Population structure of a vector‐borne plant parasite

Parasites are among the most diverse groups of life on Earth, yet complex natural histories often preclude studies of their speciation processes. The biology of parasitic plants facilitates in situ collection of data on both genetic structure and the mechanisms responsible for that structure. Here, we studied the role of mating, dispersal and establishment in host race formation of a parasitic plant. We investigated the population genetics of a vector‐borne desert mistletoe (Phoradendron californicum) across two legume host tree species (Senegalia greggii and Prosopis velutina) in the Sonoran desert using microsatellites. Consistent with host race formation, we found strong host‐associated genetic structure in sympatry, little genetic variation due to geographic site and weak isolation by distance. We hypothesize that genetic differentiation results from differences in the timing of mistletoe flowering by host species, as we found initial flowering date of individual mistletoes correlated with genetic ancestry. Hybrids with intermediate ancestry were detected genetically. Individuals likely resulting from recent, successful establishment events following dispersal between the host species were detected at frequencies similar to hybrids between host races. Therefore, barriers to gene flow between the host races may have been stronger at mating than at dispersal. We also found higher inbreeding and within‐host individual relatedness values for mistletoes on the more rare and isolated host species (S. greggii). Our study spanned spatial scales to address how interactions with both vectors and hosts influence parasitic plant structure with implications for parasite virulence evolution and speciation.     

Natural and experimental hybridization inArmeria (Plumbaginaceae):A. villosa subsp.carratracensis

An experimental crossing program was carried out in order to test the hypothesis thatArmeria villosa subsp.carratracensis, a serpentine endemic from Southern Spain, is of hybrid origin. Fruit-set and mainly pollen stainability data in artificial hybrids demonstrate that internal interspecific barriers are weak. Two generations of backcrossing with the two putative parents —A. colorata andA. villosa subsp.longiaristata— can restore pollen stainability to an average of 83.2 and 68.7, respectively. A morphometric analysis of 148 specimens belonging to parental species, different artificial hybrid generations and wild putative hybrid race shows that (1) morphological characters have a strong genetic basis and thus are good markers in identifying hybrids and introgression inArmeria; (2) artificial backcrossing produces in morphometrical characters a variation which conforms in both trend and magnitude to what is commonly attributed in the wild to introgression; (3) the position of the putative hybrid race is intermediate between the two parental species used in the program. These results are in accordance with the hypothesis of hybrid origin ofA. villosa subsp.carratracensis. Natural and experimental hybridization inArmeria, I     

Role of symbiotic and non-symbiotic bacteria in carbon dioxide production from hosts infected with Steinernema riobrave

Entomopathogenic nematodes of the family Steinernematidae and their mutualistic bacteria (Xenorhabdus spp.) are lethal endoparasites of insects. We hypothesized that growth of the nematode’s mutualistic bacteria in the insect host may contribute to the production of cues used by the infective juveniles (IJs) in responding to potential hosts for infection. Specifically, we tested if patterns of bacterial growth could explain differences in CO₂ production over the course of host infection. Growth of Xenorhabdus cabanillasii isolated from Steinernema riobrave exhibited the characteristic exponential and stationary growth phases. Other non-nematode symbiotic bacteria were also found in infected hosts and exhibited similar growth patterns to X. cabanillasii. Galleria mellonella larvae infected with S. riobrave produced two distinct peaks of CO₂ occurring at 25.6–36 h and 105–161 h post-infection, whereas larvae injected with X. cabanillasii alone showed only one peak of CO₂, occurring at 22.8–36.2 h post-injection. Tenebrio molitor larvae infected with S. riobrave or injected with bacteria alone exhibited only one peak of CO₂ production, which occurred later during S. riobrave infection (41.4–64.4 h post-infection compared to 20.4–35.9 h post-injection). These results indicate a relationship between bacterial growth and the first peak of CO₂ in both host species, but not for the second peak exhibited in G. mellonella.     

