SUBOPTIMAL VIRULENCE OF AN INSECT-PARASITIC NEMATODE

Recent considerations of parasite virulence have focused on the adverse effects that parasites can have on the survival of their hosts. Many parasites, however, reduce host fitness by an equally deleterious but different means, by causing partial or complete sterility of their hosts. A model of optimal parasite virulence is developed in which a quantity of host resources can be allocated to either host or parasite reproduction. Increases in parasite reproduction thus cause reductions in host fertility. The model shows that under a wide variety of ecological conditions, such parasites should completely sterilize their hosts. Only when opportunities for horizontal transmission are very limited should the parasites appropriate less than all of a host's reproductive resources. Field and laboratory evidence shows that the nematode parasite Howardula aoronymphium is relatively avirulent to one of its principal host species, Drosophila falleni, whereas it is much more virulent to D. putrida and D. neotestacea, suggesting that there may be substantial vertical transmission in D. falleni. However, epidemiological studies in the field and laboratory assays of host specificity strongly suggest that the three host species share a single parasite pool in natural populations, indicating that parasites in all three host species experience high levels of horizontal transmission. Thus, the low virulence of H. aoronymphium to D. falleni is not consistent with the model of optimal parasite virulence. It is proposed that this suboptimal virulence in D. falleni is a consequence of populations of H. aoronymphium being selected to exploit simultaneously several different host species. As a result, virulence may not be optimal in any one host. One must, therefore, consider the full range of host species in assessing a parasite's virulence.     

GENERAL-PURPOSE GENOTYPES FOR HOST SPECIES UTILIZATION IN A NEMATODE PARASITE OF DROSOPHILA

The nematode Howardula aoronymphium parasitizes several species of mushroom-feeding Drosophila. A survey of isofemale strains of H. aoronymphium and a 25-generation selection experiment revealed that this species does not comprise host races, and that it harbors little heritable variation for adaptation to specific hosts No tradeoffs in performance on the different host species were evident. General-purpose genotypes, which can utilize all host species, characterize H. aoronymphium. An important feature of the natural history of these nematodes—correlated epidemiology across host species—is postulated to be both a cause and a consequence of the evolution of general-purpose genotypes in this species.     

INFECTIOUS ADAPTATION: POTENTIAL HOST RANGE OF A DEFENSIVE ENDOSYMBIONT IN DROSOPHILA

Maternally transmitted symbionts persist over macroevolutionary timescales by undergoing occasional lateral transfer to new host species. To invade a new species, a symbiont must survive and reproduce in the new host, undergo maternal transmission, and confer a selective benefit sufficient to overcome losses due to imperfect maternal transmission. Drosophila neotestacea is naturally infected with a strain of Spiroplasma that restores fertility to nematode‐parasitized females, which are otherwise sterilized by parasitism. We experimentally transferred Spiroplasma from D. neotestacea to four other species of mycophagous Drosophila that vary in their ability to resist and/or tolerate nematode parasitism. In all four species, Spiroplasma achieved within‐host densities and experienced rates of maternal transmission similar to that in D. neotestacea. Spiroplasma restored fertility to nematode‐parasitized females in one of these novel host species. Based on estimates of maternal transmission fidelity and the expected benefit of Spiroplasma infection in the wild, we conclude that Spiroplasma has the potential to spread and become abundant within Drosophila putrida, which is broadly sympatric with D. neotestacea and in which females are rendered completely sterile by nematode parasitism. Thus, a major adaptation within D. putrida could arise via lateral transmission of a heritable symbiont from D. neotestacea.     

Phylogeny of chromosome races ofFestuca arundinacea andF. mairei (Poaceae) as indicated by seed protein electrophoresis

Seed protein electrophoresis of four chromosomes races ofFestuca arundinacea, F. mairei and their progenitors showed variation in banding patterns. High protein similarities betweenF. arundinacea, F. mairei, F. scariosa, andF. pratensis indicate close phylogenetic relationships of these species. The ancestry ofF. arundinacea cytotypes could be narrowed to three diploid species:F. scariosa, F. pratensis, andF. rubra or to their close relatives.     

