First Case of Dual Size Asymmetry in an Identical Arthropod Organ: Different Asymmetries of the Combative (Sexual) and Cutting (Non-Sexual) Parts of Mandibles in the Horned Stored-Product Beetle Gnatocerus cornutus (Fabricius, 1798)

Abstract

Although it is known that separate insect body structures may be asymmetrical within one species, the different functional asymmetries within a single organ as a result of differential selective regimes have not been described. Based on microscopic measurements and SEM photography, we examined the size, shape and asymmetry of the mandibular structures of males and females of the sexually dimorphic broad-horned flour beetle, Gnatocerus cornutus (Tenebrionidae, Coleoptera). It was found that sexual dimorphism only manifests in certain outgrowth parts (horns) of male mandibles, while the remaining cutting parts of the mandibles hold identical morphologies for both sexes. A more interesting finding—since this is the first published case of dual functionally selected asymmetry in an identical arthropod organ—was that the cutting part of the male mandible exhibited directional asymmetry, whereas the outgrowth horn part of the mandible showed a high degree of symmetry. Moreover, there was no relationship between the size and asymmetry of horns. The results indicate different regulatory mechanisms of sexually selected combative horns and the food-functional, more conservative (constrained by hard food and adult long life) cutting parts of mandibles.

Evolutionary radiation strategy revealed in the Scarabaeidae with evidence of continuous spatiotemporal morphology and phylogenesis

“…1a ) (Supplementary Data 2 ). The first curve was taken from the outer contour of mandible covered by the base of the left to the base of the right, a silhouette of dorsal view of mandible was used to avoid the partial asymmetry of the left and right mandible in three-dimensional space, which could lead to instability in the results 91 – 93 ; the second curve was collected from the middle of the anterior margin of the pronotum and end up at the middle of the posterior margin of the pronotum; the third curve started from the anterior margin of the left elytron and stopped at the end of elytron. 16 landmarks were taken from the right hindwing through MorphoJ for quantifying the structure of wing vein nodes (Supplementary Data 2 ), in order of numbering of landmark points: the base of ScA, the intersection of the RA 3+4 vein with the leading edge; the end of RA 3+4 ; the end of RA 3 ; the end of RA 4 ; the base of RA 1+2 ; the base of MP; the base of RP; the end of RP; the end of MP; the base of CuA; the end of CuA; the base of AA; the end of AA; the base of AP; the end of AP 81 .…”

Transcriptomic and functional screening of weapon formation genes implies significance of cell adhesion molecules and female-biased genes in broad-horned flour beetle

“…cornutus belongs to Tenebrioninae and is closely related to non-weaponed beetle genera such as Tribolium and Neomida [27]. Male mandible has bifurcated outgrowth structure that is called mandibular horn (mh, Fig 1A) and it is used for territorial combat [28,29]. Lateral region of male head (gena) is enlarged, and two head horns (hh) are located between eyes (Fig 1A ).…”

“…Our study highlighted that these common genes were independently deployed in outgrowth acquisition in G. cornutus. Morphologically, the mandibular horn of male G. cornutus is a novel branching outgrowth on the outer ridge of a mandible [29], whereas the long mandible of male stag beetle is the elongation of an existing mandible itself [6] and the horn of rhinoceros beetle is a novel outgrowth on head [20]. Therefore, the morphological origins of these weapons are also independent.…”