Evolutionary Studies on Uricases of Fungal Endosymbionts of Aphids and Planthoppers

Aphids belonging to the three genera Tuberaphis, Glyphinaphis, and Cerataphis contain extracellular fungal symbionts that resemble endocellular yeast-like symbionts of planthoppers. Whereas the symbiont of planthoppers has a uricase (urate oxidase; EC 1.7.3.3) and recycles uric acid that the host stores, no uric acid was found in Tuberaphis styraci, and its fungal symbiont did not exhibit the uricase activity. However, the fungal symbionts of these aphids, including that of T. styraci, were shown to have putative uricase genes, or pseudogenes, for the uricase. Sequence analysis of these genes revealed that deleterious mutations occurred independently on each lineage of Glyphinaphis and Tuberaphis, while no such mutation was found in the lineage of Cerataphis. These genes were almost identical to those cloned from the symbionts of planthoppers, though the host aphids and planthoppers are phylogenetically distant. To estimate the phylogenetic relationship in detail between the fungal symbionts of aphids and those of planthoppers, a gene tree was constructed based on the sequences of the uricase genes including their flanking regions. As a result, the symbionts of planthoppers and Tuberaphis aphids formed a sister group against those of Glyphinaphis and Cerataphis aphids with high bootstrap confidence levels, which strongly suggests that symbionts have been horizontally transferred from the aphids' lineage to the planthoppers'. Received: 29 March 2000 / Accepted: 31 May 2000     

福建厦门地区蚜虫病原真菌调查及其发生规律

从2014年至2015年对厦门地区蚜虫虫霉发生和流行情况进行初步调查,共鉴定出6种虫霉目病原真菌:新蚜虫疠霉Pandora neoaphidis、努利虫疠霉Pandora nouryi、普朗肯虫霉Entomophthora planchoniana、暗孢耳霉Conidiobolus obscurus、弗雷生新接霉Neozygites fresenii、根虫瘟霉Zoophthora radicans。新蚜虫疠霉和普朗肯虫霉是春季蔬菜蚜虫种群中发生、流行的最重要病原真菌,夏季未观察到虫霉的发生。     

Genetic Characterization of Plasmids Containing Genes Encoding Enzymes of Leucine Biosynthesis in Endosymbionts (Buchnera) of Aphids

The prokaryotic endosymbionts (Buchnera) of aphids are known to provision their hosts with amino acids that are limiting in the aphid diet. Buchnera from the aphids Schizaphis graminum and Diuraphis noxia have plasmids containing leuABCD, genes that encode enzymes of the leucine biosynthetic pathway, as well as genes encoding proteins probably involved in plasmid replication (repA1 and repA2) and an open reading frame (ORF1) of unknown function. The newly reported plasmids closely resemble a plasmid previously described in Buchnera of the aphid Rhopalosiphum padi [Bracho AM, Martínez-Torres D, Moya A, Latorre A (1995) J Mol Evol 41:67–73]. Nucleotide sequence comparisons indicate conserved regions which may correspond to an origin of replication and two promoters, as well as inverted repeats, one of which resembles a rho-independent terminator. Phylogenetic analyses based on amino acid sequences of leu gene products and ORF1 resulted in trees identical to those obtained from endosymbiont chromosomal genes and the plasmid-borne trpEG. These results are consistent with a single evolutionary origin of the leuABCD-containing plasmid in a common ancestor of Aphididae and the lack of plasmid exchange between endosymbionts of different aphid species. Trees for ORF1 and repA (based on both nucleotides and amino acids) are used to examine the basis for leu plasmid differences between Buchnera of Thelaxes suberi and Aphididae. The most plausible explanation is that a single transfer of the leu genes to an ancestral replicon was followed by rearrangements. The related replicon in Buchnera of Pemphigidae, which lacks leuABCD, appears to represent the ancestral condition, implying that the plasmid location of the leu genes arose after the Pemphigidae diverged from other aphid families. This conclusion parallels previously published observations for the unrelated trpEG plasmid, which is present in Aphididae and absent in Pemphigidae. Recruitment of amino acid biosynthetic genes to plasmids has been ongoing in Buchnera lineages after the infection of aphid hosts. Received: 9 March 1998 / Accepted: 18 May 1998.     

