THE GENETIC DIVERSITY OF ARBUSCULAR MYCORRHIZAL FUNGI IN NATURAL ECOSYSTEMS - A KEY TO UNDERSTANDING THE ECOLOGY AND FUNCTIONING OF THE MYCORRHIZAL SYMBIOSIS

The mycorrhizal symbiosis formed between plant roots and the arbuscula r mycorrhizal (AM) fungi or Glomales is of great interest to ecologist s 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 coexistin g plant species. Little attention, however, has been paid to the ecolo gical role of diversity of AM fungi. Difficulties in identification, t he 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 An I fungi. Nucleic acid based techniques have the potential to fi ll this gap in our knowledge by offering better means of identificatio n and the opportunity to study links between the genetic diversity of AM fungi and functional and morphological diversity. The application o f 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 population s exhibit unexpectedly high genetic diversity, despite the assumption that diversity in these seemingly asexual fungi should be low. The hig h diversity occurs in multicopy ribosomal genes and their internal tra nscribed spacers, which are normally well conserved and homogeneous wi thin an individual organism. The results show that sequence heterogene ity of the ribosomal genes can occur el en in single spores of AM fung i, and we discuss how genetic diversity may be promoted and maintained . Contrasting results, indicating that genetic diversity among replica te spores from pot-cultured material is low (even though they contain within spore sequence heterogeneity), suggest that there are mechanism s which promote high genetic diversity of AM fungi in natural ecosyste ms. We propose that AM fungi could be heterokaryotic as a result of th e exchange of nuclei following hyphal fusion with other individuals bu t that other mechanisms, such as gene turnover and molecular drive, mi ght 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 sports, among sp ores of the same morphology, and among spores of differing morphology is essential to the development of sound molecular markers for field s tudies and to the development of a phylogenetic classification. We con clude that an understanding of the mechanisms which promote and mainta in genetic diversity in the Anl 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 a nd diversity using molecular techniques are of high value they will no t 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 k nowledge of links between genetic diversity and distribution of AM fun gi in natural ecosystems, and their functional diversity.