Changes in morphological traits of the cabbage aphid (<Emphasis Type="Italic">Brevicoryne brassicae</Emphasis>) associated with the use of different host plants

The genetic and morphological differentiation of insect populations in relation to the use of different host plants is an important phenomenon that predates ecological specialisation and speciation in sympatric conditions. In this study, we describe the morphological variation of populations of Brevicoryne brassicae (Homoptera: Aphididae) associated with two host species, Brassica oleracea and Brassica campestris, which occur sympatrically in the highlands of Chiapas, Mexico. The study is aimed at obtaining evidence regarding phenotypic differentiation induced by, or associated with, the use of distinct but closely related host species. Seven morphological characters were measured in 696 wingless aphids collected from plants of the two host species at four localities. Morphological variation was summarised through principal components analysis (PCA). Sixty-two percent of morphological variation was explained by the first two PCs. The first component (PC1) was related to the general size of appendages, and PC2 was interpreted as the relationship between body size (body and leg size) and antenna length. Aphids growing on B. campestris were bigger than those collected from B. oleraceae. Significant differences between hosts were detected for PC1, whereas a significant effect of locality, host, and the interaction locality × host was detected for PC2. These results indicate that the average phenotype of B. brassicae individuals inhabiting different host-plant species differs as a consequence of the contrasting feeding environments the host species provide.     

Molecular and morphological data suggest that Spinibarbus caldwelli (Nichols) (Teleostei: Cyprinidae) is a valid species

The taxonomic problem of the cyprinid species of genus Spinibarbus, occurring in southern China and northern Vietnam, was resolved on the basis of molecular and morphological analyses. Spinibarbus caldwelli and Spinibarbus hollandi have a smooth posterior edge of the last unbranched dorsal fin ray among species in the genus. Spinibarbus caldwelli is currently regarded as a junior synonym of S. hollandi because of ambiguities in diagnostic characters. In this article, 11 mtDNA cytochrome b sequences of Spinibarbus specimens were analyzed together with Barbodes gonionotus and Puntius conchonius as outgroups. Our results showed that specimens identified as S. hollandi from Taiwan were different from those from the Asian mainland at a high level of genetic divergence (0.097–0.112), which is higher than that between the two valid species, S. sinensis and S. yunnanensis (0.089), and suggested that Taiwan specimens should be considered as a different species from the Asian mainland one. In a molecular phylogenetic analysis, the sister-group relationship between Taiwan specimens and the Asian mainland specimens was supported strongly by a high confidence level (100% in bootstrap value). Further analysis of morphological characters showed that overlap of diagnostic characters is much weaker than previously suggested. Taiwan specimens had 8 branched rays in the dorsal fin, whereas those from the mainland had almost 9–10. The molecular and morphological differences suggest S. caldwelli to be valid. The molecular divergence shows the genetic speciation of S. hollandi and S. caldwelli might have occurred 5.6–4.9 million years ago; the former could be a relict species in Taiwan, and the latter dispersed in the Asian mainland.     

The effect of isozyme selection on metric characters in an interspecific backcross of tomato — basis of an early screening procedure

Summary The extent of correlation was estimated between isozyme genotypes and the four widely segregating characters — leaf segment W/L ratio, stigma exsertion, fruit weight, and seed weight — in the first backcross of F₁ Lycopersicon esculentum x Solanum pennellii to the former parent. The inbred parents differ in their alleles at the 12 tested isozymic loci, which are known to mark a minimum of eight of the twelve tomato chromosomes. Based on the isozyme data, a mean heterozygosity value, ¯H, was calculated which estimates the proportion of pennillii alleles in each individual. Correlations between mean heterozygosity and observed levels of each quantitative trait were highly significant and positive or negative as expected from the relative parental values. Plants with the lowest mean heterozygosity — i.e., closest to the esculentum zymotype also had mean values closest to those of this parent amongst the whole backcross population for each of the quantitative traits.Bivariate and multiple regression analysis was used to evaluate the ability of isozymes vs diagnostic morphological characters to estimate the portion of recurrent parent genes carried in each backcross individual. The results suggest that isozyme data gives better estimates than single diagnostic morphological characters and approach the level obtained by combinations of three morphological traits. Since electrophoretic determinations are made on small seedlings, selection at that stage can effect great savings of space and effort by greatly deminishing the size of the population needed at maturity. As such, isozyme selection would precede morphological selection but not replace it, thus the predictive value of these biochemical markers as well as diagnostic morphological characters could be obtained.     

