Monarch miscalculation

Tuesday, February 28th, 2017

It looks like a scientific error about butterflies has persisted for more than 40 years:

A few years ago, Christopher Hamm was reading up on monarch butterflies when he noticed something peculiar. All of the scientific articles that mentioned the number of the insect’s chromosomes — 30, it seemed — referenced a 2004 paper, which in turn cited a 1975 paper. But when Hamm, then a postdoc at the University of Kansas in Lawrence, did a genetic analysis of his own, he found that his monarchs only had 28 chromosomes, suggesting that an error has pervaded the literature for more than 40 years. Another twist, however, was just around the corner.

Hamm suspected a mistake when he read the original 1975 paper. The authors, biologists N. Nageswara Rao and A. S. Murty at Andhra University in Visakhapatnam, India, had studied what they claimed was an Indian monarch butterfly in their work. But there’s a problem: Monarchs are nearly exclusively a North American species. “It’s implied they just went outside their building and collected some butterflies,” Hamm says. “I immediately thought, ‘Monarch butterflies in India? Really?’”

Sure monarchs are master travelers, with the longest-known seasonal migration of any insect. And it’s not uncommon for a few to get blown off course to Australia, the Philippines, the United Kingdom, and a handful of other places from time to time. But ending up as far away as India seemed like a stretch. Hamm, now a data scientist at Monsanto in Woodland, California, also knew that taxonomists since Carl Linnaeus have struggled to distinguish species in Lepidoptera, the order of insects to which monarchs belong. For example, the monarch (Danaus plexippus) and a similar-looking butterfly known as the common tiger butterfly (D. genutia) were thought to be the same for more than a century until they were reclassified as separate species in 1954. And guess what: D. genutia lives in India.

Common Tiger Butterfly and Monarch Butterfly

Hamm thinks that Rao and Murty, perhaps not knowing about the reclassification, netted bugs they assumed were monarchs but were actually common tiger butterflies. Back in the lab, they performed a technique known as a chromosome squash — squeezing the butterflies’ cells between thin films of glass until individual chromosomes are visible under a microscope — counted to 30, and published the results. Then, in 2004, Brazilian zoologist Keith Brown Jr. cited the work in his own research exploring the evolutionary history of butterflies; he never suspected that Rao and Murty might have been working with a misidentified species. Brown’s paper has been cited a dozen times since, and the idea that monarchs have 30 chromosomes is now well established in the literature.

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