One of the defining features of the evolutionary success of insects is the morphological diversification of their appendages, especially mouthparts. Although most insects share a common mouthpart ...ground plan, there is remarkable diversity in the relative size and shapes of these appendages among different insect lineages. One of the most prominent examples of mouthpart modification can be found in the enlargement of mandibles in stag beetles (Coleoptera, Insecta). In order to understand the proximate mechanisms of mouthpart modification, we investigated the function of appendage-patterning genes in mandibular enlargement during extreme growth of the sexually dimorphic mandibles of the stag beetle Cyclommatus metallifer. Based on knowledge from Drosophila and Tribolium studies, we focused on seven appendage patterning genes (Distal-less (Dll), aristaless (al), dachshund (dac), homothorax (hth), Epidermal growth factor receptor (Egfr), escargot (esg), and Keren (Krn). In order to characterize the developmental function of these genes, we performed functional analyses by using RNA interference (RNAi). Importantly, we found that RNAi knockdown of dac resulted in a significant mandible size reduction in males but not in female mandibles. In addition to reducing the size of mandibles, dac knockdown also resulted in a loss of the serrate teeth structures on the mandibles of males and females. We found that al and hth play a significant role during morphogenesis of the large male-specific inner mandibular tooth. On the other hand, knockdown of the distal selector gene Dll did not affect mandible development, supporting the hypothesis that mandibles likely do not contain the distal-most region of the ancestral appendage and therefore co-option of Dll expression is unlikely to be involved in mandible enlargement in stag beetles. In addition to mandible development, we explored possible roles of these genes in controlling the divergent antennal morphology of Coleoptera.
•Developmental functions of limb-patterning genes in the stag beetle were analyzed.•Male stag beetle mandibles required the dachshund expression for enlargement.
Various types of weapon traits found in insect order Coleoptera are known as outstanding examples of sexually selected exaggerated characters. It is known that the sex determination gene doublesex ...(dsx) plays a significant role in sex-specific expression of weapon traits in various beetles belonging to the superfamily Scarabaeoidea. Although sex-specific weapon traits have evolved independently in various Coleopteran groups, developmental mechanisms of sex-specific expression have not been studied outside of the Scarabaeoidea. In order to test the hypothesis that dsx-dependent sex-specific expression of weapon traits is a general mechanism among the Coleoptera, we have characterized the dsx in the sexually dimorphic broad-horned beetle Gnatocerus cornutus (Tenebrionidea, Tenebirionidae). By using molecular cloning, we identified five splicing variants of Gnatocerus cornutus dsx (Gcdsx), which are predicted to code four different isoforms. We found one male-specific variant (GcDsx-M), two female-specific variants (GcDsx-FL and GcDsx-FS) and two non-sex-specific variants (correspond to a single isoform, GcDsx-C). Knockdown of all Dsx isoforms resulted in intersex phenotype both in male and female. Also, knockdown of all female-specific isoforms transformed females to intersex phenotype, while did not affect male phenotype. Our results clearly illustrate the important function of Gcdsx in determining sex-specific trait expression in both sexes.
In this post-genomic era, genome-wide functional analysis is indispensable. The recent development of RNA interference techniques has enabled researchers to easily analyze gene function even in ...non-model organisms. On the other hand, little progress has been made in the identification and functional analyses of cis-regulatory elements in non-model organisms. In order to develop experimental platform for identification and analyses of cis-regulatory elements in a non-model organism, in this case, the ladybird beetle, Harmonia axyridis, we established transgenic transposon-tagged lines using a novel composite vector. This vector enables the generation of two types of insertion products (jumpstarter and mutator). The jumpstarter portion carries a transposase gene, while the mutator segment carries a reporter gene for detecting enhancers. The full-composite element is flanked by functional termini (required for movement); however, the mutator region has an extra terminus making it possible for the mutator to remobilize on its own, thus leaving an immobile jumpstarter element behind. Each insertion type is stable on its own, but once crossed, jumpstarters can remobilize mutators. After crossing a jumpstarter and mutator line, all tested G2 females gave rise to at least one new insertion line in the next generation. This jumping rate is equivalent to the P-element-mediated jumpstarter method in Drosophila. These established transgenic lines will offer us the ideal experimental materials for the effective screening and identification of enhancers in this species. In addition, this jumpstarter method has the potential to be as effective in other non-model insect species and thus applicable to any organism.
The three-dimensional morphologies of insects, such as spines and horns, have recently garnered attention as an effective system for elucidating the processes underlying dramatic changes in external ...morphology. Adult leaf beetles of
Dactylispa
Weise 1897 exhibit sharp spines on their pronotum and elytra, while their pupae lack such features. In order to obtain foundational data for future investigations into the developmental mechanisms governing spine formation, this study assessed variations in spine numbers across ten body regions and in spine size across three body regions in adults of
Dactylispa higoniae
(Lewis in Ann Mag Nat Hist (Ser. 6) 17:329–343, 1896.
https://doi.org/10.1080/00222939608680376
) and
D. issikii
(Chûjô in Bull Umeno Entomol Lab 6:5–13, 1938). As a result, the degree of variation in spine numbers and size was variable among body regions even within single species. However, the number of spines on the pronotal front margin in
D. higoniae
and the elytral interval 4 in
D. issikii
was stable. The spine numbers of these two species did not exhibit significant differences between the right and left sides of the body and was not likely to be influenced by sex or elytral length. Furthermore, we observed pronotal and elytral spines that formed inside the pupal cuticle of late-stage pupae of these two species. Our observations suggested neither pronotal nor elytral spines exhibit a formation pattern resembling concentric folded structures, as seen in beetle horns.
