Genetic variation in parasites has important consequences for host–parasite interactions. Prior studies of the ecologically important parasite Metschnikowia bicuspidata have suggested low genetic variation in the species. Here, we collected M. bicuspidata from two host species (Daphnia dentifera and Ceriodaphnia dubia) and two regions (Michigan and Indiana, USA). Within a lake, outbreaks tended to occur in one host species but not the other. Using microsatellite markers, we identified six parasite genotypes grouped within three distinct clades, one of which was rare. Of the two main clades, one was generally associated with D. dentifera, with lakes in both regions containing a single genotype. The other M. bicuspidata clade was mainly associated with C. dubia, with a different genotype dominating in each region. Despite these associations, both D. dentifera- and C. dubia-associated genotypes were found infecting both hosts in lakes. However, in lab experiments, the D. dentifera-associated genotype infected both D. dentifera and C. dubia, but the C. dubia-associated genotype, which had spores that were approximately 30% smaller, did not infect D. dentifera. We hypothesize that variation in spore size might help explain patterns of cross-species transmission. Future studies exploring the causes and consequences of variation in spore size may help explain patterns of infection and the maintenance of genotypic diversity in this ecologically important system.

Fungal endophytes associated with the palm, Trachycarpus fortunei, within and outside its natural geographic range were investigated. Endophytes were relatively common with colonization rates of 23–57% at the four sites sampled. The endophyte assemblages at the different sites were diverse with 75 fertile species of ascomycetes and mitosporic fungi. The assemblage composition at each site was similar and between seven and 13 species comprised 81–89% of the taxa present in relative frequencies of >1%. Glomerella cingulata and Phomopsis spp., were consistently dominant, and a large number of rare species were recorded. The diversity at each site was similar in number, but the abundance of isolates varied. The results obtained were comparable to those of previous studies of palm endophyte assemblages, but the assemblages showed more affinity with unrelated temperate hosts than with tropical palm hosts. Quantitative and qualitative differences in endophyte assemblages from old and young tissues were observed, and more isolates were recovered from old tissues independent of the age of the palm. The composition of the assemblage varied with several taxa being exclusively or more commonly isolated from old tissues (e.g. xylariaceous taxa, Oxydothis sp. nov.) or young tissues (e.g. Stagonospora spp., Phoma multirostrata). Some differences in the composition of the assemblage and in relative frequencies of various species were observed in trees and saplings. Significantly more isolates were recovered from the vein than intervein tissues, independent of leaf age or tree age. Tissue specificity was not exhibited by any taxa isolated from either leaf or petiole tissues, except for xylariaceous taxa in leaf tissues. Some other taxa showed a preference for leaf tissues or petioles, whereas others were equally distributed amongst all tissues. Endophyte assemblages of palms from continuous distributions were similar, but those from disjunct distributions (i.e. outside the natural geographic range of the palm, such as Australia and Switzerland), differed significantly from each other and from assemblages within continuous distributions. The relative importance values of endophyte species at the two sites in China indicates the site-specific nature of the endophyte assemblages. Most previous studies on the endophytes of palm hosts have concentrated on tropical palms. However, this study examines the endophytes of the warm temperate palm Trachycarpus fortunei, and investigates the relative importance of host and climate related processes.

The arbuscular mycorrhizas of bluebell (Hyacinthoides non-scripta (L.) Chouard ex Rothm.) involve several symbiotic fungi of the order Glomales. We have previously simplified the system by ignoring the taxonomic diversity of the fungi, but it is unlikely that all fungal species contribute in the same manner or to the same extent to the functioning of the symbiosis. To discover how many and which fungi take part in the bluebell mycorrhiza we sought to identify the range of arbuscular mycorrhizal (AM) fungi found in bluebell roots sampled during a complete growing season, September to June.

Although the taxonomy of the Glomales by their spores is not yet fully understood, identification is, to a large extent, possible. Arbuscular mycorrhizal communities are usually characterized by their spores but, since spores can rarely be directly associated with individual plants or plant species, a more satisfactory approach would be to identify fungal symbionts where they interact with the host plant, in the roots. Unfortunately, the intraradical mycelia of the fungi are less easily distinguished than the spores and, as yet, classification is possible only to the family level.

We have developed a method whereby different AM fungal taxa in the roots of bluebell can be distinguished by objective assessment. A large suite of morphological characteristics of the fungi in roots was recorded in samples taken at monthly intervals. Hierarchical cluster analysis of the resulting data separated six distinct AM fungal morphotypes (Scutellospora type, Acaulospora type, three Glomus types and fine endophytes) and a classification system created by which identification by eye was possible.

We compared the fungi in roots with glomalean spores in soil from the root zone of the same bluebell plants. Two species occurred in most samples, Scutellospora dipurpurescens Morton & Koske (emend. Walker, 1993) and Acaulospora koskei Blaskowski. A further six occurred sporadically, five Acaulospora spp. and Glomus rubiforme Gerdemann & Trappe) Almeida & Schenck (=Sclerocystis rubiformis). The presence of a single species of Scutellospora was consistent with a Scutellospora root morphotype which varied little. By contrast, the diversity of Acaulospora in the spore assemblage was reflected by variation within the Acaulospora morphotype. Glomus spores were very rarely found in field collections, yet Glomus morphotypes were found to be an important component of the bluebell mycorrhiza.

Because some important species are not represented in spore assemblages in the field and those that are found can only be associated with vegetational groups, not individual plants or single species, glomalean spore populations provide only a partial account of the fungi which contribute to arbuscular mycorrhizas. Although it is still not possible to identify AM fungi in roots with the same precision as their spores, the method reported here permits assessment of diversity in the roots of individual plant species, which may be applied to the investigation of mycorrhizal function and demography in natural ecosystems.