suspect that the reason we got different results is that the UCSD lab relied on DNA microsatellite markers that look virtually the same in Argentine ants throughout California," Gordon noted. "In our study, we added different markers that are more variable, which made it easier to detect genetic variation among different colonies."

The genetic work at Jasper Ridge shows that there are definite barriers to gene flow, Heller added. "We've also found that the distribution of Argentine ants on Jasper Ridge is patchy," she noted. "There are places where you just don't find any, so it's not like they're blanketing the entire state."

While agreeing that there are "big gaps" in the California population, Tsutsui maintained that the Jasper Ridge study did nothing to undermine the notion of a statewide supercolony.

"The big supercolony in California extends for almost 1,000 kilometers [600 miles]," he said. "Although it is not continuous throughout this length, it is technically a single big colony, because workers can be freely exchanged among different nests without triggering any aggression. That is, throughout this 1,000 kilometers, virtually all the nests that we looked at did not display territorial behavior toward each other."

But, asked Gordon, does it really matter if an ant in San Diego doesn't fight with an ant in San Francisco, if they will never actually encounter each other in nature? "The UCSD lab argues that the lack of aggression among California ants can be explained by the lack of microsatellite variation," she said. "But our Jasper Ridge study revealed plenty of variation, so how can we say that genetic differences have anything to do with aggression?"

European 'union'

The Stanford study at Jasper Ridge reinforced an April 2002 PNAS study by European scientists, who reported unearthing an even bigger Argentine ant supercolony stretching from Portugal to Italy alon
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Contact: Dawn Levy
dawnlevy@stanford.edu
650-725-1944
Stanford University
7-Apr-2004

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