The harvest worker makes his way through the mossy forest on eight spindly legs, delicate tent poles carry a plump body with two tiny eyes. These arachnids, sometimes called daddy longlegs, are cousins of spiders whose outdoor lifestyle sets them apart from the other creatures called daddy longlegs, better known as cellar spiders. They have other curious differences, too: the tips of a harvester’s elegant limbs are flexible so they can wrap around a branch like a monkey’s tail.
Harvesters’ distance from spiders has made them attractive to geneticists curious about how arachnids have evolved. In an article published Wednesday in the journal Proceedings of the Royal Society B, researchers who sequenced the harvester’s genome reported that the arachnids differed in key ways from spiders, and they described how certain genes tell these typical legs as they do should grow.
The size of the Harvestman genome was the team’s first focus. Ancestors of today’s spiders experienced duplication of their entire genome long ago, leaving them with more genes for evolution to work with. This could have contributed to greater diversity among spiders.
“There is this hypothesis that if the genomes are duplicated, the genes preserved can have new functions,” said Vanessa González, a scientist in computational genomics at the Smithsonian Institution and author of the new paper.
Some scientists have wondered if such duplications could help explain some of the wild diversity of the animal kingdom, said Prashant Sharma, a professor at the University of Wisconsin, Madison and author of the study. Complex genomes and more diverse organisms seem to go together.
But despite the diversity of the farmhands – there are more than 6,000 species in the group – there is no evidence of duplication in the farmhands genome, the researchers report. And horseshoe crabs, arachnids that have had at least one genome doubling in their evolution, only have a handful of species.
“Arachnids are really questioning this idea,” said Dr. Sharma. More genes could help organisms diversify, but only if environmental conditions and other factors are properly coordinated, he speculates.
In the Harvestman genome, the team identified a number of genes that are also known to control leg development in insect species. When some of the genes were suppressed in harvest workers, said Guilherme Gainett, a graduate student in Dr. Sharma’s laboratory, two or more pairs of legs did not become legs, but pedipalps, small appendages with which arachnids manipulate food and reach for companions.
It has been found that the grasping tips of a normal harvester’s legs, which Dr. Sharma compared fingers with a hundred extra knuckles, under the control of another gene. The experimental lowering of the level resulted in only one long segment that could not bend.
These changing appendages help researchers reveal the invisible map of the creature’s evolution and show how its body is built with familiar genes used in the harvester’s unique way.
In the future, the team hopes to use their newly acquired knowledge of the genome to understand how weavers and other arachnids evolve. For example, the tiny structure near the harvester’s mouth that resembles a spider’s tooth – what tells him what to become?
“We’re not the first to know what factors actually determine his identity,” said Dr. Sharma.
