Identifying potential maternal genes of Bombyx mori using digital gene expression profiling

Hemolymph Ecdysteroid Titer Affects Maternal mRNAs during Bombyx mori Oogenesis

Simple Summary

Both maternal genes and ecdysteroids play important roles during embryonic development. In this study, we aimed to characterize the dynamic landscape of maternal mRNAs and the relationship between maternal genes and ecdysteroids during silkworm oogenesis. For the first time, we determined the start of the accumulation of maternal mRNAs in the ovary at the wandering stage during the larval period. We detected the developmental expression profiles of each gene in the ovary or ovariole. We finally confirmed the role of 20-hydroxyecdysone in regulating maternal gene expression. Taken together, our findings expand the understanding of insect oogenesis and provide a perspective on the embryonic development of the silkworm.

Abstract

Silkworm larval–pupal metamorphosis and the first half of pupal–adult development occur during oogenesis from previtellogenesis to vitellogenesis and include two peaks of the hemolymph ecdysteroid titer. Moreover, a rise in 20-hydroxyecdysone titer in early pupae can trigger the first major transition from previtellogenesis to vitellogenesis in silkworm oogenesis. In this study, we first investigated the expression patterns of 66 maternal genes in the ovary at the wandering stage. We then examined the developmental expression profiles in six time-series samples of ovaries or ovarioles by reverse transcription–quantitative PCR. We found that the transcripts of 22 maternal genes were regulated by 20-hydroxyecdysone in the isolated abdomens of the pupae following a single injection of 20-hydroxyecdysone. This study is the first to determine the relationship between 20-hydroxyecdysone and maternal genes during silkworm oogenesis. These findings provide a basis for further research into the embryonic development of Bombyx mori.

Twenty-hydroxyecdysone produced by dephosphorylation and ecdysteroidogenesis regulates early embryonic development in the silkmoth, Bombyx mori

Ecdysteroids are key regulators of embryonic development as well as molting and metamorphosis in insects. Although an active form of ecdysteroids, 20-hydroxyecdysone (20E) is known to be produced through ecdysteroidogenesis from cholesterol and dephosphorylation of 20E-phosphate during embryogenesis in Lepidoptera, the importance of these production mechanisms in embryonic development has been unclear. Here, we investigated the activation timing of ecdysteroidogenesis from cholesterol and 20E-phosphate dephosphorylation during early embryogenesis in non-diapause eggs of the silkmoth Bombyx mori by observing morphological development, quantifying 20E and 20E-phosphate, measuring transcripts of enzymes involved in 20E production, and detecting activity of these enzymes using egg extracts. Stage-dependent 20E fluctuation and changes in mRNA amounts of enzymes suggest that the two 20E-producing mechanisms are activated at different stages during embryogenesis. Furthermore, knockdown of a dephosphorylation enzyme delayed development at early embryogenesis, whereas knockdown of an ecdysteroidogenic enzyme delayed development at early-middle embryogenesis. These results suggest that 20E is primarily produced initially by dephosphorylation of 20E-phosphate, and then by ecdysteroidogenesis from cholesterol to induce progression of embryonic development in B. mori.

iTRAQ-Based Quantitative Proteomic Analysis of Digestive Juice across the First 48 Hours of the Fifth Instar in Silkworm Larvae

The silkworm is an oligophagous insect for which mulberry leaves are the sole diet. The nutrients needed for vital activities of the egg, pupal, and adult stages, and the proteins formed in the cocoon, are all derived from the larval stages. The silkworm feeds and grows quickly during the larval stages. In particular, the amount of leaf ingested and digested quickly increases from the ecdysis to the gluttonous stage in the fifth instar period. In this study, we used the iTRAQ proteomic technique to identify and analyze silkworm larval digestive juice proteins during this period. A total of 227 proteins were successfully identified. These were primarily serine protease activity, esterase activity, binding, and serine protease inhibitors, which were mainly involved in the digestion and overcoming the detrimental effects of mulberry leaves. Moreover, 30 genes of the identified proteins were expressed specifically in the midgut. Temporal proteomic analysis of digestive juice revealed developmental dynamic features related to molecular mechanisms of the principal functions of digesting, resisting pathogens, and overruling the inhibitory effects of mulberry leaves protease inhibitors (PIs) with a dynamic strategy, although overruling the inhibitory effects has not yet been confirmed by previous study. These findings will help address the potential functions of digestive juice in silkworm larvae.

Comparative Proteomic Analysis Reveals Immune Competence in Hemolymph of Bombyx mori Pupa Parasitized by Silkworm Maggot Exorista sorbillans

The silkworm maggot, Exoristasorbillans, is a well-known larval endoparasitoid of the silkworm Bombyxmori that causes considerable damage to the silkworm cocoon crop. To gain insights into the response mechanism of the silkworm at the protein level, we applied a comparative proteomic approach to investigate proteomic differences in the hemolymph of the female silkworm pupae parasitized by E. sorbillans. In total, 50 differentially expressed proteins (DEPs) were successfully identified, of which 36 proteins were upregulated and 14 proteins were downregulated in response to parasitoid infection. These proteins are mainly involved in disease, energy metabolism, signaling pathways, and amino acid metabolism. Eight innate immune proteins were distinctly upregulated to resist maggot parasitism. Apoptosis-related proteins of cathepsin B and 14-3-3 zeta were significantly downregulated in E. sorbillans-parasitized silkworm pupae; their downregulation induces apoptosis. Quantitative PCR was used to further verify gene transcription of five DEPs, and the results are consistent at the transcriptional and proteomic levels. This was the first report on identification of possible proteins from the E. bombycis-parasitized silkworms at the late stage of parasitism, which contributes to furthering our understanding of the response mechanism of silkworms to parasitism and dipteran parasitoid biology.

Expression Analysis of mRNA Decay of Maternal Genes during Bombyx mori Maternal-to-Zygotic Transition

Maternal genes play an important role in the early embryonic development of the silkworm. Early embryonic development without new transcription depends on maternal components stored in the egg during oocyte maturation. The maternal-to-zygotic transition (MZT) is a tightly regulated process that includes maternal mRNAs elimination and zygotic transcription initiation. This process has been extensively studied within model species. Each model organism has a unique pattern of maternal transcriptional clearance classes in MZT. In this study, we identified 66 maternal genes through bioinformatics analysis and expression analysis in the eggs of silkworm virgin moths (Bombyx mori). All 66 maternal genes were expressed in vitellogenesis in day eight female pupae. During MZT, the degradation of maternal gene mRNAs could be divided into three clusters. We found that eight maternal genes of cluster 1 remained stable from 0 to 3.0 h, 17 maternal genes of cluster 2 were significantly decayed from 0.5 to 1.0 h and 41 maternal genes of cluster 3 were significantly decayed after 1.5 h. Therefore, the initial time-point of degradation of cluster 2 was earlier than that of cluster 3. The maternal gene mRNAs decay of clusters 2 and 3 is first initiated by maternal degradation activity. Our study expands upon the identification of silkworm maternal genes and provides a perspective for further research of the embryo development in Bombyx mori.