Adaptive Functions of Vertebrate Molting Cycles

SYNOPSIS Cyclic epidermal cellular prohfeiation,with or withoutkeratinization is a vertebrate characteristic Such activityprobably obeys an autonomous rhythm which is legulated throughneuro humoral S)stcms in response to envnonmental (piox imate)stimuliand related to adaptive (ultimate) factors In seeking causeand effect lelationships, however, it becomes apparent thatthe same environmental parameter may be both an ultimate anda pioximate factor, the latter also regulating the rate of lesponseWith regard to molting in homoio'heims, tempeiatuie acts insuch a capacity in many species Peiiodic shedding of the outer epidermis in fish amphibiansand reptiles does not appear to be coirelated with seasonalfactors to the extent that avian and mammalian molts are The evolution of vertebrate molting cycles has amounted to theentraining of inherent epidermal C)cles with seasonal demandsby the organism itself and the environment,these demands actas regulating mechanisms Pieadapted structures such as feathersand hairs function collectively as plumage and pelage in theirvarious roles but separately in their growth and leplacementcycles which, however, are coordinated for maximum functionalefficiency Molting is also synchionized with the seasonal cycleaccording to the availability of energy resources and time tocomplete the essential functions (in addition to molting) Theevolved molting systems as manifested in the gieat variety ofpatterns and types in the vertebrates, may thus be legardedas almost individual responses to selective piessures actingon a umveisil vertebrate chaiacter The basic regulatoiy system involves the neuro hvpophyseal complexwhich contiols target endocrines affecting various functionswhich themselves influence epidermal mitosis and, ultimately,molting 1 he mechanism in its simplest form controls the animalsmetabolism through the thyroid acting independently in a permissivecapacity or synergistically with the adrenal and gonadal hormoneswhich are regulated directly and/or indirectly through negativefeedback     

Physiological Control of Molting in Insects

SYNOPSIS. The initiation of a molting cycle in insects is neithera random nor a strictly periodic event. Insofar as molting canaccomplish different things under different circumstances, suchas a change in size or a change in form, it is reasonable toasume that the timing of a molt must be adapted to these functions.The onset of a metamorphic molt, in particular, must be preciselycontrolled because the onset of metamorphosis terminates thegrowth phase of a larva and establishes the body size of theadult insect. This aspect of the control of molting has receivedrelatively little attention and our knowledge of specific physiologicalmechanisms for the control of molt initiation is restrictedto three species: the blood-sucking bug, Rhodnius prolixus,the greater milkweed bug, Oncopeltus fasciatus, and the tobaccohornworm, Manduca sexta. The present review discusses the stateof our knowledge about the factors that render these insectscompetent to molt and about the stimuli that serve as a directtrigger for molting.     

In Vitro Parasitism of Rotylenchus robustus by Isolates of Hirsutella rhossiliensis

We tested the hypothesis that isolates of Hirsutella rhossiliensis from host nematodes in the family Hoplolaimidae (Rotylenchus robustus and Hoplolaimus galeatus) would be more virulent to R. robustus than would isolates from host nematodes not in the Hoplolaimidae (Heterodera schachtii and Criconemella xenoplax). Nematodes were touched to 10-20 spores of different isolates and incubated at 20 C in 4.5 mM KC1; the percentage of nematodes colonized (filled with hyphae) was determined after 2, 5, 10, 20, and 30 days. The hypothesis was rejected because isolates from H. schachtii and C. xenoplax were equivalent or better at parasitizing R. robustus than were isolates from R. robustus and H. galeatus. In addition, the R. robustus and H. galeatus isolates were as pathogenic to C. curvata as they were to R. robustus, but produced fewer spores per colonized nematode (H. schachtii) than did the other isolates.     

Hormonal Control of Molting in Decapod Crustacea

The involvement of the molting hormone, 20-hydroxyecdysone,in the mediation of molting in decapod crustaceans is brieflyreviewed. Aspects of the secretion and metabolism of its precursor,ecdysone, are discussed. Experiments are described that demonstratethe presence of a molt-inhibiting hormone (MIH) in the sinusglands of juvenile lobsters (Homarus americanus). Assays forMIH include measurement of the molt interval and radioimmunoassayof circulating titers of ecdysteroids in eyestalk-ablated lobsters.This latter assay indicates that sinus gland extracts significantlydecrease the concentration of circulating ecdysteroids 24 hrafter injection. Data are also presented on the circulatingtiters of ecdysteroids during multiple molt cycles of lobstersfollowing eyestalk ablation. These data indicate that theremust be another factor that ultimately regulates the circulatinglevels of the molting hormone.     

