Detecting Population Structure of West Coast Red Urchins
Researcher:
Ron Burton
Scripps Institution of Oceanography
UC San Diego
E.: rburton@ucsd.edu
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Revised:
May 7, 2009
A red sea urchin. Credit: Wikipedia / Kirt L. Onthank
May 7, 2009
Contact: Christina S. Johnson, csjohnson@ucsd.edu, 858-822-5334
SAN DIEGO – Despite having one of the longest larval periods of any of the common marine invertebrates, red sea urchins are, in fact, genetically differentiated along the West Coast.
Marine biology professor Ron Burton of the University of California at San Diego and former graduate student Celeste Benham analyzed 500 adult urchins from San Diego, Los Angeles and Mendocino counties across five microsatellite markers and then compared the genetic patterns to an existing, similar database of 1,400 urchins from British Columbia.
The genetic signatures strongly suggest that California and British Columbia urchins are not connected via larval dispersal and therefore comprise two distinct populations, the scientists report.
“The take-home message of this study is that if you use more markers and newer techniques you will find some population differentiation that before nobody found,” Burton says, referring to past studies that used fewer genetic markers and detected no population genetics structure in the species across large swaths of its range.
Professor Ron Burton.Credit: Scripps Institution of Oceanography
“From my evolutionary perspective, our results are important because they imply that, even on long time scales, there is no mixing," he says. "This means there is at least the potential for populations to adapt to different ocean conditions and gradually diverge. This is the first step in the two populations potentially becoming different species.”
The larval period of red sea urchins can exceed two months, compared with about a week, or less, for some species of abalones, chitons and limpets.
"The idea has been that ocean currents must be mixing urchin larvae all over the place," says Benham, now a research assistant at the marine mammal laboratory at Hubbs-SeaWorld Research Institute in San Diego. "But, there is more going on out there."
In a study of urchins from the Point Loma kelp bed in San Diego, Benham discovered that urchin "recruits" (animals less than 2 years old) are genetically different from juvenile and adult urchins in the area.
"This means that the next generation is coming from somewhere else and/or that only some adults in Point Loma are contributing to the next generation," she says.
Adult urchins may reach the age of 100. Because of this, studies based entirely on adult urchin DNA, such as the one reported here, may represent a 100-year average of larval dispersal patterns, Burton explains. The effect is a very low-resolution glimpse of the processes driving urchin population genetics.
Burton says that the next step in refining scientists' understanding of larval connectivity is to bin DNA tissue analyses according to urchin age groups. A first stab at this would be to compare DNA markers among urchins of the same size, which roughly correlates to age.
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