Differences in rectal amino acid levels determine bacteria-originated sex pheromone specificity in two closely related flies
https://doi.org/10.1038/s41396-023-01488-9
In the intricate world of nature,finding and judging a suitable mate is pivotal for reproduction in many animals. Sex pheromones are widely used by insects as a reproductive isolating mechanism to attract conspecifics and repel heterospecifics. Closely related species usually have similar sex pheromones that may only differ in the ratio of the different chemical components. However, the differentiation mechanism of sex pheromones in closely related species is still unclear.
In order to understand the mechanism, we use Bactrocera dorsalis and Bactrocera cucurbitae as the study model to investigate how the male-borne sex pheromones are different. The results demonstrated that both 2,3,5-trimethylpyrazine(TMP) and 2,3,5,6-tetramethylpyrazine (TTMP) were sex pheromones produced by rectal Bacillus using glucose and threonine (or glycine) in the two flies. However, the TMP/TTMP ratios were reversed, indicating sex pheromone specificity in the two flies (Fig 1).

Fig 1.Schematic illustrating the sex pheromone specificity generation hypothesis between B. dorsalis and B. cucurbitae. In B. dorsalis, low Sardh and ItaE gene expression results in lower threonine and glycine synthesis activity and thus leads to a higher TMP/TTMP ratio produced by rectal bacteria. In contrast, higher Sardh and ItaE gene expression results in higher threonine and glycine synthesis activity and thus leads to a lower TMP/TTMP ratio produced by rectal bacteria.
Through a series of experiments, we have shown that this specificity is not related to the difference in rectal Bacillus or the rectal glucose. Different amino acid levels determined the preference of rectal Bacillus to produce TMP or TTMP.
Modifications of sex pheromones are thought to represent a rapid means for altering mate recognition abilities during the evolution of new species. Our results provide insights into how interspecific sexual barriers arise. Our results may indicate a mode of evolution whereby rapid changes in chemical composition occur during speciation events, and greater divergence gradually occurs subsequently.
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