Divergence in larval diapause induction between the rice and water-oat populations of the striped stem borer, Chilo suppressalis (Walker) (Lepidoptera: Crambidae)

Abstract

Differences in diapause traits can result in the seasonal reproductive isolation of host plant-associated insect populations and thereby facilitate the population divergence. The striped stem borer, Chilo suppressalis, has two host plant-associated populations: rice population and water-oat population. Several studies have found evidence that seasonal reproductive isolation between these populations is at least partially due to interpopulation differences in diapause. However, there still lack unambiguous evidence comparing characteristics of diapause induction for both populations. We compared the photoperiodic response and the age of peak photoperiod sensitivity of these populations and used RNA-Seq to compare the molecular response of diapause induction between populations. The photoperiodic response of the two populations differed at 25 °C; the critical night length of larvae from the rice population was 11 h and 20 min, whereas no obvious critical night length was in those from the water-oat population. In rice population, larvae were most sensitive to photoperiod at 9-12 days of age, whereas in water-oat population, larvae were the most sensitive to photoperiod at 9-10 days of age. The RNA-Seq results indicated that there were several differences in the molecular response of diapause induction and small overlap in differentially expressed genes (DEGs) between populations. Furthermore, GO analysis indicated that both rice and water-oat population's DEGs were significantly enriched in heme and iron binding. Besides, water-oat population's DEGs were significantly enriched in metabolizing nutrients but rice population's DEGs do not. Thus, our results described differences in diapause induction between rice and water-oat populations of C. suppressalis which could affect the timing of diapause and thereby contribute to the seasonal reproductive isolation of these host plant-associated populations. In conclusion, this work suggests that difference in diapause induction could promote the population divergence in insects associated with different host plants.

Time-Series Transcriptomic Analysis Reveals the Molecular Profiles of Diapause Termination Induced by Long Photoperiods and High Temperature in Chilo suppressalis

“…Field-collected larvae were exposed to long photoperiods and high temperatures in the laboratory to terminate diapause, which contrasts with previous studies involving extreme endocrine or pharmacological manipulation. Previous studies mainly focused on comparisons of non-diapausing and diapausing states [ 20 , 24 , 25 , 26 ], or clarifying the threshold of environmental stimuli to induce or terminate diapause [ 16 , 19 , 27 ]. The results of our study provide a roadmap to understand the physiological and molecular mechanisms associated with insect diapause termination, especially in Lepidoptera.…”

“…Diapausing C. suppressalis larvae are able to endure cold and lack of food, which enables them to survive upcoming harsh winters [ 17 ]. Laboratory studies showed that C. suppressalis can be induced to enter diapause by a short photoperiod, and night length is a key component for initiating diapause [ 16 , 18 , 19 ]. In contrast, diapause can be terminated when field-collected C. suppressalis larvae (from the winter period) are exposed to long photoperiods (light:dark, 16:8) and high temperatures (15–30 °C) in the laboratory [ 16 ].…”