Selection of insect parasitic nematodes for biological control of the garden chafer,Phyllopertha horticola

Larvae ofPhyllopertha horticola L. (Coleoptera: Scarabaeidae) cause increasing problems on sports fields and lawns in NW-Europe. A biological control programme using insect parasitic nematodes is being developed. This paper contains the results of bioassays with various species and isolates of the nematode generaHeterorhabditis andSteinernema. In bioassays in small pots with moist sand, most of the nematode isolates gave 30–60% mortality against each of the three larval stages. The susceptibility of the grubs for nematode infection generally increased with larval development.H. bacteriophora, H. heliothidis, H. megidis, a DutchHeterorhabditis isolate NLH-E87.3 andS. glaseri 326 showed the highest mortality rates, with nearly 100% mortality of third instar grubs. The DutchHeterorhabditis isolate NLH-E87.3 andS. glaseri 326 were selected as candidates for further studies on their potential as biological control agents forP. horticola grubs in the field.     

Biology of theParasponia-Bradyrhizobium symbiosis

Parasponia remains the only non-legume known to nodulate withRhizobium/Bradyrhizobium. It is a pioneer plant that is capable of rapid growth and fixing large quantities of nitrogen. In addition to its high agronomic potential, the symbiosis offers the scientist the unique opportunity of studying differences at the molecular level of both partners, and to investigate any possible extension of the symbiosis to other non-legumes of importance. Haemoglobin has been found in the nodule tissue ofParasponia and other nodulated non-legumes and the gene for it has been found and expressed in non-nodulating plants such asTrema tomentosa andCeltis australis. Bradyrhizobium strains isolated from species ofParasponia growing in Papua New Guinea form a group that are more specific in their host requirements thanBradyrhizobium strains from tropical legumes from the same area. They do not effectively nodulate (except CP283) tropical legumes, andParasponia is not readily nodulated withRhizobium andBradyrhizobium strains from legumes. The effectiveness of the symbiosis is influenced by host species, theBradyrhizobium strain and the environment.Parasponia andersonii forms a more effective symbiosis than the other species tested. In competition studies with strains from legumes, isolates fromParasponia always dominate in nodules onParasponia.     

Species richness and seasonal abundance of ectomycorrhizal fungi in plantations of<Emphasis Type="Italic"> Eucalyptus dunnii</Emphasis> and<Emphasis Type="Italic"> Pinus taeda</Emphasis> in southern Brazil

The abundance and diversity of ectomycorrhizal fungi (EMF) was assessed based on the collection of basidiocarps during 12 months comprising the spring of 1995, and the summer, autumn, and winter of 1996, in three stands of young, middle-aged, and rotation age plantations of Pinus taeda and Eucalyptus dunnii, in the state of Santa Catarina, southern Brazil. A total of 3,085 collections yielded 34 presumed EMF taxa in ten genera, including mushroom-like and sequestrate species. Fruiting patterns of EMF differed with host and season, and host specificity was apparent in some. The overall relative importance (RI) and the Shannon diversity index (H) suggested that stands of E. dunnii had a more diverse aboveground EMF community than those of P. taeda. Overall, species of Scleroderma and Laccaria were not only the most abundant but also had the highest biomass values. The results show that a small number of species of abundant biomass and a larger number of species of less-abundant biomass characterize each forest class.     

Reproductive Isolation and Genetic Variation between Two “Strains” ofBracon hebetor(Hymenoptera: Braconidae)

Bracon hebetorSay(Hymenoptera: Braconidae) is known primarily as a parasitoid of pyralid moth larvae infesting stored grain. In the 1970s, a parasitoid identified asB. hebetorwas released for control ofHeliothis/Helicoverpaspp. (Lepidoptera: Noctuidae) on the island of Barbados. Because life-history traits of this parasitoid differed from those reported forB. hebetorfrom the United States, we conducted a series of laboratory experiments to determine whether this parasitoid was (i) a population ofB. hebetorthat attacks noctuids in the field or (ii) a different species fromB. hebetor.We confirmed thatHeliothis virescens(F.) was a more suitable host for the Barbados strain than forB. hebetor.However, a stored-grain infesting pyralid,Plodia interpunctella(Hübner), was a more suitable host for the Barbados strain than wasH. virescens.Reciprocal crosses between the Barbados strain andB. hebetorshowed that the two populations were reproductively isolated. No mating was observed during a series of 30-min observations of reciprocal crosses, and the crosses produced only male offspring. Examination of each female's spermatheca confirmed that females were not fertilized. Sequence analysis of a 517-bp fragment of the mitochondrial 16S rRNA gene revealed that two populations ofB. hebetorfrom our laboratory were identical but differed in sequence by 2% from the Barbados strain. Collectively, our results indicate that the Barbados strain is a distinct species fromB. hebetor.     

