Mara Heilig

The physiology of ectotherms can change substantially during acclimation to changing environmental temperature. The role of transcription in acclimation responses has been well-established, but it remains unclear whether transcriptional regulation generally reflects abrupt changes after surpassing temperature thresholds, or whether transcript abundance is a relatively monotonic, continuous function of acclimation temperature. In this study we exposed adult male Drosophila subobscura flies to four different 96-hour acclimation treatments at temperatures that were not acutely stressful but ranged from relatively cold (10°C) to relatively warm (27°C) with respect to standard rearing conditions. Transcriptome sequencing of whole-body homogenates (mRNAseq) revealed a massive, transcriptome-wide response across acclimation temperatures, with a marked overrepresentation of genes that were continuously and monotonically up- and down-regulated in response to increasing acclimation temperature. Though some genes showed more complex relationships consistent with putative threshold responses, a high percentage of the differentially expressed transcriptome (42%) showed continuous and strictly monotonic relationships. Functional enrichment suggested continuous up-regulation of spermatogenesis-related transcripts with increasing temperature and continuous up-regulation of oxidative phosphorylation-related transcripts with decreasing temperature, illustrating contrasting patterns consistent with previous studies of thermal sensitivity of male reproduction and metabolic compensation in the cold. Thus, continuous thermal sensitivity of transcription is a hallmark of acclimation in D. subobscura that likely underlies the continuous thermal sensitivity of downstream physiological processes. We also provide evidence for shared transcriptomic responses across short-term acclimation (this study) vs. published results for long-term, developmental acclimation.

Cellular stress responses are often characterized as conserved, cell-autonomous processes. However, it remains unclear whether stress responses are coordinated uniformly across tissues within complex organisms, particularly during ecologically relevant conditions. We investigated tissue- and stage-specific transcriptional responses to cold stress in Drosophila melanogaster . Adults and larvae were independently exposed to a gradual cooling and recovery time series, and three adult tissues (gut, ovary, brain) and one larval tissue (gut) were sampled at baseline, at two time points that spanned the critical thermal minimum (before and during chill coma), and after recovery to rearing temperature. Transcriptomic analyses revealed strongly tissue- and stage-specific responses to cold stress, with limited overlap in differentially expressed genes or functional enrichment across tissues. These results indicate that the organismal response to thermal stress at the transcriptional level is not coordinated by a unified transcriptional program, but rather by largely distinct, tissue-specific regulatory processes.

Transgenerational signaling allows many organisms to anticipate seasonal variation. Diapause, a hormonally-programmed period of developmental arrest, is a widespread anticipatory seasonal response that is often mediated by transgenerational signaling. Despite the well-established adaptive significance of diapause, the molecular and physiological basis of transgenerational diapause signaling remains largely unresolved. Since lipid accumulation is a hallmark of diapause preparation, we hypothesized that increased lipid provisioning and accumulation mediate transgenerational diapause signaling. To test this hypothesis, we suppressed transcripts of two lipid metabolism genes using RNA-interference in the maternal generation of the vector mosquito, Aedes albopictus. We show that a reduction in maternal lipid storage droplet 2 (lsd2) transcript abundance, but not diacyl-glycerol O-acyltransferase 1 (dgat1) transcript abundance, reduces egg triglyceride levels. Suppression of lsd2 in adult females also leads to increased egg desiccation, inhibits the starvation tolerance of larvae, and decreases egg overwinter survival. However, knockdown of lsd2 does not affect diapause incidence, the timing of diapause termination, and post-diapause development. Together, our results indicate that transgenerational lipid provisioning affects diapause-associated overwintering fitness traits, but not the regulation of entry into diapause or termination of diapause.

Transgenerational signaling allows many organisms to anticipate seasonal variation. Diapause, a hormonally-programmed period of developmental arrest, is a widespread anticipatory seasonal response that is often mediated by transgenerational signaling. Despite the well-established adaptive significance of diapause, the molecular and physiological basis of transgenerational diapause signaling remains largely unresolved. Since lipid accumulation is a hallmark of diapause preparation, we hypothesized that increased lipid provisioning and accumulation mediate transgenerational diapause signaling. To test this hypothesis, we suppressed transcripts of two lipid metabolism genes using RNA-interference in the maternal generation of the vector mosquito, Aedes albopictus . We show that a reduction in maternal lipid storage droplet 2 ( lsd2) transcript abundance, but not diacyl-glycerol O-acyltransferase 1 ( dgat1 ) transcript abundance, reduces egg triglyceride levels. Suppression of lsd2 in adult females also leads to increased egg desiccation, inhibits the starvation tolerance of larvae, and decreases egg overwinter survival. However, knockdown of lsd2 does not affect diapause incidence, the timing of diapause termination, and post-diapause development. Together, our results indicate that transgenerational lipid provisioning affects diapause-associated overwintering fitness traits, but not the regulation of entry into diapause or termination of diapause.