The effect of nitrogen supply on growth and water-use efficiency of xylem-tapping mistletoes

Xylem-tapping mistletoes are known to have normally a higher rate of transpiration and lower water-use efficiency than their hosts. The relationships between water relations, nutrients and growth were investigated for Phoradendron juniperinum growing on Juniperus osteosperma (a non-nitrogen-fixing tree) and for Phoradendron californicum growing on Acacia greggii (a nitrogen-fixing tree). Xylem sap nitrogen contents were approximately 3.5 times higher in the nitrogen-fixing host than in the non-nitrogen-fixing host. The results of the present study show that mistletoe growth rates were sevenfold greater on a nitrogen-fixing host. At the same time, however, the differences in water-use efficiency between mistletoes and their hosts, which were observed on the non-nitrogen-fixing host did not exist when mistletoes were grown on hosts with higher nitrogen contents in their xylem sap. Growth rates and the accumulation of N, P, K, and Ca as well as values for carbon-isotope ratios of mistletoe tissues support the hypothesis that the higher transpiration rates of mistletoes represent a nitrogen-gathering mechanism.Abbreviation ¹³C carbon-isotope ratio Dedicated to Professor Dr. Hubert Ziegler on the occasion of his 60th birthday     

Nutritional performance of first and penultimate-final Instar bagworm larvae on two unrelated hosts as influenced by host of origin

Nutritional performance of early and late instar bagworm (Thyridopteryx ephemeraeformis Haworth) larvae was investigated as a function of host of origin and host type, using two taxonomically distant wild hosts, Salix nigra (Marshall) and Juniperus virginiana (L.). First instar growth and survival were higher on S. nigra; overall performance was similar on the two hosts in penultimate-final instars. There was no evidence for host associated differentiation with respect to nutritional performance. There were, however, significant genotype x environment interactions for several characters studied, suggesting that the potential for host associated differentiation exists.     

Host differentiation in Orobanche foetida Poir

Orobanche foetida Poir. is a parasitic plant widely distributed in the Western Mediterranean area. It typically parasitizes wild plants but has recently been described as an agricultural problem in legume crops in Tunisia. The pattern of genetic variation within and among O. foetida populations growing on chickpea and faba bean was analyzed by RAPD markers. The UPGMA cluster analysis based on Dice distance matrix showed a clear differentiation among O. foetida samples collected on chickpea and those on faba bean, suggesting a host-differentiation process. Although an AMOVA analysis revealed substantial internal variation among individuals within O. foetida populations (69.8%), there was a significant divergence between parasites on the two hosts considered (30.2%). Moreover, germination of O. foetida seeds collected on chickpea and faba bean in the presence of both host roots was studied. Germination percentages of O. foetida seeds varied depending on the host used both for collecting the seeds and evaluating the trait. According to these results, possible explanations for the origin of this new weedy parasite and the host differentiation process are discussed.     

Electrophoretic variation in esterases and peroxidases of native greek diploidAegilops species(Ae. caudata andAe. comosa,Poaceae)

The esterase and peroxidase patterns in five varieties ofAegilops caudata (genome type C) andAe. comosa (genome type M) were studied in order to elucidate the phylogenetic relationships within and between the two groups. The electrostarch gel electrophoresis technique was applied to extracts of shoot and root of 4-day-old seedlings, and the electropherograms were evaluated by gel densitometer traces. Inspite of considerable isozyme polymorphism, closer relationships in the banding patterns were found between different varieties of a single species than between varieties of the two different species. Esterase and peroxidase patterns of the twoAe. caudata varieties (caudata andpolyathera) are very similar and prove their close phylogenetic relationship. The isozyme affinities withinAe. comosa varieties are illustrated by the seriessubventricosa—biaristata—thessalica. The latter endemic variety has quite a number of characteristic bands and is relatively isolated. Altogether, the electrophoretic data agree well with morphological and cytological similarities (Zhukovsky 1928,Eig 1929,Karataglis 1973, 1975b).     

Drei parasitische Flechten aufCaloplaca polycarpoides

No less than 3 different parasitical lichens were found on the corticolous lichenCaloplaca polycarpoides in Afghanistan. Partly, they also grow on closely allied species of the sect.Xanthoriella. Lecania triseptata begins and ends as a typical parasite, and does not form a thallus outside of its host.L. diplococca andRinodina afghanica, both described as new species, have a similar appearance; they are parasitic in the beginning, but form their own thalli outside the host later on. The 3 species are members of the small group of parasitical lichens growing on corticolous hosts. Possibly, the slow degeneration of cortices under the arid climatical conditions, has allowed the evolution of parasitic lichens of this rare type in Central Asia.

Dedicated to Hofrat Univ.-Prof. DrK. H. Rechinger on the occasion of his 80th birthday. Flechten aus Afghanistan III; Beitrag II in Nova Hedwigia42, 213–236 (1986).     

