Comparative transcriptomics of the pheromone glands provides new insights into the differentiation of sex pheromone between two host populations of Chilo suppressalis.

Reproductive isolation between different host populations is often based on intraspecific sex pheromone differences. The mechanisms underlying these differences have not been thoroughly elucidated to date. Previous studies suggested that Chilo suppressalis has differentiated into rice and water-oat host populations, and these two populations manifest clear differences in sex pheromone titer and mating rhythm. Hence, this moth is an ideal model to investigate the endogenous mechanisms of intraspecific reproductive isolation. Here, we identified a series of putative genes associated with sex pheromone biosynthesis based on the C. suppressalis pheromone gland transcriptome data. Transcripts of most genes were at higher level in the rice population. Then we obtained 11 pivotal differentially expressed genes (DEGs). The expression levels of these DEGs exhibited a distinct increase in the rice population. Moreover, we also observed the expression rhythm of these DEGs is discrepant between two host populations. Our study offers a new understanding to elucidate the mechanisms of intraspecific reproductive isolation.

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

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.

Difference in diel mating time contributes to assortative mating between host plant-associated populations of Chilo suppressalis.

Behavioral isolation in animals can be mediated by inherent mating preferences and assortative traits, such as divergence in the diel timing of mating activity. Although divergence in the diel mating time could, in principle, promote the reproductive isolation of sympatric, conspecific populations, there is currently no unequivocal evidence of this. We conducted different mate-choice experiments to investigate the contribution of differences in diel mating activity to the reproductive isolation of the rice and water-oat populations of Chilo suppressalis. The results show that inter-population difference in diel mating activity contributes to assortative mating in these populations. In the rice population, most mating activity occurred during the first half of the scotophase, whereas in the water-oat population virtually all mating activity was confined to the second half of the scotophase. However, when the photoperiod of individuals from the water-oat population was altered to more closely align their mating activity with that of the rice population, mate choice was random. We conclude that inter-population differences in diel mating time contribute to assortative mating, and thereby the partial reproductive isolation, of these host-associated populations of C. suppressalis.

Thermal effects on development and adult longevity of endoparasitoid Chelonus murakatae Munakata (Hymenoptera: Braconidae).

The effects of temperature on the development duration and longevity of adult of Chelonus murakatae were studied under five constant temperatures including 22.5, 25, 27.5, 30 and 32.5 ± 0.5 °C under laboratory conditions. It was observed that the development time was inversely proportional to the temperature within the range of 22.5 to 32.5 °C. The results indicated that the optimum temperature for development ranged from 25 to 30 °C. Thermal threshold was estimated by a linear model which was recorded as 15.5 and 18.5 °C for males and females, respectively. Number of degree days required to complete the development from egg to adult were 439.6 degree days in males and 336.8 degree days in females. Adult longevity also decreased with increase in temperature. This information can be used for optimizing mass culturing and field release for an efficient biological control of Chilo suppressalis in this specie.

Do differences in life-history traits and the timing of peak mating activity between host-associated populations of Chilo suppressalis have a genetic basis?

The development of host races, genetically distinct populations of the same species with different hosts, is considered to be the initial stage of ecological speciation. Ecological and biological differences consistent with host race formation have been reported between water-oat and rice-associated populations of Chilo suppressalis. In order to confirm whether these differences have a genetic basis, we conducted experiments to determine the extent to which various life-history traits and the time of peak mating activity of these populations were influenced by the species of host plant larvae were raised on. Individuals from each population were reared for three consecutive generations on either water-oat fruit pulp or rice seedlings. Descendants of both populations had higher larval survival rates, shorter larval developmental periods, higher pupal weight, and longer adult forewings, when reared on water-oats than when reared on rice. The time of peak of mating activity differed between the descendants of each population, irrespective of whether they were raised on water-oats or rice. These results indicate that although some life-history traits of host-associated populations of C. suppressalis are influenced by the host plant larvae are raised on, time of peak mating activity is not. Because it is a stable, objective, phenotypic trait, further research on difference in the time of peak mating activity between host-associated populations of C. suppressalis should be conducted to clarify the mechanism responsible for host race formation in this species.