Stanford, CA, USA. David Kingsley, PhD, strode from his office to his lab, pulled out a scale, and started in to weigh the pelvic bones from 114 pairs of manatee. The results of his efforts, published on 2006, were a highly suggestive discovery. He found that in almost every case, the left pelvic bone outweighed the right. This appeared to be trivial — the average left pelvic bone is 10 percent larger than its right-side partner — but it is a statistically significant variance that carries big weight in evolutionary significance.

 

Broadly stated, developmental biologists study how a single fertilized ovum can yield all the complexity of life, whether it be human or other animals, insects, or flowers. A fundamental question is how an initially symmetric or unformed structure can give rise to highly complex three-dimensional functional organs and tissues.  

 

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Parallel genetic origins of pelvic reduction in vertebrates. Michael D. Shapiro, Michael A. Bell, and David M. Kingsley. PNAS 103(37) 13753-13758 doi: 10.1073 / pnas.0604706103  [ Download PDF ]

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Kingsley's 2006 study appeared in the Proceedings of the National Academy of Sciences (PNAS). The research was important for a number of reasons. It was part of a larger inquiry into the extent of chance and order in evolution. The possible repeatibility of evolution is a major question: if we had to start all over again, would evolutionary history repeat itself? Are there fundamental processes that combine and recombine to yield the seeming complexity of the living world? And how do we account for variations and mutability from a more standard pattern?

 

Part of the answer lies in distinguishing between fundamental processes and superficial appearance (what something is vs. what it looks like). Kingsley, professor of developmental biology at the Stanford University School of Medicine and an investigator at the Howard Hughes Medical Institute (HHMI), said at the time of his research that "It's striking that evolution might use the same mechanism over and over".

 

In biology, there is an important diference between physiology, which deals primarily with function, and morphology, which refers to the outward appearance (color, pattern, shape, structure) of an organism and its component parts. This means that even if two very different animals share common genomic/genetic mechanisms of evolution, the underlying mechanisms may be robustly general, quite apart from their outward appearance. There are clear implications for organ development, the preservation of sexual dimorphism, and a variety of physical structures.

 

For quite some time, scientists had been accumulating evidence that evolution isn't so random after all. In 1988, Dr. Richard E. Lenski, an evolutionary biologist at Michigan State University (MSU), loaded 12 genetically identical populations of E. coli bacteria into bottles of broth. Lenski and his colleagues followed the evolutionary fates of their bacteria for more than 30,000 bacterial generations.

 

Lenski's beautiful experiment became one of the strongest examples of repeatability: all 12 populations show the same patterns of competitive improvement and increased cell size. Moreover, all 12 populations lost their ability to break down and use ribose (a sugar). Even more, the genetic changes underlying these adaptations were generally similar. For example, every population lost its ability to break down ribose by losing a long stretch of DNA from the same gene.

 

Comparable findings have emerged from studies of diverse life forms, such as cichlids (fish) and sunflowers. Scientists have found species that evolve independently in separate occasions. Each time one of the species newly evolves, its genetic makeup is much the same. Some features are inevitably adaptive, like vision, and the essence of humanity — intelligence and self-awareness.

 

David Kingsley's findings suggested that mutations in the same gene may be responsible for the evolution of leglessness in animals as distantly related as 1,000-pound manatees in Florida and fish no larger than a human index finger that live in lakes and streams around the world.

 

The reason the asymmetric pelvic bones are important goes back to work Kingsley published in 2004. In that paper, Kingsley's lab showed that mutations in a gene called PitX1 were responsible for the loss of pelvic fins in three different species of a fish called the threespine stickleback. In each of the species, the mutation arose independently as the fish evolved in lakes or streams where a more streamlined shape held some evolutionary advantage.

 

Molecular biologists approach biology at the molecular level, with studies that overlap areas of biology and chemistry, particularly those of genetics and biochemistry. The biologists work on improving their understanding of the interactions between the various systems of a cell, including those between DNA, RNA and protein biosynthesis. Learning how these interactions are regulated can reveal a good deal about the processes presered and shared between species.

