The genetic diversity of arbuscular mycorrhizal fungi in natural ecosystems – a key to understanding the ecology and functioning of the mycorrhizal symbiosis

Abstract

SUMMARYThe mi'corrhizal symbiosis formed between plant roots and the arbuscular mycorrhizal (AM) fungi or Glomales is of great interest to ecologists because of its potential influence on ecosystem processes, its role in determining plant diversity m natural communities and the ability of the fungi to induce a wide variety of growth responses in coexisting plant species. Little attention, however, has been paid to the ecological role of diversity of AM fungi. Difficulties in identification, the inability to grow the fungi in pure culture, problems of taxonomic classification, and a lack of basic information on the life histories of AM fungi hinder studies of the ecological significance of diversity of .AM fungi. Nucleic acid based techniques have the potential to fill this gap in our knowledge by offering better means of identification and the opportunity to study links between the genetic diversity of AM fungi and functional and morphological diversity. The application of genus-specific molecular markers has shown that different genera of AM fungi coexist in plant roots and that this is a common occurrence. Molecular techniques have alsO' shown that natural AM fungai populations exhibit unexpectedly high genetic diversity, despite the assumption that diversity in these seemingly asexual fungi should be low. The high diversity occurs in multicopy tibosomal genes and their internal transcribed spacers, which are normally well conserved and homogeneous within an individual organLsm. The results show that sequence heterogeneity of the ribosomal genes can occur even in single spores of AM fungi, and we discuss how genetic diversity m.ay be promoted and maintained. Contrasting results, indicating that genetic diversity among replicate spores from pot-cultured material is low (even though they contain within spore sequence heterogeneity), suggest that there are mechanisms which promote high genetic diversity of AM fungi in natural ecosystems.We propose that AM fungi could be heterokaryotic as a result of the exchange of nuclei following hyphal fusion with other individuals but that other mechanisms, such as gene turnover and molecular drive, might also explain the generation of high genetic diversity without any exchange of genetic material among individuals. The high diversity in ribosomal gene sequences in AM fungi might cause problems in their use as molecular markers in field studies. \ better understanding of the levels of genetic diversity of ribosomal genes within spores, among spores of the same morphology, and among spores of differing morphology is essential to the development of sound molecular markers for field studies and to the development of a phylogenetic classification.We concltide that an understanding of tbe mechanisms which promote and maintain genetic diversity in the AM iungi is crucial, not only to further advances in ecological and evolutionary studies but also to studies of the Tiolecular basis of the regulation of the symbiosis. Moreover, we predict that while observational investigations m ....

The evolution of mutualisms: exploring the paths between conflict and cooperation

“…The traditional solution for bacterial and fungal symbionts has been culturing. However, in some symbioses, what is successfully cultured does not necessarily reflect the actual community present in intact associations; and in other systems, symbionts cannot presently be cultured 11, [26][27][28] . For these reasons, molecular analyses have played a critical role both in genetically characterizing isolated mutualists and in screening assemblages directly to assess the nature of symbiont communities.…”

“…For these reasons, molecular analyses have played a critical role both in genetically characterizing isolated mutualists and in screening assemblages directly to assess the nature of symbiont communities. The resulting discoveries of stunning and unexpected diversity have transformed our understanding of mutualisms involving corals (Box 1), leaf-cutter ants (Box 4), and root symbionts 26,27,29 , among others.…”

“…At the level of different individual hosts within a species, genetically different symbionts can be found in association with different host individuals (or populations). Even within individual host organisms, several distinct symbionts can be found 12, [26][27][28] . The recognition that individual hosts can act as landscapes for communities of potentially competing symbionts (Box 1) raises the question of why competition among symbionts does not destabilize the mutualism, much as competition among parasites is believed to result in selection for increased virulence 23,25 .…”

“…Many of the themes emerging from these studies of corals characterize other symbiotic systems as well 11,26,27,29,35 .…”

Twenty years of research on community composition and species distribution of arbuscular mycorrhizal fungi in China: a review

“…Based on the lack of sexual reproduction in AMF, it has been suggested that applying the species concept could be difficult and that it would be more appropriate to base the description of biodiversity on genetic diversity (Sanders et al 1996). However, even for interpretation of the most recent molecular ecological studies, genetic variation within one population and among different isolates of one species can be quite high (Antoniolli et al 2000;Clapp et al 2001;Redecker et al 2003).…”