Hunting for moving hosts: Cephalonomia tarsalis, a parasitoid of free-living grain beetles

The parasitic wasp Cephalonomia tarsalis parasitizes larvae of the saw-toothed grain beetle Oryzaephilus surinamensis, which feed on wheat grains. In contrast to most other host–parasitoid systems studied so far, the grain beetles are highly mobile within their habitat, bulk of grains in grain stores. This should increase the wasps’ problem to locate the hosts. To study the host-finding strategy of C. tarsalis females, the reaction of wasps to different grain and host-derived odour sources was tested in a four-chamber-olfactometer. These experiments revealed that wasps were attracted by healthy grains and mechanically damaged grains. In direct comparison, healthy and mechanically damaged grains are equally attractive. Both potential sources of host-derived odours, host faeces and trail-traces of larvae on filter paper were attractive to the wasps. The response to trail-traces vanished 30 min after larvae had been removed from the filter paper. With respect to the specificity of the odours, it turned out that wasps were attracted to odours from the seed–host complexes from O. surinamensis and Oryzaephilus mercator and the non-host complex of larvae of the granary weevil Sitophilus granarius in wheat grains. Odours from the seed–host complex were preferred. From these results, we hypothesize that host habitat location in C. tarsalis is achieved by using grain-derived odours. Within the habitat, wasps search for kairomones from host faeces and host trails. Following these larval trails finally leads wasps to their hosts.     

檀香与不同豆科植物寄生关系的研究

为了解檀香(Santalum album L.)与不同豆科植物的寄生关系,选取11种具有重要经济价值的豆科植物和檀香一起种植,对檀香叶片的光合特征和株高、地径以及寄主植物根系的吸器大小进行了测定。结果表明,檀香的吸器大小、数量在不同寄主之间存在很大差异,不同寄主对檀香的株高、地径也有显著差异。檀香的优良寄主有美洲合欢(Calliandra haematocephala)、苏木(Caesalpinia sappan)、台湾相思(Acacia confusa)、龙牙花(Erythrina corallodendron);一般的寄主植物有马占相思(Acacia mangium)、降香黄檀(Dalbergia odorifera)、黄槐(Cassia surattensis);不适宜的寄主植物有凤凰木(Delonix regia)、海南红豆(Ormosia pinnata)、银合欢(Leucaena leucocephala)、洋紫荆(Bauhinia blakeana)。同时,檀香的株高、地径及光合特征和吸器数量密切相关。因此,檀香种植时应选择适宜的优良寄主植物。     

Phylogeography of island canary (Serinus canaria) populations

Island canaries (Serinus canaria) are characterised as a species living exclusively on North Atlantic islands, mainly on the Azores, Madeira and Canary Islands. Although they are very common in their habitats, their behaviour and breeding system has only recently been studied systematically. To advance the understanding of their ecology and to see if the rather isolated archipelagos are already promoting a genetic differentiation, we investigated their phylogeographic relationship as revealed by mtDNA sequences of the cytochrome b gene and investigated whether this measure corresponds to morphological characteristics within the islands. Genetic distances were very low throughout the distribution range of the species. Although the variation of genetic distances within the population of Pico (Azores) was larger than that on Madeira and Canary Islands, the genetic distances between island populations were very low throughout which prevented a clear phylogeographic differentiation. Moreover, morphological measurements did not reveal a consistent pattern to reliably separate the populations, although the measures of beak length and body weight revealed a clear island-specific differentiation. These data lead to the assumption that the colonisation of the Atlantic islands by the canaries occurred very recently, while there is no persisting gene flow between the populations.     

Population structure,gene diversity,and differentiation in natural populations of Cedar pines (Pinus subsect.Cembrae,Pinaceae) in the USSR

Twelve natural populations of four cedar pine species,Pinus sibirica, P. cembra, P. pumila, andP. koraiensis, occurring in the Soviet Union were investigated by starch-gel electrophoresis. Frequencies of 55 alleles at 19 loci were determined. Interpopulation genetic diversity inP. sibirica andP. pumila was only 2–4 per cent of the total genetic diversity. Nei's distance coefficient (Dn) was used to estimate the level of genetic differentiation among conspecific populations and among species. Dn values among populations ranged from 0.006 to 0.038. A dendrogram constructed using Dn values divided cedar pines species into 2 clusters:sibirica-cembra (Dn = 0.030) andpumila-koraiensis (Dn = 0.143). Nei's distance between these clusters was 0.232. On the basis of the data obtained it was possible to draw the following conclusion:P. sibirica, P. pumila, andP. koraiensis are distinct species, whileP. cembra should apparently be regarded as geographicalP. sibirica race.     