Diversity of the Most Commonly Reported Facultative Symbionts in Two Closely-Related Aphids with Different Host Ranges

Richness and abundance of facultative symbionts vary strongly with aphid species and genotype, symbiont strain, host plant, biogeography, and a number of abiotic factors. Despite indications that aphids in the same ecological niche show similar levels of facultative symbiont richness, existing reports do not consider the potential role of host plants on aphid microbial community. Little is known about how oligophagy and polyphagy may be influenced by secondary symbiont distribution, mainly because studies on secondary symbiont diversity are biased towards polyphagous aphids from the Northern Hemisphere. Here, we demonstrate the richness and abundance of the most common aphid-associated facultative symbionts in two tropical aphid species, the oligophagous Aphis (Toxoptera) citricidus (Kirkaldy) (Hemiptera: Aphididae) and the polyphagous Aphis aurantii (Boyer de Fonscolombe) (Hemiptera: Aphididae). Aphis citricidus is restricted to Citrus sp. host plants and closely related genera, whereas A. aurantii successfully exploits a wide variety of host plants from different families. Both were collected in the same ecological niche and our data basically indicated the same richness of secondary symbionts, but the abundance at which secondary symbionts occurred was very distinct between the two species. Spiroplasma was the most abundant facultative symbiont associated with A. citricidus and A. aurantii in the ecological niche studied. Single and multiple secondary symbiont infections were observed, but diversity of multiple infections was particularly high in A. citricidus. We discuss our findings and suggest hypotheses to explain causes and consequences of the differences in secondary symbiont diversity observed between these two aphid species.     

Evidence for Horizontal Transmission of Secondary Endosymbionts in the Bemisia tabaci Cryptic Species Complex

Bemisia tabaci (Hemiptera: Aleyrodidae) is a globally distributed pest composed of at least 34 morphologically indistinguishable cryptic species. At least seven species of endosymbiont have been found infecting some or all members of the complex. The origin(s) of the associations between specific endosymbionts and their whitefly hosts is unknown. Infection is normally vertical, but horizontal transmission does occur and is one way for new infections to be introduced into individuals. The relationships between the different members of the cryptic species complex and the endosymbionts have not been well explored. In this study, the phylogenies of different cryptic species of the host with those of their endosymbionts were compared. Of particular interest was whether there was evidence for both coevolution and horizontal transmission. Congruence was observed for the primary endosymbiont, Portiera aleyrodidarum, and partial incongruence in the case of two secondary endosymbionts, Arsenophonus and Cardinium and incongruence for a third, Wolbachia. The patterns observed for the primary endosymbiont supported cospeciation with the host while the patterns for the secondary endosymbionts, and especially Wolbachia showed evidence of host shifts and extinctions through horizontal transmission rather than cospeciation. Of particular note is the observation of several very recent host shift events in China between exotic invader and indigenous members of the complex. These shifts were from indigenous members of the complex to the invader as well as from the invader to indigenous relatives.     

Endosymbionts in paramecium

Paramecium species are extremely valuable organisms to enable experiments for the reestablishment of endosymbiosis. This is investigated in two different systems, the first with Paramecium caudatum and the endonuclear symbiotic bacterium Holospora species. Although most endosymbiotic bacteria cannot grow outside the host cell as a result of their reduced genome size, Holospora species can maintain their infectivity for a limited time. We found that an 89-kDa periplasmic protein has an important function for Holospora's invasion into the target nucleus, and that Holospora alters the host gene expression; the host thereby acquires resistance against various stresses. The second system is the symbiosis between P. bursaria and symbiotic Chlorella. Alga-free P. bursaria and the algae retain the ability to grow without a partner. Consequently, endosymbiosis between the aposymbiotic host cells and the symbiotic algae can be reestablished easily by mixing them. We now found four checkpoints for the reestablishment of the endosymbiosis between P. bursaria and the algae. The findings in the two systems provide excellent opportunities for us to elucidate not only infection processes but also to assess the associations leading to eukaryotic cell evolution. This paper summarizes recent progresses on reestablishment of the primary and the secondary endosymbiosis in Paramecium.     

Observations on the Occurrence of Entomophthorales Infecting Aphids (Aphidoidea) in Slovakia

The species spectrum of entomophthoralean fungi parasiting aphids was investigated in Slovakia. The surveys focused on aphid fauna occupying both agricultural crops and vegetation at non-crop sites. During the surveys, 15 different entomophthoralean species were identified from 66 aphid species. Of these 15 fungal species, eight had not previously been recorded in Slovakia and one species, Erynia erinacea (Ben-Ze’ev et Kenneth) Remaudière et Hennebert, had not been recorded previously in Central Europe. New hosts for some parasitic fungi were also recorded. The occurrence of fungal infection and the fungal species spectrum varied with sampling period and sampling area. In general, the fungi were observed parasiting aphid populations from April until November. Pandora neoaphidis (Remaudière et Hennebert) Humber was the predominant fungal pathogen and could effectively reduce aphid populations. Entomophthora planchoniana Cornu and Conidiobolus obscurus (Hall et Dunn) Remaudière et Keller were also common pathogens of aphids, but without an epizootic potential. On the other hand, Neozygites fresenii (Nowakowski) Remaudière et Keller had a strong tendency to establish epizootics in dense aphid colonies, especially in those of the black bean aphid. Other entomophthoralean species observed in the country were considered to be minor aphid pathogens on account of their low occurrence, lower specificity to aphids, or high specificity to a single aphid species. This is the first detailed report to date on the occurrence of Entomophthorales in Slovakia.     

