Oriented Protein in Chitinous Structures

Identification and Phylogenetic Analysis of Chitin Synthase Genes from the Deep-Sea Polychaete Branchipolynoe onnuriensis Genome

Chitin, one of the most abundant biopolymers in nature, is a crucial material that provides sufficient rigidity to the exoskeleton. In addition, chitin is a valuable substance in both the medical and industrial fields. The synthesis of chitin is catalyzed by chitin synthase (CHS) enzymes. Although the chitin synthesis pathway is highly conserved from fungi to invertebrates, CHSs have mostly only been investigated in insects and crustaceans. Especially, little is known about annelids from hydrothermal vents. To understand chitin synthesis from the evolutionary view in a deep-sea environment, we first generated the whole-genome sequencing of the parasitic polychaete Branchipolynoe onnuriensis. We identified seven putative CHS genes (BonCHS1-BonCHS7) by domain searches and phylogenetic analyses. This study showed that most crustaceans have only a single copy or two gene copies, whereas at least two independent gene duplication events occur in B. onnuriensis. This is the first study of CHS obtained from a parasitic species inhabiting a hydrothermal vent and will provide insight into various organisms’ adaptation to the deep-sea hosts.

The family of amphioxus chitin synthases offers insight into the evolution of chitin formation in chordates

Chitin is a very important and widely-used biopolymer in fungi and lower metazoans, but mysteriously disappears in mammals. Recent studies reveal that at least lower vertebrates have chitin synthases (CS) and use them to synthesize endogenous chitin. Amphioxus, a basal chordate, therefore becomes critical to understand the evolution of CS, as it occupies the transitional position from invertebrates to vertebrates, and is considered as a good proxy to the chordate ancestor. Here, by exploiting multiple genome assemblies, high-depth RNA-seq data and synteny relations, we identify 11-12 CS genes for each amphioxus species. It represents the largest CS gene pool ever found in eukaryotes so far. As comparison, most metazoans have one or two CSs. Amphioxus is the only chordate that has both the very ancient type-I CS family and the more broadly distributed type-II CS family. Specifically, amphioxus has only one type-II CS but 10-11 type-I CSs, which means that amphioxus is the only metazoan with a greatly expanded type-I CS family. Further analysis suggests that the chordate ancestor have at least one type-II CS and an expanded of type-I CS family. We hypothesize that: these ancient CSs are mostly retained in amphioxus; but the whole type-I CS family was lost in urochordates and vertebrates; the type-II CS was later duplicated into two lineages in vertebrates and followed by stochastic losses, till all type-II CSs were eventually lost in birds and mammals. Finally, our expression profiling and preliminary gene knockout analysis suggest that amphioxus CSs could have highly diverse but mildly overlapping functions in various tissues and organs. Taken together, these findings not only provide insights into the evolution of chordate CSs, lay a foundation for further functional study of the chordate CSs. After all, it is mysterious that our chordate ancestor needed so many isoenzymes for chitin formation.

Early divergence, broad distribution, and high diversity of animal chitin synthases

Even though chitin is one of the most abundant biopolymers in nature, current knowledge on chitin formation is largely based only on data from fungi and insects. This study reveals unanticipated broad taxonomic distribution and extensive diversification of chitin synthases (CSs) in Metazoa, shedding new light on the relevance of chitin in animals and suggesting unforeseen complexity of chitin synthesis in many groups. We uncovered robust orthologs to insect type CSs in several representatives of deuterostomes, which generally are not thought to possess chitin. This suggests a broader distribution and function of chitin in this branch of the animal kingdom. We characterize a new CS type present not only in basal metazoans such as sponges and cnidarians but also in several bilaterian representatives. The most extensive diversification of CSs took place during emergence of lophotrochozoans, the third large group of protostomes next to arthropods and nematodes, resulting in coexistence of up to ten CS paralogs in molluscs. Independent fusion to different kinds of myosin motor domains in fungi and lophotrochozoans points toward high relevance of CS interaction with the cytoskeleton for fine-tuned chitin secretion. Given the fundamental role that chitin plays in the morphology of many animals, the here presented CS diversification reveals many evolutionary complexities. Our findings strongly suggest a very broad and multifarious occurrence of chitin and question an ancestral role as cuticular component. The molecular mechanisms underlying regulation of animal chitin synthesis are most likely far more complex and diverse than existing data from insects suggest.

Biology and physics of locust flight. V. Strength and elasticity of locust cuticle

Elastic deformations of the cuticle play a major role in the energetics of flying locusts but the literature provides no relevant information about the elastic properties of any arthropod cuticle. The results are therefore discussed both in relation to locust flight and in relation to strength and elasticity of organic materials in general. InSchistocerca gregariaForskal there are two types of elastic cuticle, ordinarysolid cuticleandrubber-like cuticle. The characteristic material in the latter type is a newly discovered protein rubber,resilin. Samples of both were studied under static and dynamic conditions. The tensile properties of solid cuticle from various parts of the body (hind tibia, pleural wall, forewing) are similar to those of oak wood and of synthetic resins reinforced with cellulose; the static coefficient of elasticity (dcr/de) is 800 to 1000 kg/mm2and the tensile strength 8 to 10 kg/mm2, corresponding to an ultimate extension of 2 to 3 %. At moderate loads, the tensile and compressive moduli are of equal magnitude, but it is argued that the effect of tanning (hardening) is to increase the compressive strength and modulus rather than the tensile properties. Static loading results in lasting deformation. The dynamic modulus is of the same magnitude as the static modulus (forewing), at least up to 5 kg mm-2s-1and, provided the tension does not exceed 0-5 kg/mm2, the loss factor is less than 0.1. The rubber-like sample (prealar arm) consists of parallel lamellae of chitin (0.2μ thick) glued together by sheets of resilin (about 3μ thick). It behaves like a solid when extended in the direction of the lamellae but otherwise like a rubber, the elastic modulus being 0.2 kg/mm2. The swelling pressure of resilin does not play any direct role but swelling alters the geometry and, to a small extent, the elastic modulus. It is suggested that the animal controls the stiffness of its rubber-like structures by altering the swelling equilibrium chemically which, in a model experiment, is done by blocking the free amino groups. Rubber-like cuticle does not encounter any permanent deformation which is attributed to the known lack of flow of pure resilin. Within the biological rate of deformation (up to 6 unit lengths per second), the dynamic stiffness remains within 4 % of the static value and the loss factor is only 0.03, i.e. less than for other natural or synthetic rubbers. A three-component model of arthropod cuticle is suggested. It accounts for the enormous differences in mechanical properties between adjacent parts and also for the fact that strict structural and developmental continuity is observed between the parts. It has three components: (1) crystalline chitin, (2) a rubber-like protein which may act as a deformable matrix and which entraps, (3) water-soluble proteins which can undergo proper tanning.