昆虫变态的激素与基因调控

昆虫变态是一个有趣而复杂的生物学现象,有不完全变态和完全变态。昆虫通过变态获得飞翔与生殖能力,扩大了生活范围,增强了适应能力。昆虫变态主要由咽侧体分泌的保幼激素和前胸腺分泌的蜕皮激素协同调控,众多基因参与。本文综合近几年的研究进展,重点介绍昆虫变态过程中保幼激素与蜕皮激素通过级联反应基因和microRNA调控的机制,以及保幼激素和蜕皮激素的合成调节机制。     

MicroRNA在昆虫变态及生殖过程中的调控作用

MicroRNA（miRNA）是一类广泛存在于真核生物中的小分子非编码RNA,通过抑制靶基因的翻译过程或降解靶基因的mRNA,在转录后水平上调控基因表达。在昆虫中已报道了大量的miRNA,其中部分miRNA的功能得到了解析。在昆虫变态过程中,let-7, miR-100, miR-125, miR-34, miR-14, miR-8, miR-281和 miR-252-3p能够作用于保幼激素或蜕皮激素信号通路,影响昆虫蜕皮、化蛹或翅、足及神经系统的发育。在昆虫生殖阶段,bantam, miR-184和miR-275影响生殖干细胞的分化或卵子发生。本文在介绍miRNA生物合成和作用机制的基础上,重点对昆虫变态与生殖过程中miRNA的最新研究进展进行综述。     

昆虫的变态与进化

昆虫的起源与进化问题,在昆虫学界讨论颇多,许多问题已基本取得了一致的意见,如昆虫纲起源于多足纲;昆虫的增节变态是最原始的变态类型;表变态是由增节变态演化而来;有翅亚纲中的无翅类是属于后生无翅等。但仍有许多问题处于争论之中,其中之一是有翅亚纲的几种变态类型：原变态（Ｐｒｏｍｅｔａｂｏｌａ）,不完全变态（Ｈｅｍｉｍｅｔａｂｏｌａ）,完全变态（Ｈｏｌｏｍｅｔａｂｏｌａ）的起源的问题。作者认为：有翅亚纲中的三种变态类型中,不完全变态是最原始的变态形式,原变态类和完全变态类昆虫是起源于祖先的不完全变态类。…     

昆虫的变态发育研究

昆虫变态发育使得昆虫成为地球陆地上种类最多、数量最大、分布最广、生活环境最多样化的一群生物。变态使昆虫在其生命周期中的不同发育时期表现出完全不同的形态、结构、功能和生活习性的变化,有利于昆虫迁飞转移,扩大其求偶交配、生活和生存环境空间。昆虫变态发育的变化是长期自然环境适应、协同进化的结果,受激素、营养和基因的精确调控。本文简要介绍了昆虫变态的类型、激素调控、营养调控和基因调控方面的研究进展,以及研究昆虫变态发育的科学和应用意义。     

整合昆虫发育生物学和果蝇遗传学来研究昆虫发育与变态

成熟动物(昆虫)个体大小主要由生长持续时间和生长速度2个因素所决定。蜕皮激素和保幼激素协同调控昆虫发育变态,并决定昆虫生长持续时间;胰岛素、营养和细胞接触抑制等生长死亡信号及其传导途径控制细胞分裂、长大、分化、死亡,并最终决定昆虫的生长速度。最近研究成果表明,蜕皮激素信号和胰岛素信号相互影响,对昆虫个体大小起决定性的作用;脂肪体和营养代谢把这2条信号传导途径整合起来。科学家将会整合昆虫发育生物学和果蝇遗传学,抓住生长持续时间和生长速率2个关键因素,并以营养代谢和脂肪体为切入点来研究昆虫的发育变态。     

昆虫变态发育过程中的细胞自噬和凋亡

在昆虫变态期,幼虫组织发生退化或消亡,原因在于蜕皮甾醇激素（ecdysteroid）,即通常所说的蜕皮激素,诱导这些组织的细胞发生了自噬(autophagy)和凋亡(apoptosis)的程序性细胞死亡(programmed cell death,PCD)。一般情况下,自噬途径构成一种饥饿应激适应性以避免细胞的死亡,表现为低水平Cvt泡(Cvt vesicle)和自噬体(autophagosome)对部分胞质溶胶、蛋白聚集体和细胞器的吞噬和降解。昆虫进入变态发育时,由于蜕皮激素的激活,由遗传级联系统调控的PCD机制被启动,低水平的常态自噬转入高水平的自噬并同时诱发凋亡,细胞进入不可逆的死亡,导致幼虫组织在变态期退化或消亡。对果蝇Drosophila变态期PCD机制中最重要的发现是:（1）在自噬发生的PI3KⅠ- Tor 和 PI3KⅢ的分子通路中,由自噬相关蛋白Atg1引发的高水平自噬能够诱导凋亡;（2）蜕皮激素诱导表达的βFTZ-F1,E93,BR-C,E74A等转录因子不但激活凋亡的Caspases通路,还能诱导自噬的发生。     

