Summer diapause induced by high temperatures in the oriental tobacco budworm: ecological adaptation to hot summers

Evidence for an adult summer diapause in mountain pine beetle (Coleoptera: Curculionidae) that varies geographically and among haplogroups

Identifying dormancy traits is important for predicting insect population success, particularly in a changing climate that could disrupt evolved traits. The mountain pine beetle (Dendroctonus ponderosae Hopkins) is native to North America, is responsible for millions of acres of tree mortality, and is expanding northward in Canada. Research has identified thermal traits important to epidemic-phase ecology that vary among populations. Genomic research identified 3 mountain pine beetle haplogroups representing Pleistocene glacial refugia. Significant variation in generation timing aligning with the haplogroups has been observed. The adult stage was previously identified as the likely cause of differences among populations, although the mechanism(s) remain unclear. We tested for an adult summer diapause that varies among populations from 2 haplogroups, southern Colorado (CO) (central haplogroup) and southern Idaho (ID) (eastern haplogroup) using respirometry and reproduction experiments. Warm temperatures (25 °C) resulted in reduced respiration rates of central haplogroup mountain pine beetle compared to a cool temperature treatment (15 °C), whereas respiration of the eastern haplogroup did not differ between the treatments. Mated pairs of central haplogroup mountain pine beetle reared/held at 15 °C were more likely to be classified with a higher reproductive success rating compared to pairs reared/held at 25 °C. These results support a facultative summer adult diapause in southern CO central haplogroup mountain pine beetle. Manifestation of this diapause was low/absent among adults from the northerly ID location. This diapause likely serves to maintain univoltinism shown to be important for mountain pine beetle epidemic-phase ecology. The variation occurring among haplogroups highlights the long-term, evolved processes driving local adaptations in mountain pine beetle.

Clinal variation in the temperature and photoperiodic control of reproductive diapause in Drosophila montana females

Insect adaptation to climatic conditions at different latitudes has required changes in life-history traits linked with survival and reproduction. Several species, including Drosophila montana, show robust latitudinal variation in the critical day length (CDL), below which more than half of the emerging females enter reproductive diapause at a given temperature. Here we used a novel approach to find out whether D. montana also shows latitudinal variation in the critical temperature (CTemp), above which the photoperiodic regulation of diapause is disturbed so that the females develop ovaries in daylengths that are far below their CDL. We estimated CTemp for 53 strains from different latitudes on 3 continents after measuring their diapause proportions at a range of temperatures in 12 h daylength (for 29 of the strains also in continuous darkness). In 12 h daylength, CTemp increased towards high latitudes alongside an increase in CDL, and in 3 high-latitude strains diapause proportion exceeded 50% in all temperatures. In continuous darkness, the diapause proportion was above 50% in the lowest temperature(s) in only 9 strains, all of which came from high latitudes. In the second part of the study, we measured changes in CTemp and CDL in a selection experiment favouring reproduction in short daylength (photoperiodic selection) and by exercising selection for females that reproduce in LD12:12 at low temperature (photoperiodic and temperature selection). In both experiments selection induced parallel changes in CDL and CTemp, confirming correlations seen between these traits along latitudinal clines. Overall, our findings suggest that selection towards strong photoperiodic diapause and long CDL at high latitudes has decreased the dependency of D. montana diapause on environmental temperature. Accordingly, the prevalence and timing of the diapause of D. montana is likely to be less vulnerable to climate warming in high- than low-latitude populations.

