白蚁共生放线菌研究进展

白蚁与微生物的共生关系是目前较受关注的研究热点,其肠道及巢内的共生微生物在降解木质纤维素的过程中扮演着重要的角色。放线菌是这些共生微生物中的重要一类,广泛存在于肠道、蚁巢及其周围土壤中,目前已探明共生放线菌在参与白蚁碳氮循环及保护巢群免受外来病菌侵染等方面发挥着极大的作用。近年来,人们利用分子生物学技术鉴定了部分共生放线菌的类群,发现了许多具应用前景的新放线菌及相关酶和代谢产物。因此,研究与白蚁相关的放线菌不仅有助于人们了解白蚁共生菌群落间的互作及其与宿主间的关系,而且对人类开发自然资源也有较大的帮助。本文对白蚁共生放线菌的研究进展作一综述,供同行参考。     

Genetic variation of symbiotic fungi cultivated by the macrotermitine termite Odontotermes formosanus (Isoptera: Termitidae) in the Ryukyu Archipelago

Fungus-growing termites have a mutualistic relationship with their cultivated fungi. To improve understanding of genetic aspects of this relationship, we examined molecular markers in the fungus-growing termite Odontotermes formosanus and its fungi Termitomyces spp. from the Ryukyu Archipelago. Based on the polymorphic band patterns obtained from arbitrarily primed polymerase chain reaction methods, we constructed cladograms for related colonies of the termites and fungi. The resulting trees indicated that the termites display little genetic variation among the colonies, while the symbiotic fungi consist of two major genetic types. In addition, molecular phylogenetic trees of the symbiotic fungi based on internal transcribed spacer and 18S rDNA suggested that these two types of fungi are different species. We also demonstrated that the fungi comprising the fruiting bodies and fungus combs are identical, and that fungus combs are probably a monoculture within a single termite colony. Our results indicate that horizontal transmission of symbiotic fungi among termite colonies occurred during the evolutionary history of this symbiosis.     

Lignocellulose degradation by microorganisms from termite hills and termite guts: A survey on the present state of art

Abstract: In several aspects termites are a fascinating group of insects having attracted the interest of many researchers. They exhibit a complex social behavior and caste differentiation occurring elsewhere only among the hymenoptera. In an enlarged part of the hindgut, the paunch, termites have established a unique symbiotic association with prokaryotic and eukaryotic microorganisms. A similar flora is also found in wood-eating roaches of the genus Cryptocercus . The study of symbiosis between termites and their intestinal microbes is of general interest, because due to this symbiotic interaction termites can feed on complex biopolymers such as wood. Flagellates and bacteria occur in the gut of lower termites, while higher termites possess only bacteria. In particular spirochetes are abundant in the termite gut. Apart from spirochetes and other more common bacteria, actinomycetes, yeasts and fungi have also been isolated from different species of termites. This review summarizes the distinct role of the intestinal flora in degradation of wood components such as cellulose, hemicellulose and lignin.     

15N discrimination and the sensitivity of nitrogen fixation to changes in dietary nitrogen in Reticulitermes flavipes (Isoptera: Rhinotermitidae)

Xylophagous termites possess symbiotic bacteria that fix atmospheric nitrogen (N(2)). Although symbiotic N(2) fixation is central to termite nutrition and ecologically important, it is energetically costly. Using stable isotopes, we tested the hypothesis that symbiotic N(2) fixation would decrease in workers of the eastern subterranean termite, Reticulitermes flavipes Kollar, which were exposed to high nitrogen diets. To calculate the isotope discrimination factor occurring as a result of digestion, Δ(dig), and which occurs as the result of N(2) fixation, Δ(fix), symbiotic N(2) fixation was inhibited via force feeding termites the antibiotic kanamycin. Antibiotic-treated termites and control (N(2)-fixing) termites were exposed to different concentrations of dietary N (0, 0.21, and 0.94% N), their (15)N signatures were obtained, and the percent nitrogen derived from the atmosphere within termite samples was calculated. As we hypothesized, symbiotic N(2) fixation rates were negatively correlated with dietary N, suggesting that high concentrations of dietary N suppressed symbiotic N(2) fixation in R. flavipes. A comparison of the (15)N isotope signatures of antibiotic-treated termites with their food sources demonstrated that Δ(dig) = 2.284‰, and a comparison of the (15)N isotope signatures of antibiotic-treated termites with control termites indicated that Δ(fix) = -1.238‰. These are the first estimates of Δ(dig) for R. flavipes, and the first estimate of Δ(fix) for any N(2)-fixing termite species.     

