocs," he says. Soon after, the 31-year-old mother took the job in Darwin and learned how to toss a jaw rope around a large croc, yank it out of the water, and extract a blood sample from a sinus in the back of its head.
With support from the Australian government, Isberg began developing a traditional breeding program using pedigrees. Now, Isberg's colleague Lee Miles of the University of Sydney has led the effort to take that pedigree information and build the new gene map, which is published in the current issue of BMC Genomics. The map shows how 203 different genetic markers are arranged on the genome. It does not identify individual genes but rather provides a scaffold for future genome sequencing; it can also be used to further the understanding of crocodile evolution.
More crucial for Isberg is that the map will eventually help her select the best animals the day they hatch rather than waiting for them to grow and reveal their traits. "If you know what genes are responsible for performance traits," she says, "you can select for them by genetic techniques." Her team has another paper coming out in Animal Genetics that identifies the location for a gene region linked to belly scales. But they'll have to keep honing the genetic map to find genes linked to growth rate and survival. "We want to take it to the next level rather than sticking around as a gumboot [rubber boot] cottage industry," she says.