Modulation of the ecdysteroid-induced cell death by juvenile hormone during pupal wing development of Lepidoptera

Ecdysteroid-induced programmed cell death is essential for sex-specific wing degeneration of the wingless-female winter moth

The winter moth, Nyssiodes lefuarius, has a unique life history in that adults appear during early spring after a long pupal diapause from summer to winter. The moth exhibits striking sexual dimorphism in wing form; males have functional wings of normal size, whereas females lack wings. We previously found that cell death of the pupal epithelium of females appears to display condensed chromatin within phagocytes. To provide additional detailed data for interpreting the role of cell death, we performed light microscopy, transmission electron microscopy, and TUNEL assay. We consequently detected two modes of cell death, i.e., dying cells showed both DNA fragmentation derived from epithelial nuclei and autophagic vacuole formation. To elucidate the switching mechanism of sex-specific wing degeneration in females of N. lefuarius, we tested the effects of the steroid hormone 20-hydroxyecdysone (20E) on pupal diapause termination and wing morphogenesis in both sexes. When 20E (5.4 µg) was injected into both sexes within 2 days of pupation, wing degeneration started 4 days after 20E injection in females, whereas wing morphogenesis and scale formation started 6 days after 20E injection in males. We discuss two important findings: (1) degeneration of the pupal wing epithelium of females was not only due to apoptosis and phagocytotic activation but also to autophagy and epithelial cell shrinkage; and (2) 20E terminated the summer diapause of pupae, and triggered selective programmed cell death only of the female-pupal wing epithelium in the wingless female winter moth.

Two adjacent cis-regulatory elements are required for ecdysone response of ecdysone receptor (EcR) B1 transcription

Three distinct classes of nuclear receptors, EcR, E75, and HR3, are key regulators in the ecdysone-inducible gene activation cascade in insects. The transcription of these genes is induced by ecdysone (20E) differently, although the detailed mechanisms underlying their responses to 20E are largely unknown. We identified ecdysone response elements (EcREs) present in the promoters of genes coding BmEcR-B1, BmE75-A, and BHR3-B isoforms from Bombyx mori employing luciferase reporter assays in an ecdysteroid-responsive cultured cell line, NIAS-Bm-aff3 (aff3). The EcRE of BmEcR-B1 at −2800 comprises of two adjacent elements separated by 5 bp, E1 (15 bp) and E2 (21 bp), both of which are required for the 20E response. Further analysis using electrophoretic mobility shift assays showed that E1 binds to the EcR/USP heterodimer and that E2 may bind to the E-box (CACGTG) binding factor such as bHLH protein. The unique E1+E2-type EcRE is also detected in the promoter upstream regions of EcR-B1 from seven lepidopteran species studied. In contrast, both a 20 bp EcRE identified in the promoter of BmE75-A and a 18 bp EcRE identified in the BHR3-B promoter, contained only E1-type EcR/USP binding element but the E2 type element was not in the promoter regions of these genes. The combination of presence of the E2 element or other cis-regulatory elements in promoter regions explains the different 20E response of each class of nuclear receptor genes. Furthermore, the E1+E2 structure for EcR-B1 can be involved in a possible cross-talk between ecdysteroid and other regulatory pathways.

Programmed cell death of larval tissues induced by juvenile hormone in the bamboo borer, Omphisa fuscidentalis

Programmed cell death (PCD) plays a critical role during animal development through the destruction of unneeded cells and tissues. In some insects, the prothoracic glands (PGs) and anterior silk glands (ASGs) are larval-specific tissues that are normally eliminated by PCD after pupation. Previous studies report that juvenile hormone analog (JHA) terminates the larval diapause of Omphisa fuscidentalis by increasing the hemolymph ecdysteroids that trigger PCD. Because JHA may indirectly induce the PCD of the PGs and ASGs of Omphisa diapausing larvae, the effects of JHA on the induction of PCD were determined. The application of 1μg JHA induced PCD in the PGs and ASGs of larvae identified as stage G0 (prior to pupation). The injection of 1μg 20E triggered the PCD of the ASGs when the larvae expressed a G0-G1 morphology, whereas PCD occurred in the PGs on day 1 post-injection. Histological studies revealed similar patterns of morphological changes during the PG and ASG PCD in the JHA- and 20E-treated larvae. Furthermore, to confirm that PCD was induced by a high ecdysteroid level that increases after JHA application, the expression profiles of EcR-A and EcR-B1 in the PGs and ASGs from the JHA-treated larvae were examined, and the results showed that the expression levels of EcR-A and EcR-B1 mRNA increased during the G0 stage. These results suggest that JHA may be involved in PCD by increasing the ecdysteroid titer, leading to termination of the larval diapause period in Omphisa fuscidentalis.