Inheritance and diversification of symbiotic trichonymphid flagellates from a common ancestor of termites and the cockroach Cryptocercus

Cryptocercus cockroaches and lower termites harbour obligate, diverse and unique symbiotic cellulolytic flagellates in their hindgut that are considered critical in the development of social behaviour in their hosts. However, there has been controversy concerning the origin of these symbiotic flagellates. Here, molecular sequences encoding small subunit rRNA and glyceraldehyde-3-phosphate dehydrogenase were identified in the symbiotic flagellates of the order Trichonymphida (phylum Parabasalia) in the gut of Cryptocercus punctulatus and compared phylogenetically to the corresponding species in termites. In each of the monophyletic lineages that represent family-level groups in Trichonymphida, the symbionts of Cryptocercus were robustly sister to those of termites. Together with the recent evidence for the sister-group relationship of the host insects, this first comprehensive study comparing symbiont molecular phylogeny strongly suggests that a set of symbiotic flagellates representative of extant diversity was already established in an ancestor common to Cryptocercus and termites, was vertically transmitted to their offspring, and subsequently became diversified to distinct levels, depending on both the host and the symbiont lineages.     

Dietary‐driven variation effects on the symbiotic flagellate protist communities of the subterranean termite Reticulitermes grassei Clément

The ability of subterranean termites to digest lignocellulose relies not only on their digestive tract physiology, but also on the symbiotic relationships established with flagellate protists and bacteria. The objective of this work was to test the possible effect of different cellulose‐based diets on the community structure (species richness and other diversity metrics) of the flagellate protists of the subterranean termite Reticulitermes grassei. Termites belonging to the same colony were subjected to six different diets (natural diet, maritime pine wood, European beech, thermally modified European beech, cellulose powder and starvation), and their flagellate protist community was evaluated after the trials. All non‐treated sound woods produced similar flagellate protist communities that were more diverse and of high evenness (low dominance). On the contrary, flagellate protist communities from cellulose‐fed termites and starving termites were considered to be significantly different from all non‐treated woods; they were less diverse and some morphotypes became dominant as a consequence of flagellate protist communities having suffered major adaptations to these diets. The flagellate protist communities of untreated beech and thermally modified beech‐fed termites were considered to be significantly different in terms of abundance and morphotype diversity. This may be caused by a decrease in lignocellulose quality available for termites and from an interference of thermally treated wood with the chemical stability of the termite hindgut. Our study suggests that as a consequence of the strong division of labour among these protists to accomplish the intricate process of lignocellulose digestion, termite symbiotic flagellate protist communities are a dynamic assemblage able to adapt to different conditions and diets. This study is important for the community‐level alteration approach, and it is the first study to investigate the effects of thermally modified wood on the flagellate protist communities of subterranean termites.     

培菌白蚁与鸡枞菌研究进展

培菌白蚁起源于非洲,蚁巢内具有复杂的社会分工.培菌白蚁依靠独特的蚁巢结构维持内部稳态和气体循环.菌圃是白蚁培育鸡枞菌的场所.鸡枞菌隶属于担子菌亚门,但其传播方式和生活史具有区别于其它担子菌的特点.鸡枞菌协助白蚁进行植物纤维的消化,白蚁则为鸡枞菌提供合适的生长环境,并控制鸡枞菌的遗传结构.培菌白蚁和鸡枞菌形成紧密的共生关...     

Hidden cellulases in termites: revision of an old hypothesis

The intestinal flagellates of termites produce cellulases that contribute to cellulose digestion of their host termites. However, 75% of all termite species do not harbour the cellulolytic flagellates; the endogenous cellulase secreted from the midgut tissue has been considered a sole source of cellulases in these termites. Using the xylophagous flagellate-free termites Nasutitermes takasagoensis and Nasutitermes walkeri, we successfully solubilized cellulases present in the hindgut pellets. Zymograms showed that the hindguts of these termites possessed several cellulases and contained up to 59% cellulase activity against crystalline cellulose when compared with the midgut. Antibiotic treatment administered to N. takasagoensis significantly reduced cellulase activity in the hindgut, suggesting that these cellulases were produced by symbiotic bacteria.     

Symbiotic fungi produce laccases potentially involved in phenol degradation in fungus combs of fungus-growing termites in Thailand