Neue Beobachtungen zur Bestäubung vonOphrys (Orchidaceae) in Südspanien,mit besonderer Berücksichtigung des FormenkreisesOphrys fusca agg.

An investigation of pseudocopulation behaviour in species ofOphrys from southern Spain confirms the close relationship betweenCampsoscolia ciliata (Scoliidae) andOphrys speculum, and betweenEucera nigrilabris (Apoidea) andOphrys tenthredinifera. It could be demonstrated thatEucera barbiventris is the pollinator ofOphrys scolopax subsp.scolopax, whereas other species ofEucera andTetralonia which are active at the same time show no interest at all for the flowers of this species. Special attention was paid to the forms of the taxonomically confusedOphrys fusca group:O. fusca s. str.,O. iricolor, O. omegaifera andO. atlantica. WhileO. fusca s. str. is widespread, small-flowered and has late anthesis,O. iricolor has very large flowers and early anthesis. Each of the four members ofO. fusca agg. in S. Spain is pollinated by a different bee, and selective experiments show that each of these four species of bees is specifically attracted only to one of theOphrys species:Andrena flavipes is the pollinator ofO. fusca s. str.,Colletes cunicularius infuscatus ofO. iricolor, Anthophora atroalba ofO. omegaifera, andChalicodoma parietina ofO. atlantica. These four pollinators belong to 4 different bee families (Andrenidae, Colletidae, Anthophoridae, Megachilidae). As this type of pollination represents a very effective pregamic isolation mechanism, these four taxa ofOphrys fusca agg., at least in southern Spain, behave and should be regarded as genuine species.

     

Two new species ofDelitschia from submerged wood

Two new species ofDelitschia are described from submerged wood;D. palmietensis collected in the Palmiet River in Durban Westville, South Africa andD. fasciatispora from the Black River, Mauritius. This brings the total ofDelitschia species known from non-coprophilous habitats to four.Delitschia palmietensis andD. fasciatispora are described and illustrated with light micrographs and discussed in relation to other species in the genus. A key to the non-coprophilousDelitschia species is provided.     

Phage specificity and lipopolysaccarides of stem- and root-nodulating bacteria (Azorhizobium caulinodans, Sinorhizobium spp., and Rhizobium spp.) of Sesbania spp

Phage susceptibility pattern and its correlation with lipopolysaccharide (LPS) and plasmid profiles may help in understanding the phenotypic and genotypic diversity among highly promiscuous group of rhizobia nodulating Sesbania spp.; 43 phages were from two stem-nodulating bacteria of S. rostrata and 16 phages were from root-nodulating bacteria of S. sesban, S. aegyptica and S. rostrata. Phage susceptibility pattern of 38 Sesbania nodulating bacteria was correlated with their LPS rather than plasmid profiles. Different species of bacteria (A. caulinodans- ORS571, SRS1-3 and Sinorhizobium saheli- SRR907, SRR912) showing distinct LPS subtypes were susceptible to different group of phages. Phages could also discriminate the strains of Si. saheli (SSR312, SAR610) possessing distinct LPS subtypes. Phages of Si. meliloti (SSR302) were strain-specific. All the strains of R. huautlense having incomplete LPS (insignificant O-chain) were phage-resistant. In in vitro assay, 100% of the phages were adsorbed to LPS of indicator bacterium or its closely related strain(s) only. These observations suggest the significance of LPS in phage specificity of Sesbania nodulating rhizobia. Highly specific phages may serve as biological marker for monitoring the susceptible bacterial strains in culture collections and environment.     

Relationships between species ofLeymus,Psathyrostachys, andHordeum (Poaceae,Triticeae) inferred from Southern hybridization of genomic and cloned DNA probes

We have used total genomic DNA as a probe to size-fractionated restriction enzyme digests of genomic DNA from a range ofTriticeae species from the generaLeymus Hochst.,Psathyrostachys Nevski, andHordeum L., and hybrids betweenHordeum andLeymus to investigate their taxonomic relationships. Genomic Southern hybridization was found to be an effective and simple way to assess the distribution and diversity of essentially species-specific and common, repetitive DNA sequences, and is hence especially useful in evolutionary studies. The DNA sequences ofH. vulgare seem to diverge substantially from those ofH. brachyantherum, H. lechleri, H. procerum, andH. depressum. The genome ofThinopyron bessarabicum shows little homology to those of theLeymus species investigated, confirming thatT. bessarabicum is not an ancestral genome inLeymus. Although the genomes ofLeymus andPsathyrostachys share substantial proportions of DNA sequences, they include divergent repeated sequences as well. Hybridization with a ribosomal DNA probe (pTa 71) showed that the coding regions containing structural genes encoding the 18 S, 5.8 S, and 26 S ribosomal RNA were conserved among the species investigated, whereas the intergenic spacer region was more variable, presenting different sizes of restriction fragments and enabling a classification of the species. The rye heterochromatin probe pSc 119.2 hybridized to DNA fromH. lechleri andT. bessarabicum, but not to DNA from the other species investigated.     

