Ticks and mites closer relatives than thought
Genomic analysis finds two groups of parasites share a single lineage. Andrew Masterson reports.
Genomic evidence reveals that mites and ticks are much more closely related to each other than previously thought.
The discovery that the two types of arachnid share a common lineage will prompt a major revision of the massive group of arthropods known as chelicerates – the group that contains not only ticks and mites, but also spiders, scorpions, horseshoe crabs and others. There are roughly 77,000 living chelicerate species.
Historically, the evolutionary relationship between ticks and mites has been the subject of much debate among entomologists.
One school of thought positioned them as only distantly related, on the grounds that they have significantly divergent shapes, while the other saw them as sister-groups – not quite so divergent – that should be clumped together in a subclass of the arachnids, called the Acari.
The second position has gained favour over the years, but until now the matter remained unresolved.
A 2007 review published in the Journal of Zoological Systematics and Evolutionary Research noted that the idea of the Acari is “controversial” because some of the measures proposed to define it “do not stand up to detailed scrutiny when compared with Arachnida in general”.
On the other hand, the review noted, “earlier supporters of two independent origins for mites [and ticks] largely failed to demonstrate” sufficient evidence to allocate separate lineages.
One of the primary difficulties in determining the precise evolutionary relationship between the two groups lay in the fact that species within each show an astonishingly wide range of physical forms, making classification based on morphology extremely challenging.
The latest research turns to DNA data to make a determination.
Researchers led by Jesus Lozano Fernandez from the UK’s University of Bristol sequenced the genomes of 10 species of mites and 11 species of ticks. The results appear to end the debate for good.
“Regardless of the methods we used, our results converge on the same answer – mites and ticks really do form a natural group,” says Lozano Fernandez.
“Evolutionary trees like the one we've reconstructed provide us with the background information we need to interpret processes of genomic change.”
The findings have significant implications for how the massive arthropod group is formally classified – and also, on a more shallow level, generate extra work for record-book writers.
“Spiders, with more than 48,000 described species, have long been considered the most biodiverse chelicerate lineage, but 42,000 mite and 12,000 tick species have been described,” says co-author Davide Pisani.
“So, if mites and ticks are a single evolutionary entity rather than two distantly related ones, they are more diverse than the spiders.”
On a more serious level, the researchers say the confirmation of the close relationship between the two groups will help guide a wide range of future research in fields as diverse as agricultural pest management and human disease treatment.
The research is published in the journal Nature Communications.