Juvenile hormone regulates photoperiod-mediated male reproductive diapause via the methoprene-tolerant gene in the ladybeetle Harmonia axyridis.

Juvenile hormone (JH) absence induces photoperiod-mediated reproductive diapause, which is characterized by reproductive cessation. Although the role of methoprene-tolerant (Met)-mediated JH signaling in photoperiod-mediated female reproduction has been well documented, its role in male reproduction remains unclear. In this study, we investigated the role of JH in regulating photoperiod-mediated development of the male internal reproductive system (IRS) in the predatory ladybeetle Harmonia axyridis (Pallas). In a previous study, we found that adult male H. axyridis reared under either a short-day (SD) or long-day (LD) photoperiod had obvious differences in IRS development, but we were unable to identify the regulators of male reproductive diapause. In this study, we found that beetles reared under an SD photoperiod had significantly lower JH titer and a relatively undeveloped male IRS compared with those reared under an LD photoperiod. Additionally, application of the JH analog (JHA) methoprene promoted IRS development. Furthermore, Met knockdown strongly blocked JH signaling in males reared under the LD photoperiod, thereby slowing IRS development. Moreover, exogenous JHA did not reverse the suppressed development of the male IRS caused by Met knockdown. These results indicate that photoperiod regulates male IRS development in H. axyridis through a conserved Met-dependent JH signaling pathway.

Differences in the Development of Internal Reproductive Organs, Feeding Amount and Nutrient Storage between Pre-Diapause and Pre-Reproductive Harmonia axyridis Adults.

Diapause control is one of the successful methods for long-term cold storage of biological control organisms without decreasing their fitness. Sufficient preparation before diapause is essential for successful diapause initiation and maintenance. Harmonia axyridis, an important biocontrol agent in its native place, can enter reproductive diapause for overwintering. However, the key preparatory events before diapause in H. axyridis, such as specific developmental trajectory, timing, and physiological changes, remain unclear. We compared differences in the development of internal reproductive organs, feeding amount and nutrient storage between pre-diapause and pre-reproductive adult H. axyridis which had been reared at 20 °C under a short-day (10L:14D) and long-day (14L:10D) condition, respectively. The results showed that there were obvious morphological differences in internal reproductive organs of diapause and reproductive H. axyridis. The development of internal reproductive organs was suppressed at early adult stage in pre-diapause beetles compared to pre-reproductive beetles. Feeding amount in both pre-diapause and pre-reproductive beetles increased for more than ten days after eclosion. The feeding amount of pre-diapause beetles sharply decreased from the 15th day after eclosion in females and the 14th day after eclosion in males, which implied the initiation of diapause. During the pre-diapause stage, carbohydrates and lipids were mainly accumulated by females, whereas males mainly accumulated carbohydrates. Our results not only provide basic information about the diapause process of pre-diapause stage, but also give tips to better utilization of diapause strategy during mass production and storage of H. axyridis.