We evaluated miRNA and mRNA expression differences in head tissues between avid-biting vs. reluctant-biting Aedes albopictus (Skuse) females from a single population over a 20-min timescale. We found no differences in miRNA expression between avid vs. reluctant biters, indicating that translational modulation of blood-feeding behavior occurs on a longer timescale than mRNA transcription. In contrast, we detected 19 differentially expressed mRNAs. Of the 19 differentially expressed genes at the mRNA level between avid-biting vs. reluctant-biting A. albopictus, 9 are implicated in olfaction, consistent with the well-documented role of olfaction in mosquito host-seeking. Additionally, several of the genes that we identified as differentially expressed in association with phenotypic variation in biting behavior share similar functions with or are inferred orthologues of, genes associated with evolutionary variation in biting behaviors of Wyeomyia smithii (Coq.) and Culex pipiens (Lin.). A future goal is to determine whether these genes are involved in the evolutionary transition from a biting to a non-biting life history.

RNA interference (RNAi) is a conserved cellular pathway found in nearly all eukaryotes that leads to the silencing of target mRNAs. Using RNAi as a mechanism to knockdown specific genes has enabled functional and reverse genetics studies in a wide range of eukaryotes. Previous work suggests that RNAi is inhibited at lower temperatures, potentially limiting the possibility to perform knockdown studies on ecologically relevant phenotypes that are only expressed at low temperatures. To determine whether RNAi is inhibited at low temperatures in Aedes albopictus (Skuse), we injected mosquitoes reared at 20 ± 1 °C, with dsRNA targeting yellow-g2 and compared knockdown efficacy to mosquitoes injected at 26.5 ± 1 °C. Our results demonstrate efficient knockdown at both temperatures, thereby establishing the feasibility of RNAi for functional genetic studies in A. albopictus at low temperatures.

Creating transgenic insects is a key technology in insect genetics and molecular biology. A widely used instrument in insect transgenesis is the piggyBac transposase, resulting in essentially random genomic integrations. In contrast, site-specific recombinases allow the targeted integration of the transgene construct into a specific genomic target site. Both strategies, however, often face limitations due to low transgenesis efficiencies. We aimed to enhance transgenesis efficiencies by utilizing capped mRNA as a source of transposase or recombinase instead of a helper plasmid. A systematic comparison of transgenesis efficiencies in Aedes mosquitoes, as models for hard-to-transform insects, showed that suppling piggyBac transposase as mRNA increased the average transformation efficiency in Aedes aegypti from less than 5% with the plasmid source to about 50% with mRNA. Similar high activity was observed in Ae. albopictus with pBac mRNA. No efficiency differences between plasmid and mRNA were observed in recombination experiments. Furthermore, a hyperactive version of piggyBac transposase delivered as a plasmid did not improve the transformation efficiency in Ae. aegypti or the agricultural pest Drosophila suzukii. We believe that the use of mRNA has strong potential for enhancing piggyBac transformation efficiencies in other mosquitoes and important agricultural pests, such as tephritids.

Creating transgenic insects is a key technology in insect genetics and molecular biology. A widely used instrument in insect transgenesis is the piggyBac transposase, resulting in essentially ran-dom genomic integrations. In contrast, site-specific recombinases allow the targeted integration of the transgene construct into a specific genomic target site. Both strategies, however, often face limitations due to low transgenesis rates. We aimed to enhance transgenesis rates by utilizing a capped mRNA as a source of transposase or recombinase instead of a helper plasmid. A system-atic comparison of transgenesis rates in Aedes mosquitoes, as models for hard to transform in-sects, showed that suppling piggyBac transposase as mRNA increased the average transfor-mation efficiency in Aedes aegypti from less than 5% with the plasmid source to about 50% with mRNA. Similar high transformation activity was observed in Ae. albopictus with pBac mRNA. No efficiency differences between plasmid and mRNA were observed in recombination experi-ments. Furthermore, a codon-optimized version of piggyBac transposase delivered as plasmid didn’t improve the transformation efficiency in Ae. aegypti or the agricultural pest D. suzukii. We believe that the use of mRNA has strong potential for enhancing pBac transformation efficiencies in other mosquitoes and important agricultural pests such as tephritids.