Variability and genetic components of innate fruit odour recognition in a parasitoid of Drosophila

This study aims to investigate the genetic variability of olfactory responses to odours from the host habitat, involved in host selection by insect parasitoids. The probing response of females to fruit and non-fruit odours was studied in Leptopilina boulardi Barbotin et al. (Hymenoptera, Figitidae), a parasitoid of the frugivorous Drosophila melanogaster larvae. Responses of two strains (from Nasrallah, Tunisia, and from Brazzaville, Congo) and their hybrids were compared. Probing is a searching mode by repeated ovipositor insertions into the breeding substrate to locate digging larvae. Most Brazzaville females exhibited innate ovipositor probing into agar in response to the fruit aromas (banana, orange, pear, prickly pear), whereas this was rare in the Nasrallah strain. These four fruits are all breeding sites for D. melanogaster, whereas mushroom is not. Mushroom odour did not trigger any probing response in either strain. In addition, the latency of the probing response was shorter in Brazzaville females. Results from hybrid females (F1 and Backcross) showed that both probability to probe in response to a fruit aroma and latency of the response were strongly heritable. Regarding the probability to probe, there was an apparent complete dominance of Nasrallah characteristics, whereas additive inheritance was found in the Brazzaville lineage. The contrast between reciprocal F1 hybrids suggested a strong interaction with non-chromosomal factors. It is suggested that a cytoplasmic factor in the Nasrallah strain decreases females' propensity to probe in response to significant odours. The latency of probing appeared to be under a chromosomal influence. Thus, this study identifies a strong genetic component in the responses to odours from the host habitat. It also provides information on the genetic basis of the two characters studied.