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

Abstract

The mycorrhizal 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 in 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 fungal 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 ribosomal genes and their internal transcribed spacers, which are normally well conserved and homogeneous within an individual organism. 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 may 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. A 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 conclude that an understanding of the mechanisms which promote and maintain genetic diversity in the AM fungi is crucial, not only to further advances in ecological and evolutionary studies but also to studies of the molecular basis of the regulation of the symbiosis. Moreover, we predict that while observational investigations on AM fungal ecology and diversity using molecular techniques are of high value they will not give an understanding of the role of AM fungi in natural ecosystems and that further studies should also aim to fill the gaps in current knowledge of links between genetic diversity and distribution of AM fungi in natural ecosystems, and their functional diversity.