Phylogenetic analysis of the generaCladonia andCladina (Cladoniaceae, lichenizedAscomycota)

A cladistic analysis of 44 species of the generaCladina andCladonia is presented.Pycnothelia papillaria, Cladia aggregata andC. retipora were used as outgroup taxa. The consensus of all equally parsimonious trees suggests a common ancestral origin for species inCladonia andCladina while the generaPycnothelia andCladia cluster outside this group. The results do not support distinction ofCladina at genus level although it is distinguished as a monophyletic group but within the genusCladonia. The current sectional division ofCladonia is not supported. SectionsCocciferae andHelopodium are best supported but even these groups as currently delimited seem to in include elements that are more closely related to species of the other sections, e.g.C. carneola of sect.Cocciferae andC. pityrophylla of sect.Helopodium. Sections such asUnciales andPerviae seem to be artificial assemblages. In general the evolutionary scheme of the genus seems to be more complicated than revealed by the current sectional division.     

Laboratory culture and taxonomy of two species ofDerbesia (Class Chlorophyceae) in Japan

Two species ofDerbesia (Class Chlorophyceae),D. marina andD. tenuissima, have been studied for the purpose of obtaining a better understanding of their morphological details and life histories, using preserved and living specimens as well as laboratory cultures. The life histories of both species were completed in the laboratory, starting from both zoospores and zygotes. Specimens were collected at Asamushi, Aomori-ken, and Shimoda, Shizuoka-ken. Their life history types are fundamentally identical, zoospores giving rise upon germination toa Halicystis-phase, while zygotes grow into aDerbesia-phase. The thallus of theHalicystis-phase which alternates withD. marina is the same as that ofH. ovalis which grows in the northern regions of Japan. On the other hand, the thallus of theHalicystis-phase alternating withD. tenuissima is the same as that ofH. parvula known to occur in the temperate to subtropical regions of Japan. These results coincide with those obtained withD. marina andD. tenuissima in Europe, where the type localities of both species are located. Specimens assignable to these two species were collected at several localities in Japan and, as a result of detailed examination of the morphology, they are believed to be identical with eitherD. marina orD. tenuissima.     

Nematodes (Longidorus sp. andTylenchorhynchus microphasmis loof) in growth and nodulation of sea buckthorn (Hippophaë rhamnoides L.)

Summary Hippophaë rhamnoides seedlings were grown in sterilized and unsterilized soil from a decliningH. rhamnoides scrub, to which different numbers ofLongidorus sp. andTylenchorhynchus microphasmis were added. In sterilized and unsterilized soil, retardation of growth, content of dry matter in the shoots, and incidence of deformed short lateral roots of test plants were positively correlated with counts of both nematode species. Nitrogen content in the shoots, nodulation on the roots of test plants and increase increase in nematodes were negatively correlated with the initial number of both nematode species in sterilized soil. In unsterilized soil, an unknown biotic factor was present that reduces growth ofH. rhamnoides, nodulation and multiplication of the nematodes. This factor seems to interact with the nematodes in reducing growth ofH. rhamnoides.Deceased.     

Epibiota communities of the introduced and indigenous macroalgal relatives<Emphasis Type="Italic"> Sargassum muticum</Emphasis> and<Emphasis Type="Italic"> Halidrys siliquosa</Emphasis> in Limfjorden (Denmark)

Sargassum muticum (Phaeophyceae, Fucales) has recently been introduced to Limfjorden (Denmark) where its closest relative is the indigenous Halidrys siliquosa. Previous studies have demonstrated large quantitative (canopy biomass) and qualitative (canopy persistence) differences in the habitat available to epibiota within the canopies of these two macroalgae. We therefore hypothesised that these algae would support different epibiota communities and tested this by sampling the epibiota of S. muticum and H. siliquosa on seven occasions throughout 1997 by enclosing entire thalli in mesh bags. We found 53 epibiota taxa and, with only one exception, they were all recorded on both host species. Species richness and abundance of epibiota exhibited clear seasonal variation on both host species, although epibiota biomass was seasonally constant on H. siliquosa but not on S. muticum. These patterns were consistent with the different life histories of the host species. There was a weakly negative correlation between thallus size and epibiota biomass for both host species. When taking species-specific seasonal variation in thallus size into consideration, S. muticum and H. siliquosa were found to support significantly different epibiota biomasses. Multivariate analyses showed that epibiota community structure was different, although highly overlapping, between the two species, whereas there was an almost parallel temporal development in epibiota community structure. We conclude that it is unlikely that the introduction of S. muticum to Limfjorden has caused major changes in local epibiota community structure. However, the standing stock of epibiota is likely to have increased.Communicated by H.-D. Franke     

