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Mites Alter Reproductive Behavior

Reproduction of tiny orabatid mites raises questions about evolutionary biology.

SYRACUSE, N.Y., — A group of researchers has discovered that a family of tiny mites found in the Southern Hemisphere has taken the unusual step of resuming sexual reproduction after years of producing offspring through asexual means, raising intriguing questions about evolutionary biology.

imageIn a paper published this week in the Proceedings of the National Academy of Sciences, Dr. Roy A. Norton of the SUNY College of Environmental Science and Forestry (ESF) and three German colleagues describe their discovery regarding a family of oribatid mites (also known as beetle mites because of their hard shells).

The publication marks the first time that researchers have formally reported the revival of a complex trait such as sexual reproduction after it had been long dormant, Norton said.

“They found a way to re-evolve sex,” Norton said.

The discovery touches on questions that have puzzled biologists for years:  Why do some organisms continue to reproduce asexually, given the distinct evolutionary advantages that a species derives — especially defenses against antagonists such as parasites, predators and competitors—from mixing genomes? And how can an organism jump-start a group of genes, such as those specific to sexual reproduction, after many millions of years of not being used?

“Now the question is: How can these genes reverse selectively and then come back to work?” Norton said. “We’re talking about something that involves a lot of elements in both males and females.”

Norton said the research team discovered both males and females within a family of mites, known by its scientific name of Crotoniidae, that lives in trees and on rocks. Oribatid mites are most commonly found in soil, where they perform a vital role as decomposition agents, grazing on fungi, decaying leaves and other organic matter.

Norton speculates that moving from their ancestral home in the ground, where organic material for nourishment is plentiful in soil, to trees and rocks, where food can be scarce, could account for the change in reproduction. The increased exposure and competition for food may create a need for more responsive genetic defense mechanisms, he said.

Oribatid mites, most of which are not more than three-quarters of a millimeter long, provide a rich subject for the study of sexual vs. asexual reproduction. In most taxonomic groups, less than one-tenth of one percent of the species are asexual, Norton said. In oribatid mites, the total is closer to 10 percent.

“That makes them a beautiful test group to examine this question,” he said.

And there is no lack of mites to study.

“When you walk in the woods, under your foot there could be upwards of 30 species of oribatid mites. And right nearby, there might be 100 more,” he said. “They are both diverse and abundant.”

Norton worked on a team with three researchers from the Technical University of Darmstadt in Germany. They are Katja Domes, Mark Maraun and Stefan Scheu.