The harvestman makes its way through the mossy forest on eight spindly legs, delicate tent poles supporting a plump body with two small eyes. These arachnids, sometimes called papa longlegs, are cousins of spiders whose outdoor lifestyle sets them apart from the other creatures called papa longlegs, which are more commonly known as basement spiders. They also have other curious differences: The tips of a harvestman’s elegant limbs are flexible, allowing them to wrap around a twig like a monkey tail.
The harvestmen’s distance from spiders has made them attractive to geneticists curious about how arachnids evolved. In an article published Wednesday in the journal Proceedings of the Royal Society B, researchers who sequenced the harvestman’s genome reported that the arachnids differ from spiders in important ways, describing how certain genes tell those distinctive legs how to grow.
The size of the harvestman’s genome was the team’s first focus. Ancestors of today’s spiders experienced a doubling of their entire genomes at some point long ago, giving them more evolution genes to work with. This may have contributed to greater diversity among spiders.
“There’s a hypothesis that when you have duplicated genomes, the genes that are preserved can have new functions,” said Vanessa González, a computational genomics scientist at the Smithsonian Institution who authored the new paper.
Some scientists have questioned whether such duplications could explain some of the wild variety of the animal kingdom, said Prashant Sharma, a professor at the University of Wisconsin, Madison and also an author of the study. Complex genomes and more varied organisms may seem to coexist.
But despite the variety of harvestmen — there are more than 6,000 species in the group — there’s no sign of duplication in the harvestman’s genome, the researchers report. And horseshoe crabs, arachnids that had at least one genome duplication in their evolution, have only a handful of species.
“Arachnids are really challenging this idea,” said Dr. Sharma. Having more genes can help organisms diversify, but only if environmental conditions and other factors also align correctly, he speculates.
In the harvestman’s genome, the team identified a number of genes that are also known to control leg development in insect species. When some genes were suppressed in harvestmen, said Guilherme Gainett, a graduate student in Dr. Sharma, two or more pairs of legs became not legs, but rather pedipalps, tiny appendages that arachnids use to manipulate food and grab mates.
The gripping points of the legs of an ordinary harvestman, which Dr. Sharma compared fingers with a hundred extra knuckles, turned out to be controlled by a different gene. Lowering the level experimentally resulted in only one long segment that failed to bend.
These changing appendages help the researchers reveal the invisible map of the creature’s development, showing how its body is built with known genes used in the harvestman’s own particular ways.
In the future, the team hopes to use their new knowledge of the genome to understand the development of harvestmen and other arachnids. For example, the small structure at the harvestman’s mouth that resembles a spider’s canine tooth – what tells him what to become?
“We don’t know the first thing about what factors actually determine his identity,” said Dr. Sharma.