David Kingsley is passionate about vertebrates.
He is fascinated about how nature can take cartilage and bone and mold it into the different sizes and shapes seen in both living and fossil animals.
“How do you take the same tissue type and exquisitely control its formation, growth, and patterning to produce useful structures that allow animals to hop, swim, fly, or walk upright, as in humans?” Kingsley asks.
To satisfy his driving curiosity, for more than two decades Kingsley has been studying the genetic mechanisms controlling vertebrate skeletal formation and evolution. In that time, he identified key genes controlling bone and joint formation in mice; uncovered molecular mechanisms for major evolutionary changes in stickleback fish; and showed that similar mechanisms are used repeatedly when similar traits evolve in very different animals, including humans.
Kingsley says he focused on studying skeletons because of the “intrinsic interest in understanding ourselves as vertebrates” and the importance of bones in human health, given the high incidence of osteoporosis, bone fractures, osteoarthritis, and other joint diseases.
Kingsley first started doing vertebrate skeletal genetics in 1987 as a postdoctoral fellow at the National Cancer Institute. At that time, methods for tracking traits to genes and molecules were just being developed for humans and mice. Kingsley surveyed a collection of skeletal anomalies in mice, and decided to find the genetic basis of the “short-ear” trait that altered skeletal patterning and bone healing.
To find the gene, Kingsley combined classical genetics with the then new tools of molecular biology. Crossing laboratory animals allowed him to zero in on a particular chromosome region controlling the trait. Molecular biology enabled him to isolate and study all the genes in the corresponding region.
Given the large genome size of vertebrates, the genetic sleuthing took almost five years. In 1992, Kingsley, who by then had moved to Stanford, showed that the short-ear gene encoded a secreted signaling molecule called a bone morphogenetic protein (BMP). The work was among the first to track a vertebrate morphological trait to a specific gene, and it provided strong genetic evidence that BMPs are the endogenous signals used to induce cartilage and bone formation.
Two years later, Kingsley’s lab studied a different mouse mutant, one with short feet instead of short ears. Those studies showed that a different member of the BMP family played a key role in the formation of joints between skeletal structures. In 2000, Kingsley’s lab isolated a third trait, showing that the progressive ankylosis gene encodes a novel molecule controlling arthritis susceptibility in mice and humans.
Convinced of the power of genetics, Kingsley wondered if similar approaches could be used to understand the molecular basis of vertebrate evolution. In 1998, he chose the stickleback fish as an animal model that would lend itself to genetic analysis of evolutionary changes in natural species. Sticklebacks are small fish that live in the ocean but migrate each spring into freshwater to spawn. At the end of the last ice age, sticklebacks colonized countless new streams and lakes, and have since had about 10,000 generations to adapt to a range of new environments.
Today, many sticklebacks look very different from their ocean ancestors. To understand how they evolved major changes in teeth, jaws, spines, fins, armor plates, and colors, Kingsley’s lab again turned to genetics. Because different populations evolved so recently, different fish could still be crossed using artificial fertilization. Kingsley’s lab developed the first comprehensive set of molecular genetic markers for sticklebacks. By comparing the inheritance of traits and chromosomes, his lab showed that major differences between fish could be mapped to particular chromosome regions.
Kingsley then tracked down the specific genes that control the number of armor plates on the side of the fish, the presence or absence of an entire pelvis, or the color of gills and skin. Soon the results began to reveal common mechanisms underlying evolutionary change in multiple populations. For each trait, rapid evolutionary changes occurred by big effects in a few genes, not by countless small changes occurring in many genes. For each trait, the key genes turned out to encode major developmental regulators required for the formation of multiple tissues. And for each trait, evolution occurred by making small changes where those genes were expressed during development, producing major alterations in plate, or fin, or skin color formation.
Most interesting to Kingsley has been the evidence that particular genes underlying evolution are used repeatedly, not only in different lakes, but also in different species. The genetic mechanisms found in one lake usually predict what happens in others. And Kingsley’s recent studies show that the same mechanism that controls skin color evolution in sticklebacks also plays an important role in skin color variation in different human populations.
With classical genetics and the amazing advances in genome technologies, including today’s high-throughput gene analysis methods, Kingsley is confident he will reveal more interesting findings about the skeletal system and vertebrate evolution. “I like genetics,” Kingsley says. “It works. And the answers coming from genetics are turning out to be surprisingly general, whether we start with mice, fish, or people.”