Regulation of Crustacean Molting: A Multi-Hormonal System

SYNOPSIS. In order to increase in size, arthropods must firstmolt (shed) their confining exoskeleton. This molting processis under the immediate control of the steroid molting hormone20-hydroxyecdysone (20-HE). Both the initial rise in circulatinghormone concentration and a coordinated decline are necessaryfor successful molting. Synthesis and/or release of ecdysone,the precursor to 20-HE, is regulated by the neuropeptide molt-inhibitinghormone (MIH). We have determined the primary amino acid sequenceof MIH in the lobster, Homarus americanus. This peptide hasa high degree of identity with the lobster hyperglycemic hormone.Another endocrine factor that appears to be involved in moltingis the juvenile hormone-like terpenoid methyl farnesoate (MF).We have characterized hemolymph MF binding proteins during themolt cycle. In addition, recent data indicate that MF may stimulatethe secretion of 20-HE in vivo and in vitro.     

Neurosecretion and Molting in Some Parasitic Nematodes

The adult female of Ascaris lumbricoides possesses a numberof nerve cells containing material which stains with paraldehyde-fuchsin.Among others, most of the primary sensory cells in the lipsare fuchsinophilic. Ascaris does not survive outside its host,so that it is impossible to ascribe a function to these cells. Phocanema decipiens possesses similar cells in the dorsal andventral ganglia which exhibit a cycle of secretion correlatedwith the burst of cytological activity which accompanies thedeposition of the new cuticle. Ligation experiments have demonstratedthat a new cuticle can be deposited in the absence of theseneurosecretory cells. Our most recent experiments suggest thatthe neurosecretory cells may control the release of leucineaminopeptidase in the excretory gland, a substance which isthought to be responsible for ecdysis.     

Hormonal Control of Molting and Reproduction in Ticks

SYNOPSIS. Among ticks there are two developmental and threereproductive patterns that correlate with taxonomic groupings(Argasidae, prostriate and metastriate Ixodidae). Feeding isa prerequisite for molting; feeding and mating are necessaryfor reproduction in all except a few parthenogenetic species.Growth and development in ticks and other chelicerates appearto be controlled by molting hormones (ecdysteroids), as theyare in insects and crustaceans. Ecdysone and 20-hydroxyecdysoneappear to be present in most or all of the major cheliceratetaxa. Epidermis is the site of ecdysone production and fat bodythe site of 20-hydroxylation in the argasid Ornithodoros parkeri,as is probably the case in all ticks. Ecdysteroids influenceearly stages of spermatogenesis by stimulation of DNA synthesisin spermatocytes, but controls for later stages of meiosis areunknown. A polypeptide (12,000 daltons) from male genital accessoryglands stimulates capacitation (maturation) of spermatids intosperm at the time of spermatid transfer to females. Knowledgeof control of egg development and oviposition is incomplete.Stimuli from the synganglion are necessary for completion ofoogenesis and two synganglial factors have been proposed. AnEgg Development Stimulation Factor (EDSF) in O. parkeri is synthesizedand/or released three to six days after feeding. VitellogenesisInducing Factor (VIF) in O. moubata is synthesized and/ or releasedwithin one hour after feeding. The VIF is hypothesized to impactan unidentified tissue which in turn produces a Fat Body StimulationFactor (FSF) that stimulates fat body to synthesize vitellogenin(Vg). Roles of ecdysteroids and juvenile hormones during eggdevelopment and oviposition are unclear.     

