Weak sinks could cradle mutualistic symbioses – strong sources should harbour parasitic symbioses

Using a population model of selection on an obligate symbiont and its host, we evaluate how demographic differences across geographical landscapes can produce selection mosaics in interacting species. The model assumes that the host populations vary geographically from demographic sources to sinks in the absence of effects by the symbionts, and that a virulent and a relatively avirulent form of the symbiont compete with one another across all habitats. Our results indicate that productivity gradients can create selection mosaics across habitats, resulting in complex fitness landscapes over which evolution occurs. We find that relatively virulent symbionts only persist if they have an advantage over avirulent strains or species in terms of interference (i.e. competition, and/or cross‐transmission) interactions. When such a trade‐off exists, we predict that the more virulent symbiont is most likely to be found in habitats where host population growth is highest, whereas the more avirulent symbiont should tend to persist in more marginal habitats or even habitat sinks for symbiont‐free hosts. Demographic sinks may be the habitats most likely to favour the origin of new mutualisms. Very productive mutualisms can be exploited by hyperparasites or cheaters. We discuss our findings in terms of geographical scenarios for the emergence of mutualisms, and the long‐standing debate about geographical patterns in the maintenance of sex.     

Nodulation of legumes by rhizobium

Abstract. The formation of nitrogen-fixing nodules on leguminous plants is the culmination of an integrated development involving many plant and bacterial genes. This review focuses on the structure, function and regulation of the bacterial genes involved in the nodulation process. We attempt to interpret recent observations on these genes in the context of signal exchanges which occur between the macro-and micro-symbionts.     

Unciliated body surface in three species of the Umagillidae (Dalyellioida,Platyhelminthes)

Anoplodiera voluta Westblad, Seritia elegans (Westblad) and Wahlia macrostylifera Westblad are species of the family Umagillidae living in the intestine of the holothurian Parastichopus tremulus. In all three species, part of the epidermis is unciliated, but unlike unciliated epidermis in the major parasitic flat-worm groups, it is cellular and has intraepithelial nuclei. The surface of the unciliated cells in A. voluta is convex with the cells separated by lateral gaps. The cells have two distinct regions: the basal, organelle-rich part and the apical part which contains few identifiable organelles except vesicles. In W. macrostylifera the unciliated cells have a flat surface and, between them, narrower gaps that in some cases widen to paracellular compartments below the cell surface. Apically the cells contain electron-dense vesicles, often in contact with the surface. S. elegans has unciliated cells separated by gaps. In all these species, apical vesicles indicate secretory activity. Comparison of the epidermis of these three species with the neodermis of the major parasitic flatworm groups within the Neodermata does not support a close relationship of the three to the Neodermata.     

Developmental biology in marine invertebrate symbioses

Associations between marine invertebrates and their cooperative bacterial symbionts offer access to an understanding of the roots of host-microbe interaction; for example, several symbioses like the squid-vibrio light organ association serve as models for investigating how each partner affects the developmental biology of the other. Previous results have identified a program of specific developmental events that unfolds as the association is initiated. In the past year, published studies have focused primarily on describing the mechanisms underlying the signaling processes that occur between the juvenile squid and the luminous bacteria that colonize it.     

Genetic diversity in epichloid endophytes of Hordelymus europaeus suggests repeated host jumps and interspecific hybridizations

Epichloid fungal endophytes (Epichloë and Neotyphodium spp.) are excellent model systems for studying speciation processes because of their variable life history traits that are linked to host grass fitness. Presumed jumps to new hosts and subsequent somatic hybridizations appear to be common among epichloid endophytes resulting in increased genetic variation upon which selection can act and speciation be initiated. In this study, we explored the endophyte diversity of a rare European native woodland grass species, Hordelymus europaeus, along a latitudinal transect covering the entire distribution range of H. europaeus. From 28 populations in six countries, isolates were sampled and molecularly characterized. Based on the sequences of tubB and tefA, six distinct epichloid taxa (interspecific hybrid or cryptic haploid species) were found, of which four were novel and two have been previously reported from this host. Of the novel endophytes, two were presumed to be interspecific hybrids and two of nonhybrid origin. While previously known endophytes of H. europaeus are seed‐born and strictly asexual, one of the novel nonhybrid endophytes found in the glacial refugium of the Apennine peninsula reproduced sexually in cultured plants. This is the first case of a seed‐borne, but sexually reproducing endophyte of this host. We discuss the origin, and possible ancestral species, of the six epichloid taxa using phylogenetic analyses. Repeated host jumps and somatic hybridizations characterize the diversity of the endophytes. To date, no other grass species is known to host a larger diversity of endophytes than H. europaeus.     

Mycoheterotrophic interactions are not limited to a narrow phylogenetic range of arbuscular mycorrhizal fungi

The majority of achlorophyllous mycoheterotrophic plant species associate with arbuscular mycorrhizal fungi (AMF). Previous studies have shown that some species are highly specialized towards narrow lineages of AMF and have suggested that only particular lineages of these fungi are targeted by mycoheterotrophic plants. To test this hypothesis, we analyzed all available partial SSU sequences of AMF associated with mycoheterotrophic plants including data from 13 additional specimens from French Guiana, Gabon and Australia. Sequences were assigned to 'virtual taxa' (VT) according to the MaarjAM database. We found that 20% of all known Glomeromycota VT are involved in mycoheterotrophic interactions and the majority of associations involve Glomeraceae (Glomus Group A) fungi. While some mycoheterotrophic plant species have been found growing with only a single VT, many species are able to associate with a wide range of AMF. We calculated significant phylogenetic clustering of Glomeromycota VT involved in mycoheterotrophic interactions, suggesting that associations between mycoheterotrophic plants and AMF are influenced by the phylogenetic relationships of the fungi. Our results demonstrate that many lineages of AMF are prone to exploitation by mycoheterotrophic plants. However, mycoheterotrophs from different plant lineages and different geographical regions tend to be dependent on lineages of AMF that are phylogenetically related.