Fungus-growing termites efficiently decompose plant litter through their symbiotic relationship with basidiomycete fungi of the genus Termitomyces. Here, we investigated phenol-oxidizing enzymes in symbiotic fungi and fungus combs (a substrate used to cultivate symbiotic fungi) from termites belonging to the genera Macrotermes, Odontotermes, and Microtermes in Thailand, because these enzymes are potentially involved in the degradation of phenolic compounds during fungus comb aging. Laccase activity was detected in all the fungus combs examined as well as in the culture supernatants of isolated symbiotic fungi. Conversely, no peroxidase activity was detected in any of the fungus combs or the symbiotic fungal cultures. The laccase cDNA fragments were amplified directly from RNA extracted from fungus combs of five termite species and a fungal isolate using degenerate primers targeting conserved copper binding domains of basidiomycete laccases, resulting in a total of 13 putative laccase cDNA sequences being identified. The full-length sequences of the laccase cDNA and the corresponding gene, lcc1-2, were identified from the fungus comb of Macrotermes gilvus and a Termitomyces strain isolated from the same fungus comb, respectively. Partial purification of laccase from the fungus comb showed that the lcc1-2 gene product was a dominant laccase in the fungus comb. These findings indicate that the symbiotic fungus secretes laccase to the fungus comb. In addition to laccase, we report novel genes that showed a significant similarity with fungal laccases, but the gene product lacked laccase activity. Interestingly, these genes were highly expressed in symbiotic fungi of all the termite hosts examined.     

白蚁及共生微生物木质纤维素水解酶的种类

白蚁是热带生态系统重要的木质纤维素降解者。白蚁种类丰富,可分成高等白蚁和低等白蚁,食性也具有各自特点。白蚁自身可以产生纤维素酶,主要是GHF9的内切葡聚糖酶(EG),也有β-葡萄糖苷酶(GB)。低等白蚁共生的原虫中已发现丰富的纤维素酶基因,属于GHF5,7和45。同时还有其他相关功能基因,如木聚糖酶和果胶类物质水解酶。高等白蚁肠道中没有共生原虫。高等培菌白蚁可以利用共生蚁巢伞属真菌促进木质纤维素降解,真菌可以产生纤维素酶,果胶质水解酶类、木聚糖酶,同时还产生可能与木质素分解相关的一种漆酶,但是从分子水平,关于共生真菌纤维素水解酶的研究还较少。白蚁肠道已分离出许多具有木质纤维素降解能力的菌株,最近的研究也发现了大量细菌纤维素酶基因。白蚁-共生系统丰富的木质纤维素水解酶类为发展生物方法开发纤维素乙醇这一思路提供有价值的资源。     

低等白蚁肠道共生微生物的多样性及其功能

低等白蚁肠道里存在着复杂的微生物区系,包括真核微生物鞭毛虫和原核生物,细菌及古细菌。低等白蚁的后肠以特别膨大的囊形胃及其氢氧浓度的明显梯度分布和丰富的微生物区系为特征,是白蚁进行木质纤维素消化的主要器官。后肠内的鞭毛虫能将纤维素水解并发酵为乙酸,二氧化碳和氢,为白蚁提供营养和能源。系统发育研究表明,低等白蚁肠道共生细菌的主要类群为白蚁菌群1、螺旋体、拟杆菌,低G C mol%含量的革兰氏阳性菌和紫细菌等。而古细菌主要为甲烷短杆菌属的产甲烷菌。共生原核生物与二氧化碳的还原和氮的循环等代谢有关。但肠道共生微生物的具体功能和作用机制还有待进一步的揭示。     

Cospeciation in the triplex symbiosis of termite gut protists (Pseudotrichonympha spp.), their hosts, and their bacterial endosymbionts

A number of cophylogenetic relationships between two organisms namely a host and a symbiont or parasite have been studied to date; however, organismal interactions in nature usually involve multiple members. Here, we investigated the cospeciation of a triplex symbiotic system comprising a hierarchy of three organisms -- termites of the family Rhinotermitidae, cellulolytic protists of the genus Pseudotrichonympha in the guts of these termites, and intracellular bacterial symbionts of the protists. The molecular phylogeny was inferred based on two mitochondrial genes for the termites and nuclear small-subunit rRNA genes for the protists and their endosymbionts, and these were compared. Although intestinal microorganisms are generally considered to have looser associations with the host than intracellular symbionts, the Pseudotrichonympha protists showed almost complete codivergence with the host termites, probably due to strict transmissions by proctodeal trophallaxis or coprophagy based on the social behaviour of the termites. Except for one case, the endosymbiotic bacteria of the protists formed a monophyletic lineage in the order Bacteroidales, and the branching pattern was almost identical to those of the protists and the termites. However, some non-codivergent evolutionary events were evident. The members of this triplex symbiotic system appear to have cospeciated during their evolution with minor exceptions; the evolutionary relationships were probably established by termite sociality and the complex microbial community in the gut.     

Molecular phylogeny of symbiotic basidiomycetes of fungus-growing termites in Thailand and their relationship with the host

Termitomyces-related symbiotic basidiomycetes in the nests of fungus-growing termites (Macrotermitinae) of several genera in Thailand were cultivated and analyzed phylogenetically based on the DNA sequence of nuclear ribosomal RNA genes. The relationships of the symbiotic fungi with host termites and their locality were apparently complex, supporting intricate mechanisms for the termites to acquire the symbionts.     

