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No mixing for European corn borers

Researchers at the biodiversity dynamics laboratory (Ladybio)1 in Toulouse are attempting to understand speciation, which occurs when two groups of the same species stop reproducing with each other and diverge genetically, sometimes going as far as creating new species. Thibaut Malausa and his colleagues have been studying the European corn borer.2  “This moth eats corn, but also other plants like mugwort,” says Malausa. “We are trying to evaluate the role of differing food resources in speciation.”

 Studies have already shown that there are at least two groups of genetically distinct moths, one living on corn and the other on mugwort. It is also known that, in the laboratory, adults prefer to mate with individuals from their own group. This is difficult to verify in a natural environment, since the moths fly away once they are adults, leaving their original plant. Moths from both groups are morphologically identical. To discover who was mating with whom, the researchers used a biogeochemical marker: the proportions of two carbon isotopes in the plants, which differ from one plant to the other. This marker, transmitted to the organism that eats the plant, shows what a particular moth fed on and therefore to which group it belongs. By analyzing the tissues of females captured in the field and the spermatophores (the little solidified droplets of sperm that they carry, left by the male during mating), Malausa quantified cross-fertilization between individuals from the two groups.3 The result is very clear: the probability that a corn borer will mate with a mugwort borer is less than five percent, even when the two groups are in the same place at the same time. “There is therefore almost total reproductive isolation in this species,” he explains. “This is one of the rare cases in which we can quantify cross-breeding directly and in a natural environment.” This raises many questions about the evolution of the species. The researchers are now investigating the part played by the plant in this evolutionary process.

Stéphanie Belaud

Notes :

1. Dynamique de la biodiversité. Joint lab: CNRS / University of Toulouse-III.
2. Collaboration between Ladybio and Montpellier's INRA Center for population biology and management.
3. T. Malausa et al., “Assortative mating in sympatric host races of the European corn borer,” Science 308: 258-60. 2005.

Contacts :

Thibaut Malausa
Laboratoire Dynamique de la biodiversité, Toulouse
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