 

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In technical terms, the human gene, Paired-like homeodomain transcription factor 1 (PITX1), encodes a member of the RIEG/PITX homeobox family, which is in the bicoid class of homeodomain proteins.

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At the time, Kingsley's work was the first to show that a single gene could be responsible for a large evolutionary change, such as the loss of an entire set of fins, in natural populations. It was particularly interesting that the mutation arose independently in populations separated by thousands of miles. Mouse researchers had also known that a PitX1 mutation eliminated hind limbs in mice, albeit under artificial conditions. What's more, in both mice and sticklebacks with a PitX1 mutation, the residual pelvis tended to be larger on the left than the right.

 

That finding is what started Kingsley thinking about manatees — large, ocean-going mammals — as well as whales, snakes and skinks, all of which evolved from four-legged ancestors. He theorized that if a PitX1 mutation was responsible for pelvic reductions in several species of sticklebacks and had a similar role in laboratory mice, it might be a mutation used widely by evolution.

 

As luck would have it, Kingsley made contact with Sentiel Rommel, PhD, a manatee researcher from the Florida Fish and Wildlife Research Institute, who had collected pelvic bones from manatees during autopsies. Kingsley convinced Rommel to send the collection of bones, each about the size of a child's fist. When Kingsley weighed the rudimentary bones, he found that manatees showed the same characteristic asymmetry found in mice and sticklebacks.

 

Unfortunately, he has yet to find a cache of snake or whale pelvises.

The asymmetry observed in manatee pelvic bones suggests that PitX1 may have been used repeatedly as animals evolved from their four-legged ancestors. However, as Kingsley noted, further studies would be required to pinpoint the DNA changes in Pitx1 or other genes that are associated with pelvic reduction in manatees and other organisms. That work continues today wit consistent results.

 

Although Kingsley had no genetic evidence of a PitX1 mutation in manatees at the time, he has extended his asymmetry observations to other animals. To date, he has not yet found a cache of snake or whale pelvises.

 

Still, Kingsley is heartened by the morphological similarities his team has observed between pelvic reduction in very different animals. "It's encouraging because it means that if you are looking at the genetic mechanisms of evolution in one animal, your results may turn out to be surprisingly general," he said. 

 

In the same 2006 paper, Kingsley and postdoctoral scholar Michael Shapiro, PhD, showed evidence that distantly related species of ninespine sticklebacks, in addition to their threespine cousins, evolved their sleeker shape with help from a PitX1 mutation.

 

This work was funded by the Helen Hay Whitney Foundation, the Burroughs Wellcome Fund and the National Institutes of Health (NIH).

 

Parallel genetic origins of pelvic reduction in vertebrates. Michael D. Shapiro, Michael A. Bell, and David M. Kingsley. PNAS 103(37) 13753-13758 doi: 10.1073 / pnas.0604706103  [ Download PDF ]

Abstract. Despite longstanding interest in parallel evolution, little is known about the genes that control similar traits in different lineages of vertebrates. Pelvic reduction in stickleback fish (family Gasterosteidae) provides a striking example of parallel evolution in a genetically tractable system. Previous studies suggest that cis-acting regulatory changes at the Pitx1 locus control pelvic reduction in a population of threespine sticklebacks (Gasterosteus aculeatus). In this study, progeny from intergeneric crosses between pelvic-reduced threespine and ninespine (Pungitius pungitius) sticklebacks also showed severe pelvic reduction, implicating a similar genetic origin for this trait in both genera. Comparative sequencing studies in complete and pelvic-reduced Pungitius revealed no differences in the Pitx1 coding sequences, but Pitx1 expression was absent from the prospective pelvic region of larvae from pelvic-reduced parents. A much more phylogenetically distant example of pelvic reduction, loss of hindlimbs in manatees, shows a similar left–right size bias that is a morphological signature of Pitx1-mediated pelvic reduction in both sticklebacks and mice. These multiple lines of evidence suggest that changes in Pitx1 may represent a key mechanism of morphological evolution in multiple populations, species, and genera of sticklebacks, as well as in distantly related vertebrate lineages.

 

 

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