培菌白蚁菌圃微生物降解木质纤维素的研究进展

白蚁及其共生微生物协同降解植物细胞壁的机理一直被世界各国科学家所关注。培菌白蚁作为高等白蚁,相比低等食木白蚁具有更多样化的食性,其利用外共生系统“菌圃”,对多种植物材料进行处理。本文综述了菌圃微生物降解木质纤维素的研究进展,以期为深入研究菌圃中木质纤维素降解过程及其机制,并挖掘利用菌圃降解木质纤维素的能力及仿生模拟菌圃开发新的生物质利用系统提供参考。培 菌白蚁在其巢内利用由植物材料修建的多孔海绵状结构——“菌圃”来培养共生真菌鸡枞菌Termitomyces spp.,形成了独特的木质纤维素食物降解和消化策略,使木质纤维素在培菌白蚁及其共生微生物协同作用下被逐步降解。幼年工蚁取食菌圃上的共生真菌菌丝组成的小白球和老年工蚁觅得食物并排出粪便堆积到菌圃上成为上层菌圃。这一过程中,被幼年工蚁取食的共生真菌释放木质素降解酶对包裹在植物多糖外部的木质素屏障进行解聚。菌圃微生物(包括共生真菌)对解聚的木质素基团进一步降解,将多糖长链或主链剪切成短链,使菌圃基质自下而上被逐步降解。最后下层的老熟菌圃被老年工蚁取食,其中肠的内源酶系及后肠微生物将这些短链进一步剪切和利用。因此,蚁巢菌圃及其微生物是培菌白蚁高效转化利用木质纤维素的基础。化学层面的研究表明,菌圃能够实现对植物次生物质解毒和植 物纤维化学结构解构。对共生真菌相关酶系的研究显示可能其在菌圃的植物纤维化学结构和植物次生物质的降解中发挥了作用,但不同属共生真菌间其效率和具体功能不尽相同。而菌圃中的细菌是否发挥了作用和哪些细菌类群发挥了作用等仍有待进一步的研究。相比于低等食木白蚁利用其后肠共生微生物降解木质纤维素,培菌白蚁利用菌圃降解木质纤维素具有非厌氧和能处理多种类型食物两大优势,仿生模拟菌圃降解木质纤维素的机制对林地表面枯枝落叶的资源化利用具有重要意义。     

白蚁肠道微生物研究方法概述

白蚁是木质纤维素的主要降解者,在森林生态系统碳氮循环过程中发挥着重要作用。白蚁肠道共生微生物主要包括原生生物、细菌、古菌和真菌。在白蚁对木质纤维素进行降解、发酵,从而产生乙酸、氢气和甲烷以及对氮的固定过程中,白蚁肠道共生微生物起着重要的作用。本文对白蚁肠道微生物的研究方法进行总结,概述了各种方法的优缺点,同时对肠道微生物的研究进展进行了总结,以期为白蚁肠道微生物的进一步研究和利用提供参考。     

Symbiotic Fungi Produce Laccases Potentially Involved in Phenol Degradation in Fungus Combs of Fungus-Growing Termites in Thailand

Fungus-growing termites efficiently decompose plant litter through their symbiotic relationship with basidiomycete fungi of the genus Termitomyces. Here, we investigated phenol-oxidizing enzymes in symbiotic fungi and fungus combs (a substrate used to cultivate symbiotic fungi) from termites belonging to the genera Macrotermes, Odontotermes, and Microtermes in Thailand, because these enzymes are potentially involved in the degradation of phenolic compounds during fungus comb aging. Laccase activity was detected in all the fungus combs examined as well as in the culture supernatants of isolated symbiotic fungi. Conversely, no peroxidase activity was detected in any of the fungus combs or the symbiotic fungal cultures. The laccase cDNA fragments were amplified directly from RNA extracted from fungus combs of five termite species and a fungal isolate using degenerate primers targeting conserved copper binding domains of basidiomycete laccases, resulting in a total of 13 putative laccase cDNA sequences being identified. The full-length sequences of the laccase cDNA and the corresponding gene, lcc1-2, were identified from the fungus comb of Macrotermes gilvus and a Termitomyces strain isolated from the same fungus comb, respectively. Partial purification of laccase from the fungus comb showed that the lcc1-2 gene product was a dominant laccase in the fungus comb. These findings indicate that the symbiotic fungus secretes laccase to the fungus comb. In addition to laccase, we report novel genes that showed a significant similarity with fungal laccases, but the gene product lacked laccase activity. Interestingly, these genes were highly expressed in symbiotic fungi of all the termite hosts examined.