Cathepsin L Contributes to Reproductive Diapause by Regulating Lipid Storage and Survival of Coccinella septempunctata (Linnaeus)

Cathepsin L protease, which belongs to the papain-like cysteine proteases family, is an important player in many physiological and pathological processes. However, little was known about the role of cathepsin L in ladybird beetles (Coccinella septempuctata Linnaeus) during diapause. Here, we analyzed the characteristics of cathepsin L (CsCatL) in the females of C. septempunctata and its role during the diapause of the ladybeetle. CsCatL was cloned and identified from beetle specimens by rapid amplification of cDNA-ends (RACE). The cDNA sequence of CsCatL was 971 bp in length, including an 843 bp open reading frame encoding a protein of 280 amino acids. It was identified as the cathepsin L group by phylogenetic analysis. Knockdown of CsCatL by RNA interference led to decreased expression levels of fatty acid synthase 2 (fas 2) genes and suppressed lipid accumulation. Furthermore, silencing the CsCatL gene distinctly reduced diapause-related features and the survival of female C. spetempunctata under diapause-inducing conditions. The results suggested that the CsCatL gene was involved in fatty acid biosynthesis and played a crucial role in the survival of adult C. septempunctata during the diapause preparation stage.

Emergent Protective Organogenesis in Date Palms: A Morpho-Devo-Dynamic Adaptive Strategy during Early Development

Desert plants have developed mechanisms for adapting to hostile desert conditions, yet these mechanisms remain poorly understood. Here, we describe two unique modes used by desert date palms (Phoenix dactylifera) to protect their meristematic tissues during early organogenesis. We used x-ray micro-computed tomography combined with high-resolution tissue imaging to reveal that, after germination, development of the embryo pauses while it remains inside a dividing and growing cotyledonary petiole. Transcriptomic and hormone analyses show that this developmental arrest is associated with the low expression of development-related genes and accumulation of hormones that promote dormancy and confer resistance to stress. Furthermore, organ-specific cell-type mapping demonstrates that organogenesis occurs inside the cotyledonary petiole, with identifiable root and shoot meristems and their respective stem cells. The plant body emerges from the surrounding tissues with developed leaves and a complex root system that maximizes efficient nutrient and water uptake. We further show that, similar to its role in Arabidopsis (Arabidopsis thaliana), the SHORT-ROOT homolog from date palms functions in maintaining stem cell activity and promoting formative divisions in the root ground tissue. Our findings provide insight into developmental programs that confer adaptive advantages in desert plants that thrive in hostile habitats.

Developmental plasticity in metabolism but not in energy reserve accumulation in a seasonally polyphenic butterfly

The evolution of seasonal polyphenisms (discrete phenotypes in different annual generations) associated with alternative developmental pathways of diapause (overwintering) and direct development is favoured in temperate insects. Seasonal life history polyphenisms are common and include faster growth and development under direct development than in diapause. However, the physiological underpinnings of this difference remain poorly known despite its significance for understanding the evolution of polyphenisms. We measured respiration and metabolic rates through the penultimate and final larval instars in the butterfly Pieris napi and show that directly developing larvae grew and developed faster and had a higher metabolic rate than larvae entering pupal diapause. The metabolic divergence appeared only in the final instar, that is, after induction of the developmental pathway that takes place in the penultimate instar in P. napi. The accumulation of fat reserves during the final larval instar was similar under diapause and direct development, which was unexpected as diapause is predicted to select for exaggerated reserve accumulation. This suggests that overwinter survival in diapause does not require larger energy reserves than direct development, likely because of metabolic suppression in diapause pupae. The results, nevertheless, demonstrate that physiological changes coincide with the divergence of life histories between the alternative developmental pathways, thus elucidating the proximate basis of seasonal life history polyphenisms.

Evidence that microRNAs are part of the molecular toolkit regulating adult reproductive diapause in the mosquito, Culex pipiens