Some insects possess complex three-dimensional (3D) structures that develop under the old cuticle prior to the last imaginal molt. Adult treehoppers (Insecta: Hemiptera: Auchenorrhyncha: Membracidae) ...have one such complex 3D structure, known as a helmet, on their dorsal side. The adult helmet likely forms inside the nymphal pronotum during the final instar nymphal stage. Previous morphological studies have reported that the adult helmet is a large, bi-layered, plywood-like structure, whereas the nymphal pronotum is a monolayer, sheath-like structure. The adult helmet is much larger than nymphal helmet. Thus, the emergence of the adult helmet involves two structural transitions: a transition from a monolayer, sheath-like pronotum to a bi-layer, plywood-like helmet, and a transition in size from small to large. However, when, how, and in what order these transitions occur within the nymphal cuticle is largely unknown. To determine how adult helmet development occurs under the nymphal cuticle, in the present study we describe the morphology of the final adult helmet and investigate developmental trajectories of the helmet during the final instar nymphal stage. We used micro-CT, scanning electron microscope and paraffin sections for morphological observations, and used
as a model species. We found that the structural transition (from monolayer, sheath-like structure to bi-layer, roof-like structure) occurs through the formation of a "miniature" of the adult helmet during the middle stage of development and that subsequently, extensive folding and furrows form, which account for the increase in size. We suggest that the making of a "miniature" is the key developmental step for the formation of various 3D structures of treehopper helmets.
•Sexually dimorphic plasticity has evolved to limit the impact of sexual conflict.•Many insects have traits that are sexually dimorphic and plastically responsive to condition.•Signals of condition ...and the downstream responses to condition differ between species.•The gene doublesex seems to be critical to sexually dimorphic plasticity in all organisms studied.•Next-generation technology allows the investigation of regulatory responses to doublesex.
Many traits that are sexually dimorphic, appearing either differently or uniquely in one sex, are also sensitive to an organism's condition. This phenomenon seems to have evolved to limit genetic conflict between traits that are under different selective pressures in each sex. Recent work has shed light on the molecular and developmental mechanisms that govern this condition sensitive growth, and this work has now expanded to encompass both sexual dimorphism as well as conditionally plastic growth, as it seems the two phenomena are linked on a molecular level. In all cases studied the gene doublesex, a conserved regulator of sex differentiation, controls both sexual dimorphism as well as the condition-dependent plastic responses common to these traits. However, the advent of next-generation -omics technologies has allowed researchers to decipher the common and diverged mechanisms of sexually dimorphic plasticity and expand investigations beyond the foundation laid by studies utilizing beetle weapons.
Clarifying the mechanisms underlying shape alterations during insect metamorphosis is important for understanding exoskeletal morphogenesis. The large horn of the Japanese rhinoceros beetle ...Trypoxylus dichotomus is the result of drastic metamorphosis, wherein it appears as a rounded shape during pupation and then undergoes remodeling into an angular adult shape. However, the mechanical mechanisms underlying this remodeling process remain unknown. In this study, we investigated the remodeling mechanisms of the Japanese rhinoceros beetle horn by developing a physical simulation. We identified three factors contributing to remodeling by biological experiments - ventral adhesion, uneven shrinkage, and volume reduction - which were demonstrated to be crucial for transformation using a physical simulation. Furthermore, we corroborated our findings by applying the simulation to the mandibular remodeling of stag beetles. These results indicated that physical simulation applies to pupal remodeling in other beetles, and the morphogenic mechanism could explain various exoskeletal shapes.
Numerous coleopteran species express male‐specific “weapon traits” that often show size variations among males, even within a single population. Many empirical studies have demonstrated that ...environmental conditions during development affect absolute weapon size. However, relatively few studies in horned beetles support the hypothesis that the relationship between weapon size and body size, also referred to as a “scaling relationship” or “static allometry”, is largely determined by genetic factors. In this study, the heritability of absolute mandible length and static allometry between mandible length and body size were estimated in the stag beetle Cyclommatus metallifer. While no significant heritable variation was observed in absolute mandible length, high heritability (h2 = 0.57 ± 0.25) was detected in the static allometry between mandible length and body size. This is the first report on the genetic effect on male mandible size in Lucanidae, suggesting that absolute mandible size is largely determined by environmental conditions while the static allometry between weapon size and body size is primarily determined by genetic factors.
Dimorphic sperm formation by Sex-lethal Sakai, Hiroki; Oshima, Hiroyuki; Yuri, Kodai ...
Proceedings of the National Academy of Sciences - PNAS,
05/2019, Volume:
116, Issue:
21
Journal Article
Peer reviewed
Open access
Sex is determined by diverse mechanisms and master sex-determination genes are highly divergent, even among closely related species. Therefore, it is possible that homologs of master ...sex-determination genes might have alternative functions in different species. Herein, we focused on Sex-lethal (Sxl), which is the master sex-determination gene in Drosophila melanogaster and is necessary for female germline development. It has been widely shown that the sex-determination function of Sxl in Drosophilidae species is not conserved in other insects of different orders. We investigated the function of Sxl in the lepidopteran insect Bombyx mori. In lepidopteran insects (moths and butterflies), spermatogenesis results in two different types of sperm: nucleated fertile eupyrene sperm and anucleate nonfertile parasperm, also known as apyrene sperm. Genetic analyses using Sxl mutants revealed that the gene is indispensable for proper morphogenesis of apyrene sperm. Similarly, our analyses using Sxl mutants clearly demonstrate that apyrene sperm are necessary for eupyrene sperm migration from the bursa copulatrix to the spermatheca. Therefore, apyrene sperm is necessary for successful fertilization of eupyrene sperm in B. mori. Although Sxl is essential for oogenesis in D. melanogaster, it also plays important roles in spermatogenesis in B. mori. Therefore, the ancestral function of Sxl might be related to germline development.
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