Effects of Pollutants on Molting and Regeneration in Crustacea

Many environmental pollutants have toxic effects that can alternormal limb regeneration and molting in Crustacea. The mostcommon effect of heavy metals is that of retardation of regenerationof limbs accompanied by a delay in ecdysis; in some cases regenerationis affected without altering the molt cycle. Chlorophenols anddithiocarbamates caused inhibition of regeneration without affectingmolting in shrimp. Organic toxicants such as aromatic hydrocarbonsanddioxins also result in a decrease in the growth increment permolt, while DDT was found to accelerate regeneration and molting.A number of toxicants also produce morphological alterationsin the regenerated limbs of crabs. These may be relatively minor,such as reduced number of pigment cells, setae, or tuberclesin the regenerated limbs (mercury and cadmium), or may be moremajor deformities, such as abnormal bending in the limb or claw(tributyltin), or defects inchitin formation in the exoskeleton(diflubenzuron).     

Effects of Induced Molting on the Well-Being of Egg-Laying Hens

Induced molting in egg-laying hens is an important method for maximizing hen egg production and quality as well as hen health in commercial settings; however, there is growing societal concern over its effects on hen well-being. Using individual hens as their own controls, this research examined the behavior of hens subjected to different treatments of induced molting under premolt, molt, and postmolt conditions. Cage pecking increased in fast-induced subjects and aggression increased in fast-induced and nonfast-induced subjects during the molt. Gakel calling and several aspects of its acoustic structure were much higher during the molt condition in fast-induced subjects only. These data suggest that nonfast-induced molting treatments provide an effective method for inducing molting in hens and improving their well-being by minimizing discomfort due to food deprivation. In addition, these data further support that gakel vocalizations in hens may serve as an effective indicator for assessing well-being in a species otherwise behaviorally stoic in expressing stress or discomfort.     

Hemolymph Proteins and Molting in Crustaceans and Insects

The exoskeleton of crustaceans and insects is formed by cellsof the hypodermis, but several hemolymph proteins contributeto the synthesis of the new exoskeleton. These hemolymph proteinsshare a surprising degree of sequence similarity and are membersof the hemocyanin gene family. Copper-containing prophenoloxidasesof crustaceans and insects are directly involved in cross-linkingand hardening of the exoskeleton during molting and repair.Crustacean cryptocyanin and insect hexamerins lack copper andhave probably evolved from a copper-free product of an earlyhemocyanin gene duplication. These proteins have been implicatedin transport of hormones and phenols, and may be used directlyas structural components of the new exoskeleton. They are synthesizedelsewhere in the body, transported in the hemolymph, and probablytaken up by the hypodermis via specific receptors. Hemocyaninshave some residual phenoloxidase activity, in addition to theirprimary role of supplying oxygen to the metabolizing tissues.Thus multiple members of the hemocyanin gene family play vitalroles during molting, and a molecular phytogeny of these proteinswill contribute to our understanding of the evolution of formand function of these molecules from oxygen transport to molt-relatedactivities. Further studies on the expression of prophenoloxidase,cryptocyanin, hexamerins and hemocyanin, potential marker proteins,may extend our understanding of the relationship between othermolting animals in the proposed clade, Ecdysozoa.     

人工饲养条件下胎生蜥蜴蜕皮行为的观察

2014年5~8月,采用直接观察的方法,在实验室条件下,对胎生蜥蜴(Zootoca vivipara)的蜕皮行为进行了观察和研究。结果表明,胎生蜥蜴成体在繁殖期间(6~7月份)的蜕皮次数为0~3次,不同性别之间的蜕皮次数没有显著差异,且蜕皮次数与个体的体长、体重无关。成体的蜕皮周期不恒定,最短为8 d左右,与仔蜥的蜕皮周期基本相同。仔蜥的蜕皮行为与其异速生长有关,且随着蜕皮行为次数的增加,蜕皮过程的持续时间逐渐减少。胎生蜥蜴成体的蜕皮一般较为完整,除一小部分外,均可翻转后完成蜕皮,而仔蜥不翻转,这种特殊的蜕皮方式可能与人工饲养条件有关,也可能与胎生蜥蜴栖息地的气候条件、行为策略等因素相关。     

