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New Findings: How Do Scorpions Make Their Tails?

by AMNH on

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A new study led by scientists at the American Museum of Natural History reveals the genetic blueprint behind the patterning of scorpion tails. Scientists have long been puzzled by the development of scorpion tails—which in addition to venom-producing glands also have light-sensing capabilities—because there weren’t enough known genes to code for their many segments. But the new research, which was published today in Proceedings of The Royal Society B, reveals that scorpions have more “body-planning” genes than previously thought, potentially solving the scorpion tail mystery.

A close-up of an Arizona bark scorpion in brown dirt.
An Arizona bark scorpion (Centruroides sculpturatus), the species the researchers used for this work. 
Image credit: G. Giribet

“Scorpions have six segment-types in the back-end of their body, almost double the number seen in their closest relatives. They also are the only arthropods to have a group of segments exclusively dedicated to prey capture and defense,” says Prashant Sharma, a postdoctoral researcher in the Museum’s Division of Invertebrate Zoology and lead author of the paper. “The question is how to pattern this kind of complexity.”

Along with Ward Wheeler, a curator in the Museum’s Division of Invertebrate Zoology, and colleagues at Harvard University, Sharma focused on a group of genes known as the Hox family, which encode the body plan in numerous organisms from worms to humans. By acting in different combinations, these genes control whether a given portion of the embryo will develop mouthparts, wings, or gills, for example. 

Diagram of gene expression in spiders and harvestmen
Hox genes guide the body plan in many organisms and are expressed head to tail in the organism in the same order they appear on the chromosome. The system works by “staggering” the expression of the gene family. This illustration shows the normal expression of four Hox genes on the back end, or opisthosoma, of the body in harvestmen and spiders and the resulting development.
Image credit: P.P. Sharma

Hox genes are expressed, from head to tail, in the same order as they appear in the genetic code. The system works by “staggering” the expression of the gene family. The first gene in the Hox family will be expressed starting in the head. Subsequent genes, however, begin to be expressed one section of the embryo at a time, causing each section to have a unique genetic cocktail: If , say, a given section has genes X and Y, for example, it may produce legs, while if it had genes X, Y, and Z, it would make lungs.  The staggering of these Hox genes allows many different segment types to develop. Manipulation of a single Hox gene can turn what would be a fly’s antenna into a leg, or even be used to create a 10-legged spider.

mutated spider embryo and adult, with legs numbered
In this spider embryo (left), the Hox gene Antennapedia was mutated so it was no longer expressed. The section in which Antennapedia would normally have appeared instead has the genetic cocktail of the leg-making segment, leading to a spider with 10 legs (right). 
Image credit: Khadjeh et al. (2012)

Arachnids—the group of arthropods that includes scorpions, spiders, and daddy-long-legs—are presumed to have 10 Hox genes. In non-scorpion arthropods, six of the 10 Hox genes have been shown to aid in the patterning of the front part of the organism that includes the legs and feeding appendages. This leaves only four to control the back end. Four genes, however, are not enough to pattern the scorpion’s tail. 

“If the previous model were true, we couldn’t actually make a scorpion,” Sharma said. “We would need either more genes or a different model.”

The researchers used the Arizona bark scorpion to investigate Hox gene makeup. By taking tissue samples of scorpion embryos and determining the genes that were being expressed at a given developmental stage, they discovered that 19 different Hox genes are active during development, instead of the typical 10. 

“But just because scorpions have a lot of genes, it doesn’t mean those genes have anything to do with body patterning,” Sharma said. 

The group began the process of testing whether the genes are all actually involved in shaping the scorpion’s tail. First, the researchers bathed embryonic tissue samples with probes that change color if a certain gene was being expressed. Their results uphold the model: the appearance of each gene in the family is staggered and coincides with a shift in segment identity. While further mutative experiments would be required to definitively prove the connection between genetic code and body form, it seems that the scorpion’s extra genes do in fact pattern its tail.