The pollen morphology ofOdontites (Scrophulariaceae) and its taxonomic significance

The pollen morphology of 29 species ofOdontites and related genera was examined by light and scanning electron microscopy. Three major pollen types differing fundamentally in exine sculpturing were found. In the parasitic tribePedicularieae retipilate sculpturing is the most widespread type representing a plesiomorphic character state from which the two other types are derived. Of these, reticulate sculpturing is confined toOdontites, whereas a complex retirugulate pattern was found only in the monotypic Near East genusBornmuellerantha. Within the retipilate and the reticulate major types eight minor types were distinguished based on the differential correlation of exine surface morphology, size, shape and amb form. The pollen data are generally well correlated with macromorphological features and furnish important taxonomic characters at the genus—as well as at the specieslevel.     

Phylogeny of the ectomycorrhizal mushroom genus Alnicola (Basidiomycota, Cortinariaceae) based on rDNA sequences with special emphasis on host specificity and morphological characters

Alnicola (=Naucoria, pro parte) is a mushroom genus of strictly temperate, obligately ectomycorrhizal species, traditionally included in the family Cortinariaceae. Most Alnicola spp. are primarily host specific on Alnus, although a few are mycobionts of Salix or other hosts. The different species of Alnicola exhibit unique morphological (cystidia, pileipellis) and cytological (dikaryotic or monokaryotic hyphae) characters. This makes the genus Alnicola of particular interest for studying the evolution of host specificity and morphological characters in ectomycorrhizal basidiomycetes. We used a combination of classical morphological and phylogenetic methods (rDNA ITS and LSU sequences) to address the following questions: (i) Is Alnicola monophyletic? And (ii) Are characters like host specificity or microscopical structures synapomorphic for certain clades? The study included nearly all currently known European Alnicola sp. Our results demonstrated that, on one hand, the genus Alnicola is polyphyletic, with sistergroup relationships to Hebeloma, Anamika or the clades /Hymenogaster I and /Hymenogaster II. On the other hand, Alnicola splits into three well-supported clades corresponding to the sections Alnicola, Submelinoides, and Salicicolae. The strict host-specificity to Alnus is a derived character and has occurred at least twice. The following morphological characters are synapomorphic for defined clades: the spindle-shaped hymenial cystidia for sect. Alnicola, the hymeniform pileipellis for sect. Submelinoides, and monocaryotic/clampless hyphae for sect. Salicicolae and its sistergroup /Hymenogaster II. As a taxonomical consequence, polyphyly of Alnicola implies that the sects. Submelinoides and Salicicolae need to be segregated from Alnicola.     

Regulation of vole populations by the nematode <Emphasis Type="Italic">Trichuris arvicolae</Emphasis>: insights from modelling

The goal of this study is to determine whether a parasitic nematode may regulate, or destabilise by inducing demographic cycles, its host populations. We explore three host–parasite systems through population dynamic models. The hosts considered are the fossorial water vole, Arvicola terrestris, the common vole Microtus arvalis and the bank vole Myodes (Clethrionomys) glareolus and the parasitic nematode is Trichuris arvicolae. Three differential equation-based mathematical models are developed including host immunity and the existence of trade-off between immunity and host survival. Using parameters estimated from field data and laboratory observations, all these models show that T. arvicolae can induce host population regulation but not demographic cycles. The regulation effect of the nematode is un-ambiguous for the water vole (reduction of 50.2% of the host population size), but less obvious for the common vole (5.9%) and even less for the bank vole (1.4%). Important biological parameters to be taken into account in such models are discussed. Experimental confirmation of the regulatory potential of the nematode and of the costs of mounting an immune response against this nematode are now required. Communicated by W. Lutz     

Differences in allozyme patterns between Diaphanosoma brachyurum and Diaphanosoma mongolianum,as revealed in Central European populations

Recent taxonomic revisions in the genus Diaphanosoma have made the uncertainty in species discrimination with morphological characters obvious. Therefore species characterization on a genetic basis seems to be required.Here we examined the genetic structure of two Diaphanosoma mongolianum populations and three Diaphanosoma brachyurum populations in Central Europe by allozyme electrophoresis. A genetic differentiation between both species was evident. 5 out of 9 tested loci carried diagnostic alleles. Both species differed in their habitat choice: D. mongolianum was adapted to higher trophic levels than D. brachyurum. Co-occurrence was observed in a eutrophic lake. Populations sampled from their preferred habitats showed high genotype diversities.Most genotype frequency distributions were in agreement with Hardy-Weinberg expectations and temporal (between years) changes in genotypic compositions were not common. This indicated annually occurring bouts of sexual reproduction, similar to intermittent Daphnia populations. Genetic differentiation among lakes suggested restricted gene flow.