水生甲壳类蜕皮发生过程及其影响因素的研究与进展

甲壳类的蜕皮发生贯穿甲壳类的整个生命周期,与其生长、发育及繁殖息息相关。相对于其他生物,甲壳类的生长是非连续性的,每次蜕皮过程都需要蜕去旧表皮,合成新表皮,同时体宽与体重在较短时间内实现一次跨越式增长。影响甲壳类的蜕皮发生的因素众多,本文重点从内源因子(蜕皮激素、蜕皮抑制激素、蜕皮激素受体与维甲类X受体、性腺发育)与外源因子(温度、光照、盐度、钙离子、饵料与污染物)两个方面概括总结了影响水生甲壳类蜕皮发生的关键因子及其国内外研究进展,分析了其关键因子对水生甲壳类蜕皮发生的影响机制,并尝试对水生甲壳类蜕皮发生当前存在的主要问题进行讨论。     

Changing Activities of the Crustacean Epidermis during the Molting Cycle

The criteria established by Drach for subdividing the moltingcycle into stages are reviewed, and a suggestion is made forimproving the uniformity of postmolt staging of different species.Changes in the epidermis during the molting cycle of the crayfishOrconectes obscurus and O. sanborni are described. EpidermalDNA content was measured throughout the cycle and found to dropsharply at stage D₀ and to rise sharply at stage A. Proteincontent declined during postmolt and rose during premolt, asexpected. Protein synthesis remained more or less constant duringpostmolt, rose during premolt, and dropped to the postmolt levelat ecdysis. Chitin synthesis appeared to follow two differentcurves depending upon whether labelled glucose or acetylglucosaminewas used as precursor. This, and the presence of a separateenzyme capable of phosphorylating acetylglucosamine and notglucose, suggests that acetylglucosamine may be utilized directlywithout prior conversion to glucose. Actinomycin D was foundto prevent increases in rate of chitin biosynthesis during premoltbut not to inhibit chitin biosynthesis already underway. Duringthe same period, actinomycin stimulated general protein biosynthesis.By utilizing the molt staging criteria described, we were ableto detect induction of premolt by ecdysone.     

Phocanema decipiens: Molting in seals

Infective larvae of the anisakine nematode Phocanema decipiens from the muscle of Atlantic cod (Gadus morhua) were fed to harbor seals (Phoca vitulina) and gray seals (Halichoerus grypus). During maturation in the stomach of seal hosts, P. decipiens molted twice; these molts are the third and fourth of its life cycle. The third molt occurred between the second and fifth days of infection. The third stage, i.e., infective larva entering the third molt, had a cuticular tooth ventral to the mouth; the fourth stage larva emerging from the third molt had three bilobed lips with dentigerous ridges. The fourth molt occurred between the 5th and 15th days in seals. A female nematode emerging from the fourth molt possesses a vulva and a vagina; a male possesses caudal alae, pre- and postanal papillae. Significant morphometric changes in nematodes were associated with both molts. Females and males of P. decipiens reached maturity after 15 to 25 days in seals. Ova were detected in the feces of the seal hosts as early as the 16th day.     

Molting in workers of the Formosan subterranean termite Coptotermes formosanus

The Formosan subterranean termite, Coptotermes formosanus, with its huge colonies, is a major urban pest in several southern states and Hawaii as well as in South Asia. Because of their cryptic nature (underground habitat) and very long life cycle, not much is known about molting in termite workers. In C. formosanus, the workers stop foraging and lose their gut fauna, respectively, approximately 10 and 5 days prior to ecdysis. In any given colony an average of 1.01% (range 0.6-1.8) of the workers were found to molt each day under laboratory conditions. Workers destined to molt become sluggish and their head capsules develop a mottled texture one day prior to ecdysis. Ecdysis was generally accomplished with the assistance of other workers, which also fed on the exuviae. Immediately after molting worker mandibles were light pink in color and became fully melanized approximately two days later. Gut fauna were acquired on the fourth day after molting. Flagellates were transferred as small encysted cells from other workers through proctodeal feeding. Juvenile hormone III titer ranged between 30-41 pg/mg bodyweight in all stages except in workers sampled 6 days prior to ecdysis. In these workers the titer was 80.5 pg/mg. The high juvenile hormones (JH) titer may also be involved in causing defaunation. Ecdysteroid titer increased from 2.1 pg/mg in non-molting workers to 359.5 and 332.4 pg/mg one and two days following defaunation, respectively. There was a second smaller peak two days prior to ecdysis.