Spiders and silk go hand in hand, but the lines may exist thanks to an ancient error
The ability of spiders to weave webs could be one of the consequences of a very serious genetic error.
A close look at the genetics and development of spinnerets – the silk-making organs of spiders – reveals that an early arachnid doubled all of its DNA hundreds of millions of years ago. The results, published on January 14 in Scientific advancessuggest key, doubled genes were responsible for the evolution of the sectors.
The specific genetic changes underlying the evolution of these “biological factories” that make silk have remained elusive, says Shuqiang Li, an evolutionary developmental biologist at Anhui Normal University in Wuhu, China. One hypothesis was that the spindles could have formed following a duplication of genes responsible for limb configuration.
Li and his team analyzed the complete genetic instruction manuals of two species of spiders and a whip scorpion, a clawed arachnid with a slender tail. They compared these genomes to those of other groups of arachnids and discovered that about 400 million years ago, an early ancestor of the group that gave rise to spiders, scorpions and whip scorpions underwent a massive mutation that duplicated its entire genome. This single event led to extra copies of genes throughout the arachnid genome.
A pair of genes is abdominal-Awho, through a series of genetic and developmental experiments on spider embryos, determined that Li and his colleagues were strongly involved in the formation of spinnerets from protolimbs. Inactivation of the gene caused the embryos to lose their lifelines. Each gene copy appears to have evolved different roles after duplication, working together to create pathways.
“The genomic work is beautiful,” says Prashant Sharma, an invertebrate biologist at the University of Wisconsin-Madison, but he doesn’t think the study has completely ruled out another explanation for the origins of the pathways. Other arthropods like sea spiders and the mites are missing abdominal-A gene and have visibly truncated bodies with missing rear parts. It may be that the genes govern all of the hind segments of the body rather than the spines specifically.
Researchers may have stumbled upon a genetic pathway to create an animal that resembles a sea spider or mite, Sharma says.
Whole genome duplication is common in plant evolution, but rarer in animals. The process could create a massive increase in genomic real estate and an opportunity to develop new genetic interactions. This has been associated with major evolutionary changes, such as the increase in body plan diversity of jawed vertebrates.
“This powerful evolutionary mechanism is a recurring theme in the generation of animal diversity,” says Li.