昆虫基因表达的调控

昆虫的基因表达过程十分复杂,受多种调控因子的调节,随着分子生物学研究新技术在昆虫学上的应用,昆虫基因表达的调控也取得了较快进展,本文以一些特征清楚的系统为例,讨论昆虫基因表达调控的研究现状,旨在为其它昆虫表达的调控研究提供借鉴。     

MicroRNA对不完全变态昆虫变态及生殖调控作用的研究进展

MicroRNAs(miRNAs)是一类在真核生物中由内源基因编码的小分子RNA,参与昆虫变态、生殖发育、细胞分化等多种重要生物学过程,在转录水平上发挥重要作用.在完全变态昆虫中,大量调控其变态及生殖的miRNA被广泛报道且进行了深入的研究,但miRNA调控不完全变态昆虫的变态及生殖发育的研究仍比较少,对其具体的调控机制也需要进一步阐明.为此,本文综述了近年来miRNA在调控不完全变态昆虫(以直翅目飞蝗Locusta migratoria、半翅目褐飞虱Nilaparvata lugens和蚜虫以及部分蜚蠊目昆虫为代表)的变态及生殖方面的研究进展,以期深化理解miRNA对不完全变态昆虫变态和生殖发育方面的机制,为害虫生态治理和综合防控提供科学依据.     

昆虫细胞生长之调控机制的探讨

细胞生长调控机制的探讨是近年来发育生物学中一个十分热门的研究领域。生物体内之细胞是如何得知何时该生长及分裂?何时该停止生长?何时该死亡?对生物体来说至关重大。研究显示调控细胞生长之信息传递系统出现差错,将导致生长异常,从而产生组织细胞之异常增生而诱发癌症或产生其它重大疾病。而人类也只有在充分理解细胞生长之机制的基础上,我们才能了解癌症等重大疾病发生的细胞生物学基础。在探讨细胞生长调控机制的研究中,昆虫特别是果蝇Drosophilamelanogaster一直是一个十分理想的实验材料。文章介绍了如何从昆虫看细胞之生长调控机制。     

Developmental neuroethology of insect metamorphosis

During metamorphosis, the insect nervous system must change to accomodate alterations in body form and behavior. Studies primarily on moths have shown that these changes involve the death of some larval neurons, the conservation and remodeling of others, and the maturation of new, adult-specific cells. The motor and sensory sides of the adult CNS vary in this regard with the former being constructed primarily from remodeled larval components, whereas the latter arises primarily from new neurons. Neuronal remodeling has received considerable attention. Larval-specific dendritic fields are pruned back during the larval–pupal transition, followed by the sprouting of adult-specific dendrites. Simple reflexes have been used to correlate these neuronal changes with the acquisition or loss of particular behaviors. The loss of the proleg retraction reflex is associated with the regression of the dendritic arbors of the proleg motoneurons. By contrast, expansion of axon arbors of the gin-trap afferents is necessary, but not sufficient, for the assembly of the gin-trap reflex in the pupal stage. The stretch receptor reflex provides a third example in which a new dendritic field in the adult form of a neuron is associated with new adult-specific connections. Interestingly, these connections are masked by persisting larval contacts until the emergence of the adult moth. For the metamorphosis of more complex behavioral circuits, some, such as that for flight behavior, seem to be assembled de novo, whereas others, like that for adult ecdysis behavior, show conservation of some circuit elements from the larval stage but with the superposition of some adult-specific components. © 1992 John Wiley & Sons, Inc.     

Active muscle migration during insect metamorphosis

The R1 abdominal retractor muscles of the insect Tenebrio molitor change position during the course of metamorphosis. These muscles detach from the epidermal tendon cells at their anterior ends, and migrate in a posterior direction, parallel to the body axis, to form completely new attachments shortly before adult emergence. Movement is preceded by the loss of sarcomere structure, and the muscles migrate in a partially dedifferentiated condition, closely accompanied by satellite cells and haemocytes. Movement appears to result from the extension of muscle processes towards the epidermis posterior to the larval attachment sites, which contact reciprocal processes extended from the epidermis. Contacts at the new posterior sites are then reinforced, and relinquished at the anterior. This cycle is subsequently repeated. It is envisaged that migration ceases when the muscles encounter a contour in the epidermal gradient known to specify the position of the adult muscle attachment sites. This positional information may be encoded in the epidermal basal lamina. The muscles then redifferentiate, with concurrent differentiation of new epidermal tendon cells. Development of adult muscle attachments appears to require reciprocal morphogenetic interactions between muscle and epidermis.     

