How Smells Can Split Species

An altered sense of smell could drive the formation of new species, as seen in hawthorn flies and apple flies.

AsianScientist (Feb. 2, 2017) – A small switch in the brains of flies is enough to drive the evolution of two distinct species, according to a study published in the Proceedings of the Royal Society B. The research, by scientists at the National Centre for Biological Sciences (NCBS), Bangalore, provides a neurosensory explanation of how hawthorn files and apple flies evolved.

Hawthorn flies and apple flies are considered to be two races of the species complex Rhagoletis pomonella. The flies are textbook examples for the process of sympatric speciation, a process by which new species evolve in the same geographic region from a common ancestor species. The two races of flies maintain separate populations on the basis of preferred host fruits, which they detect through smells; apple flies prefer apple scents, while hawthorn flies prefer hawthorn fruit smells.

“Changes in behavior can lead to the evolution of new species, particularly when these behaviors influence habitat choice. Yet the neural bases for such changes are relatively unknown,” said Dr. Shannon Olsson, who heads a laboratory on chemical ecology at NCBS and was involved in this study.

A major factor that has limited scientists’ understanding of how these races could be evolving was a prior inability to study their nervous systems closely.

Now, researchers have investigated the differences at a sensory level between two populations in the process of differentiating into distinct species. To gain a neurosensory perspective on how the distinct preferences for apples and hawthorn fruits arose in these flies, the scientists began by examining how nerve cells on the antennae of apple flies and hawthorn flies react to the different fruits’ scents.

The team identified 28 classes of nerve cells called olfactory sensory neurons (OSNs) that responded to different combinations of odors. Among this set, were a small collection of OSNs that responded to key chemicals from apple and hawthorn fruit. Previous studies had identified the two chemicals—butyl hexanoate in apple odours and 3-methyl-1-butanol in hawthorn fruit odours—to be most important in attracting apple and hawthorn flies respectively.

While testing this smaller subset of nerve cells with these chemicals, the researchers found a startling pattern. Just two pairs of OSNs located in the same area of the fly antenna could be the cause of apple and hawthorn flies’ specific preferences.

These findings suggest that a tiny switch in the wiring of two channels in the brain—one coding for detection of hawthorn odors, and one for apple—could have created a change in host fruit preference. This change in behavioral preference has seeded the beginnings of speciation by keeping apple flies and hawthorn flies as separated populations, isolated from each other.

“Our work is significant in its implication that even for such complex behaviors as host choice, tiny changes in the nervous system can have dramatic effects on a species, even on an evolutionary timescale,” said Olsson.

“This finding thus has implications beyond evolution for our understanding of the relationships between the brain, behavior, and animal ecology—especially in the case of native species’ response to an introduced, foreign species—a big issue for us in India,” she adds.

The article can be found at: Tait et al. (2016) Sensory Specificity and Speciation: A Potential Neuronal Pathway for Host Fruit Odor Discrimination in Rhagoletis pomonella.


Source: National Center for Biological Sciences.
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