2008-11-01 – L’evoluzione di ragni e scorpioni (spider and scorpion evolution)

Un nuovo studio condotto sul DNA mitocondriale di 25 taxa di artropodi ha permesso di stimare un’età più antica di quella che ci raccontano i fossili per la differenziazione del taxa dei Chelicherata di cui fanno parte ragni e scorpioni.

L’età stimata del progenitore comune varia da 400 a 450 milioni di anni fa, mentre fino ad ora i reperti fossili ci dicono che il più antico ragno fossile ritrovato è stato datato 120-135 milioni di anni fa, mentre per gli scorpioni si scende a 200 milioni di anni fa.

Il dato è di per sè significativo, anche se probabilmente estendendo l’analisi a più taxa in futuro si avranno risultati più precisi.

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UF study suggests spiders, scorpions, ticks, mites far older than fossil record indicates

GAINESVILLE, Fla. – Halloween is the only holiday when spiders and other arachnids get a little respect from humans, and a new University of Florida study suggests they deserve more, because they’ve apparently managed to survive a very, very long time. By analyzing gene sequences in modern-day spiders, scorpions, ticks and mites, researchers have estimated that these invertebrates first appeared on Earth roughly 400 million to 450 million years ago.

GAINESVILLE, Fla. — Halloween is the only holiday when spiders and other arachnids get a little respect from humans, and a new University of Florida study suggests they deserve more, because they’ve apparently managed to survive a very, very long time.

By analyzing gene sequences in modern-day spiders, scorpions, ticks and mites, researchers have estimated that these invertebrates first appeared on Earth roughly 400 million to 450 million years ago.

The study, published online this month by the journal Experimental and Applied Acarology, is the first large-scale attempt to use genetic analysis to make projections suggesting when various arachnids evolved, said entomologist Marjorie Hoy, an eminent scholar with UF’s Institute of Food and Agricultural Sciences.

“This is a first estimate, it’s not the be-all and end-all,” said Hoy, who co-authored the paper with Ayyamperumal Jeyaprakash, a senior biological scientist with the entomology department.

“It suggests for the first time that these creatures are much older than the fossil record indicates,” Hoy said.

Fossil arachnids are rare because their bodies contain no bones and typically decompose altogether, so many questions remain unanswered about their early history.

The oldest known fossil spider is 125 million to 135 million years old; the oldest fossil scorpion is about 200 million years old, Hoy said.

Jeyaprakash and Hoy used a relatively new type of computer software in their study, analyzing all the genetic sequences from the mitochondria, cell components sometimes called “cellular power plants” because they produce chemical energy. Mitochondria provide a good yardstick for calculating when related organisms diverged from a common ancestor.

“Not too many people are using this strategy,” Hoy said. “It’s been used on fish but not on invertebrates.”

The UF researchers obtained complete genetic sequences for the mitochondria of 25 different invertebrates, including four spiders, three scorpions, 10 ticks and four mites. Then Jeyaprakash identified genetic sequences common to the organisms and used two sophisticated computer models to calculate how much time had passed since the sequences evolved from a common origin.

The results also supported the hypothesis that spiders, scorpions, ticks and mites all descended from a common ancestor, something scientists have generally believed for decades, Hoy said. The study didn’t provide any hints regarding that creature’s appearance, but it probably lived in the ocean, she said.

Although the findings are subject to debate, the UF study will be helpful to researchers making similar estimates in the future, said Hans Klompen, an associate professor with The Ohio State University’s entomology department.

“I think this is a great start,” said Klompen, who studies mite evolution. “Now we have something specific to test, and that’s always a good thing.”

The computer models Hoy and Jeyaprakash used may not fully compensate for variations in the speed of mitochondrial evolution, he said. So the results may indicate that some arachnids appeared on Earth earlier or later than they actually did.

“It might work better if the sampling size was bigger,” Klompen said. “But we can’t wait for the perfect data set. We have to work with what’s available.”

Credits
Writer
Tom Nordlie, tnordlie@ufl.edu, 352-392-0400
Source
Marjorie Hoy, mahoy@ifas.ufl.edu, 352-392-1901, ext. 153
Ayyamperumal Jeyaprakash, margret@ufl.edu, 352-392-1901, ext. 170
Hans Klompen, Klompen.l@osu.edu

link: http://media-newswire.com/release_1077847.html

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Scientific article:

Experimental and Applied Acarology, Saturday, October 18, 2008

First divergence time estimate of spiders, scorpions, mites and ticks (subphylum: Chelicerata) inferred from mitochondrial phylogeny

Ayyamperumal Jeyaprakash and Marjorie A. Hoy

A. Jeyaprakash e-mail: ajey@ifas.ufl.edu

PDF (365.8 KB), HTML, Supplemental HTML

Abstract  Spiders, scorpions, mites and ticks (chelicerates) form one of the most diverse groups of arthropods on land, but their origin and times of diversification are not yet established. We estimated, for the first time, the molecular divergence times for these chelicerates using complete mitochondrial sequences from 25 taxa. All mitochondrial genes were evaluated individually or after concatenation. Sequences belonging to three missing genes (ND3, 6, and tRNA-Asp) from three taxa, as well as the faster-evolving ribosomal RNAs (12S and 16S), tRNAs, and the third base of each codon from 11 protein-coding genes (PCGs) (COI-III, CYTB, ATP8, 6, ND1-2, 4L, and 4-5), were identified and removed. The remaining concatenated sequences from 11 PCGs produced a completely resolved phylogenetic tree and confirmed that all chelicerates are monophyletic. Removing the third base from each codon was essential to resolve the phylogeny, which allowed deep divergence times to be calculated using three nodes calibrated with upper and lower priors. Our estimates indicate that the orders and classes of spiders, scorpions, mites, and ticks diversified in the late Paleozoic, much earlier than previously reported from fossil date estimates. The divergence time estimated for ticks suggests that their first land hosts could have been amphibians rather than reptiles. Using molecular data, we separated the spider-scorpion clades and estimated their divergence times at 397 ± 23 million years ago. Algae, fungi, plants, and animals, including insects, were well established on land when these chelicerates diversified. Future analyses, involving mitochondrial sequences from additional chelicerate taxa and the inclusion of nuclear genes (or entire genomes) will provide a more complete picture of the evolution of the Chelicerata, the second most abundant group of animals on earth.

Electronic supplementary material  The online version of this article (doi:10.1007/s10493-008-9203-5) contains supplementary material, which is available to authorized users.