Testing the Physiological Barriers to Viral Transmission in Aphids Using Microinjection

Potato loafroll virus (PLRV), from the family Luteoviridae infects solanaceous plants. It is transmitted by aphids, primarily, the green peach aphid. When an uninfected aphid feeds on an infected plant it contracts the virus through the plant phloem. Once ingested, the virus must pass from the insect gut to the hemolymph (the insect blood ) and then must pass through the salivary gland, in order to be transmitted back to a new plant. An aphid may take up different viruses when munching on a plant, however only a small fraction will pass through the gut and salivary gland, the two main barriers for transmission to infect more plants. In the lab, we use physalis plants to study PLRV transmission. In this host, symptoms are characterized by stunting and interveinal chlorosis (yellowing of the leaves between the veins with the veins remaining green). The video that we present demonstrates a method for performing aphid microinjection on insects that do not vector PLVR viruses and tests whether the gut is preventing viral transmission. The video that we present demonstrates a method for performing Aphid microinjection on insects that do not vector PLVR viruses and tests whether the gut or salivary gland is preventing viral transmission.Download video file.^{(138M, mov)}     

Role of Host Nutrition in Symbiont Regulation: Impact of Dietary Nitrogen on Proliferation of Obligate and Facultative Bacterial Endosymbionts of the Pea Aphid Acyrthosiphon pisum

The impact of host nutrition on symbiont regulation in the pea aphid Acyrthosiphon pisum was investigated. The population density of the obligate symbiont Buchnera aphidicola positively correlated with dietary nitrogen levels. In contrast, the population density of the facultative symbiont Serratia symbiotica increased in aphids reared on low-nitrogen diets, indicating distinct regulatory mechanisms in the same insect host.     

Equilibrium Frequency of Endosymbionts in Multiple Infections Based on the Balance between Vertical Transmission and Cytoplasmic Incompatibility

Cytoplasmic incompatibility (CI)-inducing endosymbiotic bacteria, such as Wolbachia and Cardinium, have been well studied through field data and validations on the basis of numerical simulations. However, the analytically derived equilibrium frequency of multiple infections has not yet been determined, although the equilibrium for cases of single infection has been reported. In this study, we considered the difference equation for endosymbionts using three parameters: the probability of the failure of vertical transmission (), CI strength (), and the level of host inbreeding (). To analyze this model, we particularly focused on , i.e., the frequency of host individuals completely infected with all -bacterial strains in the population. , at the equilibrium state, was analytically calculated in the cases where and is any arbitrary value. We found that can be described using two parameters: and , which is identical to . has a larger value in a system with a smaller . In addition, determines the maximum number of strains that infect a single host. Our results revealed the following: i) three parameters can be reduced to a single parameter, i.e., and ii) the threshold of the maximum number of infections is defined by , which prevents additional invasions by endosymbionts.     

Endosymbionts and mitochondrial origins

Summary The possibility is put forth that the mitochondrion did not originate from an endosymbiosis, 1–2 billion years ago, involving an aerobic bacterium. Rather, it arose by endosymbiosis in a much early, anaerobic period, and was initially a photosynthetic organelle, analogous to the modern chloroplast. This suggestion arises from a reconsideration of the nature of endosymbiosis. It ex-plains the remarkable diversity in mitochondrial information storage and processing systems.     