For many insects, diapause is the primary mechanism for surviving unfavorable seasons. Some aspects of diapause regulation are well known, but we still lack a mechanistic understanding of molecular mechanisms that control the diapause pathway. Accumulating evidence suggests microRNAs regulate diapause in evolutionarily diverse insect species including flesh flies and moths, and, it is likely that microRNAs regulate multiple characteristics of diapause, including arrested egg follicle development and fat hypertrophy, in females of the Northern house mosquito, Culex pipiens. To investigate microRNA regulation of diapause in this species, we measured changes in egg follicle development and total lipid content over 22 days following adult emergence. We also evaluated changes in the abundance of candidate microRNAs associated with these physical changes during the same time frame. We found egg follicle size and lipid content were nearly the same in diapausing and nondiapausing females on the day of adult emergence, and then diverged over time such that by day 22 diapausing females had significantly smaller egg follicles and higher total lipids than their nondiapausing counterparts. Several microRNAs associated with lipid metabolism in insects, including miR-14-3p, miR-277-3p, and miR-305-5p, were underexpressed in diapausing females compared to nondiapausing females on the day of adult emergence, which suggests microRNA regulation occurs ahead of observed changes in these two features of the diapause phenotype. We also found miR-309-3p, miR-375-3p which stimulate ovarian development in other mosquito species, were underexpressed in diapausing females of Cx. pipiens at times after diapause is fully established and may be responsible for the arrest in ovarian development in this species. Taken together, our results demonstrate that changes in the abundance of some microRNAs is associated with phenotypic changes in diapause Cx. pipiens and suggests this epigenetic mechanism is part of the molecular toolkit regulating diapause.

Biological Traits of Cadra cautella (Lepidoptera: Pyralidae) Reared on Khodari Date Fruits Under Different Temperature Regimes

The almond moth, Cadra cautella (Walker) (Lepidoptera: Pyralidae), is a serious worldwide pest of dates and other crops, both in the field and in storage. Infestation by this pest significantly limits the sale and export of dates. Modified atmospheres, the temperature manipulations, are the best eco-friendly alternatives in stored product protection. We examined temperature regulation as an ecologically friendly method of pest management in stored food commodities, in particular, its effects on characteristics of C. cautella biology, including development time, oviposition period, fecundity, fertility, and longevity of adults reared on date cultivar 'Khodari,' at temperatures of 15, 25, and 35 ± 1 °C, with 65 ± 5% relative humidity and a photoperiod of 15:9 (L:D) h. Six larval instars were recorded at each temperature. Larval development was sluggish at 15 °C; therefore, daily observations were terminated after 180 d of the larval span for this temperature. Total larval spans of 48.95 ± 0.76 and 32.12 ± 1.08 d were recorded at 25 °C and 35 °C, respectively. A mean ovipositional period of 5.80 ± 0.44 d yielding 213.60 ± 13.41 eggs per female, with hatchability of 85.94 ± 2.97%, was recorded at 25 °C, whereas at 35 °C, a mean ovipositional period of 1.60 ± 0.26 d yielded 19.80 ± 5.11 eggs per female, with no hatchability. These differences were highly significant. Our observations of poor development at 15 °C, lower numbers of eggs and absence of hatchability at 35 °C, together with head capsule measurements and growth ratios, can be of benefit when planning management strategies to reduce C. cautella infestation.

Fatty acid synthase 2 contributes to diapause preparation in a beetle by regulating lipid accumulation and stress tolerance genes expression

Diapause, also known as dormancy, is a state of arrested development that allows insects to survive unfavorable environmental conditions. Diapause-destined insects store large amounts of fat when preparing for diapause. However, the extent to which these accumulated fat reserves influence diapause remains unclear. To address this question, we investigated the function of fatty acid synthase (FAS), which plays a central role in lipid synthesis, in stress tolerance, the duration of diapause preparation, and whether insects enter diapause or not. In diapause-destined adult female cabbage beetles, Colaphellus bowringi, FAS2 was more highly expressed than FAS1 at the peak stage of diapause preparation. FAS2 knockdown suppressed lipid accumulation and subsequently affected stress tolerance genes expression and water content. However, silencing FAS2 had no significant effects on the duration of diapause preparation or the incidence of diapause. FAS2 transcription was suppressed by juvenile hormone (JH) and the JH receptor methoprene-tolerant (Met). These results suggest that the absence of JH-Met induces FAS2 expression, thereby promoting lipid storage in diapause-destined female beetles. These results demonstrate that fat reserves regulate stress tolerance genes expression and water content, but have no significant effect on the duration of diapause preparation or the incidence of diapause.