Larval fat body cells die during the early pupal stage in the frame of metamorphosis remodelation in Bombyx mori

In holometabolus insects, morphology of the larval fat body is remodeled during metamorphosis. In higher Diptera, remodeling of the fat body is achieved by cell death of larval fat body cells and differentiation of the adult fat body from primordial cells. However, little is known about remodeling of the fat body at pupal metamorphosis in Lepidoptera. In this study, we found that cell death of the larval fat body in Bombyx mori occurs at shortly after pupation. About 30% of the fat body cells underwent cell death on days 1 and 2 after pupation. The cell death involved genomic DNA fragmentation, a characteristic of apoptosis. Surgical manipulation and in vitro culture of fat body cells revealed that 20-hydroxyecdysone and juvenile hormone had no effect on either initiation or progression of cell death. During cell death, a large increase in activity of caspase-3, a key enzyme of cell death, was observed. Western blot analysis of the active form of caspase-3-like protein revealed that the length of caspase-3 of B. mori was much larger than that of caspase-3 in other species. The results suggest that larval fat body cells of B. mori are removed through cell death, which is mediated by a caspase probably categorized in a novel family.

Programed cell death shapes the expression of horns within and between species of horned beetles

Holometabolous insects provide an excellent opportunity to study both the properties of development as well as their evolution and diversification across taxa. Here we investigate the developmental basis and evolutionary diversification of secondary trait loss during development in the expression of beetle horns, a novel and highly diverse class of secondary sexual traits. In many species, horn growth during late larval development is followed by a period of dramatic remodeling during the pupal stage, including the complete resorption of horns in many cases. Here we show that programed cell death plays an important and dynamic role in the secondary resorption of pupal horn primordia during pupal development. Surprisingly, the degree of cell death mediated horn resorption depended on species, sex, and body region, suggesting the existence of regulatory mechanisms that can diversify quickly over short phylogenetic distances. More generally, our results illustrate that secondary, differential loss of structures during development can be a powerful mechanism for generating considerable morphological diversity both within and between species.

Chapter 6. The origin and diversification of complex traits through micro- and macroevolution of development: insights from horned beetles

Understanding how development and ecology shape organismal evolution is a central goal of evolutionary developmental biology. This chapter highlights a class of traits and organisms that are emerging as new models in evo-devo and eco-devo research: beetle horns and horned beetles. Horned beetles are morphologically diverse, ecologically rich, and developmentally and genetically increasingly accessible. Recent studies have begun to take advantage of these attributes and are starting to link the microevolution of horned beetle development to the macroevolution of novel features, and to identify the genetic, developmental, and ecological mechanisms, and the interactions between them, that mediate organismal innovation and diversification in natural populations. Here, I review the most significant recent findings and their contributions to current frontiers in evolutionary developmental biology.

Positional cloning of a Bombyx wingless locus flugellos (fl) reveals a crucial role for fringe that is specific for wing morphogenesis

Mutations at the flügellos (fl) locus in Bombyx mori produce wingless pupae and moths because of the repressed response of wing discs to ecdysteroid. Four recessive fl alleles occurred spontaneously and were mapped at 13.0 of the silkworm genetic linkage group 10. By positional cloning, we confirmed that the gene responsible for fl is fringe (fng) encoding Fng glycosyltransferase, which is involved in regulating the Notch signaling pathway. In four different fl alleles, we detected a large deletion of the fng gene in fl(k) and nonsense mutations in fl, fl(o), and fl(n). In the wild-type (WT) silkworm, fng is expressed actively in the wing discs, brain, and reproductive organs from the fourth to final instars but barely in the other tissues tested. In situ hybridization showed that fng mRNA is expressed in the dorsal layer of the WT wing discs. The wingless (wg) mRNA, a downstream marker of Fng-mediated Notch signaling, is localized at the dorsoventral boundary in the WT wing discs but repressed markedly in the fl wing discs. Although null mutants of Drosophila fng result in postembryonic lethality, loss of fng function in Bombyx affects only wing morphogenesis, suggesting different essential roles for fng in tissue differentiation among insects.