Endosymbionts (Buchnera) from the Aphids Schizaphis graminum and Diuraphis noxia Have Different Copy Numbers of the Plasmid Containing the Leucine Biosynthetic Genes

The prokaryotic endosymbiont (Buchnera) of the aphid Schizaphis graminum contains 24 copies of a plasmid that has genes encoding enzymes of the leucine biosynthetic pathway while the endosymbiont of the related aphid Diuraphis noxia has only one copy of this plasmid. These results, in conjunction with similar results for the trpEG-containing plasmids, suggest that D. noxia has a reduced demand for endosymbiont-derived essential amino acids. Received: 11 September 1997 / Accepted: 23 September 1997     

Chromosome Stability and Gene Loss in Cockroach Endosymbionts

Insect endosymbiont genomes reflect massive gene loss. Two Blattabacterium genomes display colinearity and similar gene contents, despite high orthologous gene divergence, reflecting over 140 million years of independent evolution in separate cockroach lineages. We speculate that distant homologs may replace the functions of some eliminated genes through broadened substrate specificity.Obligate symbionts of insects exhibit extreme patterns of genome evolution and include the smallest known bacterial genomes (10, 11, 14). Two recently published sequences of Blattabacterium, the obligate symbiont of cockroaches (7, 16), present the opportunity to analyze genome evolution in an additional symbiont lineage with extreme genome reduction.     

昆虫内共生菌研究概况

内共生菌与昆虫形成互惠共生关系,在宿主昆虫的生长、生殖、传播植物病害以及探讨生命起源与进化等等方面都有很重要的意义。从昆虫内共生菌的特点、内共生菌在昆虫体内的分布、对昆虫的影响、获得少或缺内共生菌的方法及其研究内共生菌的潜在意义等方面进行了综述。     

Facultative parthenogenesis and sex-ratio evolution

Summary Phenotypic models of selection are used to determine the effect of facultative parthenogenesis on the production of males in a spatially variable environment when (i) sex determination is under strict genetic control, and (ii) when sex may be environmentally determined. The results show that when sex is under strict genetic control and there is some chance of maturing in isolation, selection favors a female-biased sex ratio. When sex can be environmentally induced by cues which indicate high density, selection favors a mixture of genetic and environmental control, such that half the individuals always become female and the other half become females when isolated and become males when not isolated.     

Cospeciation of Psyllids and Their Primary Prokaryotic Endosymbionts

Psyllids are plant sap-feeding insects that harbor prokaryotic endosymbionts in specialized cells within the body cavity. Four-kilobase DNA fragments containing 16S and 23S ribosomal DNA (rDNA) were amplified from the primary (P) endosymbiont of 32 species of psyllids representing three psyllid families and eight subfamilies. In addition, 0.54-kb fragments of the psyllid nuclear gene wingless were also amplified from 26 species. Phylogenetic trees derived from 16S-23S rDNA and from the host wingless gene are very similar, and tests of compatibility of the data sets show no significant conflict between host and endosymbiont phylogenies. This result is consistent with a single infection of a shared psyllid ancestor and subsequent cospeciation of the host and the endosymbiont. In addition, the phylogenies based on DNA sequences generally agreed with psyllid taxonomy based on morphology. The 3′ end of the 16S rDNA of the P endosymbionts differs from that of other members of the domain Bacteria in the lack of a sequence complementary to the mRNA ribosome binding site. The rate of sequence change in the 16S-23S rDNA of the psyllid P endosymbiont was considerably higher than that of other bacteria, including other fast-evolving insect endosymbionts. The lineage consisting of the P endosymbionts of psyllids was given the designation Candidatus Carsonella (gen. nov.) with a single species, Candidatus Carsonella ruddii (sp. nov.).     

Acquisition and Retention of Potato virus Y Helper Component in the Transmission of Potato aucuba mosaic virus by Aphids

Potato aucuba mosaic virus (PAMV) is transmissible by aphids in a non-persistent manner but only in the presence of a Potyvirus helper component (HC) such as that of Potato virus Y (PVY) that must be acquired beforehand or simultaneously. To compare the acquisition and persistence of PAMV and PVY-HC, the PVY-HC was acquired first and then PAMV. The acquisition access and post-acquisition fasting periods were varied. The results show that PAMV and PVY particles, and also PVY-HC could be acquired in 10 s. Some PVY-HC activity could be detected in fasting aphids at least 6 h after acquisition whereas PAMV infectivity was retained for only 2 h. These findings are compatible with the non-persistent transmission concept being controlled by enzymatic release of PAMV particles bound to aphid mouthparts by PVY-HC.     

Ants and Habitat Specificity in Aphids

A number of rare butterflies are known to only utilise their host plant species when they grow in particular situations. Field data are presented showing that two rare species of aphid also only utilised their host plants when they grew in particular situations, namely near ant nests. The oak-feeding aphid Stomaphis quercus only occupied trees within 17 m of a Lasius fulignosus nest. The thyme-feeding aphid Aphis serpylli only occurred in a region of a Breckland grass heath where the sward contained a high density of Lasius niger nests, and then most commonly only in quadrats containing ant nests. The sward that was suitable for A. serpylli was generated by the action of farm traffic. It is concluded that ants indirectly impose habitat specificity on these aphids.