Puccinia kraussiana (Uredinales) onHeterosmilax gaudichaudiana,A new record of rust in Eastern Asia

Puccinia kraussiana, a rust fungus of Africa, was found in Taiwan for the first time in eastern Asia. It was parasiting on leaves ofHeterosmilax gaudichaudiana, on which all four stages were characteristically found. The host species,H. gaudichaudiana, newly added to the host range ofP. kraussiana, constitutes also a new entry to the phanerogamic flora of Taiwan.     

Host habitat finding and host selection of theDrosophila parasitoidLeptopilina australis (Hymenoptera,Eucoilidae), with a comparison of the niches of EuropeanLeptopilina species

Summary Decaying petioles of giant hogweed,Heracleum mantegazzianum Sommier & Levier, are used as a breeding site by six species ofDrosophila and the drosophilidScaptomyza pallida. The most numerous parasitoid species associated with this community isLeptopilina australis. BecauseL. australis was previously unknown in western Europe, we present the characters to distinguish it form its close relativeL. clavipes. Experiments on host species selection and survival ofL. australis showed that this parasitoid mainly usesD. limbata as host. Olfactometer experiments showed thatL. australis is attracted by the odour of decaying hogweed stalks, especially when these contain larvae ofD. limbata. L. australis is also strongly attracted by the odour of stinkhorns, a habitat in which it has never been found in nature.D. phalerata is the dominant fly species in stinkhorns, and is not a host ofL. australis. We offer a possible functional explanation for this unexpected habitat choice, by showing thatD. transversa andD. kuntzei, both species found to breed in fungi, are also suitable hosts forL. australis. We also discuss habitat choice with regard to a proposed phylogeny of theLeptopilina species in temperate Europe. Finally, we discuss niche overlap ofL. australis with the otherLeptopilina species.     

Phylogeny of the GenusTrichodermaBased on Sequence Analysis of the Internal Transcribed Spacer Region 1 of the rDNA Cluster

Sequences of the internal transcribed spacer region 1 (ITS1) of the ribosomal DNA were used to determine the phylogenetic relationships of species ofTrichodermasect.Pachybasium.To this end, 85 strains—including all the availableex-type strains—were analyzed. Parsimony analysis demonstrated that the section is nonmonophyletic, distributing the 85 strains among three main groups that were supported by bootstrap values. Group A comprises two clades (A1 and A2), with A1 includingT. polysporum, T. piluliferum,andT. minutisporum,while A2 includedT. hamatum, T. pubescens,andT. strigosumin addition to species previously included in sect.Trichoderma(i.e.,T. viride, T. atroviride,andT. koningii). Theex-type strain ofT. fasciculatumformed a separate branch basal to clade A. Clade B contained the sect.PachybasiummembersT. harzianum, T. fertile, T. croceum, T. longipile, T. strictipile, T. tomentosum, T. oblongisporum, T. flavofuscum, T. spirale,and the anamorphs ofHypocrea semiorbisandH. cf. gelatinosa.Sequence differences among clades A1, A2, and B were in the same order of magnitude as between each of them andT. longibrachiatum,which was used as an outgroup in these analyses. Sequence differences within clades A1, A2, and B were considerably smaller: in some cases (i.e.,T. virensandT. flavofuscum; T. strictipileandH. cf. gelatinosa), the ITS1-sequences were identical, suggesting conspecifity. In other cases (e.g.,T. crassumandT. longipile; T. harzianum, T. inhamatum, T. croceum, T. fertile,andH. semiorbis; T. hamatumandT. pubescens;andT. viride, T. atroviride,andT. koningii) differences were in the range of 1–3 nt only, suggesting a very close phylogenetic relationship. The sequence of a previously described aggressive mushroom competitor group ofT. harzianumstrains (Th2) was strikingly different from that of theex-type strain ofT. harzianumand closely related species and is likely to be a separate species.