LmCht5-1 and LmCht5-2 Promote the Degradation of Serosal and Pro-Nymphal Cuticles during Locust Embryonic Development

The success of the degradation of the extraembryonic serosal cuticle and the second embryonic cuticle (pro-nymphal cuticle) is essential for the development and molting of nymph from egg in Orthoptera Locusta migratoria. Chitinase 5 is an important gene for chitin degradation in nymphs and in the egg stage. In this study, we investigated the important roles of chitinase 5-1 (LmCht5-1) and chitinase 5-2 (LmCht5-2) in the degradation of the serosal and pro-nymphal cuticles during locust embryonic development. The serosal cuticle degrades from 7-day-old embryos (E7) to E13, along with the degradation of the pro-nymphal cuticle, which begins at E12 to E14. The mRNA and protein of LmCht5-1 and LmCht5-2 are expressed during the degradation process of the serosal cuticle and the pro-nymphal cuticle. RNAi experiments at the embryonic stage show that both dsLmCht5-1 and dsLmCht5-2 contribute to the failure of development in early and late embryogenesis. Further, during the serosal cuticle molting process, ultra-structure analysis indicated that dsLmCht5-1 prevented the loss of the coarse chitin layer in the upper part in both early and late embryogenesis. Meanwhile, dsLmCht5-2 blocked the degradation of the lower fine chitin layer at the early stage and blocked the chitin degradation of loose coarse chitin in the late molting process. During the degradation of the pro-nymphal cuticle, dsLmCht5-1 suppresses chitin degradation between layers in the procuticle, while dsLmCht5-2 suppresses chitin degradation into filaments inside of the layer. In summary, our results suggest that both LmCht5-1 and LmCht5-2 contribute to the degradation of the serosal and pro-nymphal cuticles during the locust embryonic stage.

An odorant-binding protein mediates sexually dimorphic behaviors via binding male-specific 2-heptanone in migratory locust

Migratory locusts (Locusta migratoria) frequently aggregate into huge swarms that cause serious economic losses for the agricultural sector. Differential behaviors of male and female insects may contribute to such population explosions. However, the key olfactory mechanisms underlying different behaviors associated with sex-related pheromones are unclear. Here, we report that male-specific odor, 2-heptanone plays different roles in relation to the behavior of migratory locust males and females, and that this sexual dimorphism involves a soluble odorant-binding protein (OBP) in the peripheral olfactory processes. This odor strongly binds to LmigOBP4, a novel OBP, present in antennal trichoid sensilla, and elicits opposite locomotor tendencies between the sexes: attracting females and repelling males. Furthermore, an adult male group mimicked a high dosage of 2-heptanone by promoting their attractiveness to single females. Additionally, RNAi suppression of Lmigobp4 expression reduced the physiological responses to 2-heptanone to levels that were indistinguishable between the sexes. This suppression reversed the adult behavioral responses to 2-heptanone, i.e., females were repelled and males were attracted. We conclude that LmigOBP4 is associated with olfactory recognition of male-specific 2-heptanone, which plays dual roles that differ between adult male and female migratory locusts.

Two chitinase 5 genes from Locusta migratoria: molecular characteristics and functional differentiation

The duplication of chitinase 5 (Cht5) into two to five different genes has been reported only in mosquito species to date. Here, we report the duplication of Cht5 genes (LmCht5-1 and LmCht5-2) in the migratory locust (Locusta migratoria). Both LmCht5-1 (505 aa) and LmCht5-2 (492 aa) possess a signal peptide and a catalytic domain with four conserved motifs, but only LmCht5-1 contains a chitin-binding domain. Structural and phylogenetic analyses suggest that LmCht5-1 is orthologous to other insect Cht5 genes, whereas LmCht5-2 might be newly duplicated. Both LmCht5 genes were expressed in all tested tissues with LmCht5-1 highly expressed in hindgut and LmCht5-2 highly expressed in integument, foregut, hindgut and fat bodies. From the fourth-instar nymphs to the adults, LmCht5-1 and LmCht5-2 showed similar developmental expression patterns with transcript peaks prior to each nymphal molting, suggesting that their expression levels are similarly regulated. Treatment with 20-hydroxyecdysone (20E; the most active molting hormone) and reducing expression of EcR (ecdysone receptor gene) by RNAi increased and decreased expression of both LmCht5 genes, respectively, indicating that both genes are responsive to 20E. Although transcript level of LmCht5-2 is generally 10-fold higher than that of LmCht5-1, RNAi-mediated suppression of LmCht5-1 transcript led to severe molting defects and lethality, but such effects were not seen with RNAi of LmCht5-2, suggesting that the newly duplicated LmCht5-2 is not essential for development and survivorship of the locust.