Dynamics and metamorphosis of an identifiable peptidergic neuron in an insect

Eclosion hormone (EH) is a 7000 Da peptide that triggers ecdysis behavior in insects. In the moth, Manduca sexta, EH is found in two pairs of ventromedial (VM) cells in the brain which send their axons down the ventral nerve cord to a neurohemal site in the proctodeal nerve in the larva and pupa. During adult development, these cells send axon collaterals to the corpora cardiaca where they form a new release site used for adult eclosion. Studies of bioassayable peptide during the 5th larval instar and the larval-pupal transformation revealed that after depletion at ecdysis, the VM cells showed a transient increase in EH found in their cell bodies and axons. By contrast, their terminals in the proctodeal nerve showed a gradual accumulation of peptide followed by a release of over 90% of the stored material at pupal ecdysis. In situ hybridization analysis on whole mounts of the brains showed that the VM cells always contained EH mRNA with increased accumulation during the larval and pupal molting periods with a slight decline just before ecdysis. High levels of EH mRNA were found in brains of diapausing pupae. During the first two-thirds of adult development, mRNA accumulated to high levels, then slowly declined until ecdysis. EH mRNA levels up to 3 days after adult eclosion. At no time was EH mRNA found in the lateral neurosecretory cell cluster previously reported to produce EH for adult eclosion. 1994 John Wiley & Sons, Inc.     

The molting hormone activity in Sarcophaga buttata in relation to metamorphosis and reproduction

Ecdysteroid levels were determined by radioimmunoassay, using an antiserum with higher affinity for ecdysterone than for ecdysone. In both sexes, the ecdysteroid level remains low until day 3 after eclosion. When fed liver on day 3, the ecdysteroid titre declines within 4 h in both sexes, stays low in males, but increases again after 12 h in females. The content in non-liver-fed animals, specially females, was significantly higher than if given liver. The level in liver-fed whole animals increased during oogenesis from the onset of follicular growth until the 4B—4C stage, and decreased thereafter.

During embryogenesis, a new high level was found in females, due to the developing embryos which contained a high titre. In isolated ovaries, the ecdysteroid content increased during follicular growth, with a steep increase during the last phase (4C—M and from M to uterine eggs). The ecdysteroid content in hemolymph of 7-day-old females, fed liver or not, was around 10 ng/ml, but in hemolymph of males of the same age no activity was found.

Ovariectomized females contained 12.4 ng/g ecdysteroid. The level during metamorphosis was also determined, mainly to be able to compare our radioimmunoassay results with known bioassay data. Our results support the statement that the ovary cannot be the only site of synthesis of molting hormone in adult S. buttata.     

Amphibian metamorphosis as a model for studying the developmental actions of thyroid hormone

The thyroid hormones L-thyroxine and triiodo-Lthyronine have profound effects on postenbryonic development of most vertebrates.Analysis of their action in mammals is vitiated by the exposure of the developing foetus to a number of maternal factors which do not allow one to specifically define the role of thyroid hormone (TH) or that of other hormones and factors that modulate its action.Amphibian metamorphosis is obligatorily dependent on TH which can initiate all the diverse physiological manifestations of this postembryonic developmental process(morphogenesis,cell death,re-structuring,etc.) in free-living embryos and larvas of most anurans.This article will first describe the salient features of metamorphosis and its control by TH and other hormones.Emphasis will be laid on the key role played by TH receptor (TR),in particular the phenomenon of TR gene autoinduction,in initiating the developmental action of TH.Finally,it will be argued that the findings on the control of amphibian metamorphosis enhance our understanding of the regulation of postembryonic development by TH in other vertebrate species.     

Induction of metamorphosis in the sand dollar Peronella japonica by thyroid hormones

The larva of the sand dollar Peronella japonica lacks a mouth and gut, and undergoes metamorphosis into a juvenile sand dollar without feeding. In the present study, it was found that thyroid hormones accelerate the metamorphosis of P. japonica larvae. The contents of thyroid hormones in larvae increased gradually during development. Thiourea and potassium perchlorate, inhibitors of thyroid hormone synthesis, delayed larval metamorphosis and simultaneously repressed an increase in the content of thyroxine in the larval body. These results suggest that the P. japonica larva has a system for synthesis of thyroid hormones that act as factors for inducing metamorphosis.