Encarna Velázquez and Martha
Trujillo
Departamento de Microbiología y Genética. Universidad de
Salamanca. SPAIN.
__________________
Among filamentous fungi that solubilise phosphate, the genera Aspergillus and Penicillium are the most representative although strains of Trichoderma have also been reported as P-solubilizer. Among the yeasts, only a few studies have been conducted to assess their ability to solubilise phosphate; these include Yarrowia lipolytica. As more studies are conducted a wider diversity of phosphate-solubilising filamentous fungi are expected to be described. Therefore, the knowledge of taxonomy of fungi is very important for those that work with these microorganisms.
Fungal systematics is still based mainly on morphological criteria, and most fungi are usually recognized and identified basically on their phenotypes. Numerous alternative approaches have been developed, including nutritional and physiological studies, serologic tests, secondary metabolites, and fatty acids. Although some of these are very useful for identifying poorly differentiated fungi such as yeasts and black yeasts, they are only complementary tools of morphological data in most cases. Molecular biology techniques, especially the analysis of rDNA sequences, are currently used for reliable phylogenetic studies, which enable a more natural classification system to be established.
The dual modality of fungal propagation (sexual and asexual), has meant that since the last century there has been a dual nomenclature. Since each phase has been described in total ignorance of the existence of the other in many cases, the International Code of Botanical Nomenclature maintains that it is legal to give them separate binomials. With the advent of molecular approaches in fungal taxonomy, some mycologists have advocated abandoning the dual system of naming because unified classification of all fungi may be possible on the basis of the rDNA sequences of the anamorphs.
Phylogenetic information gained from sequence analyses, especially of ribosomal DNA, has revolutionized our knowledge of the classification of this group over the last decade. Comparisons of the 18S rDNA sequences have been performed to assess the relationships of the major groups of living organisms. For phylogeny of filamentous fungi, the 18S sequence is mostly used completely or in subunits of over 600 bp. In the yeasts, the D1 and D2 variable regions of the 25S rDNA are almost exclusively used.
The correct identification of fungi is of great practical importance in clinical mycology, plant pathology, biodeterioration, biotechnology, and environmental studies. Fungi comprise one of the largest eukaryotic microbial groups where a significant number of species have been described predominately on phenotypic characteristics, metabolic features, and the morphology of the vegetative and sexual states of these organisms. Since the distinguishing morphological characteristics are frequently too limited and difficult to apply by non-experienced researchers in the identification of a fungus, physiological and biochemical techniques are applied, as has been routinely done for the yeasts. However, for poorly differentiated filamentous fungi, these methods are laborious, time-consuming, variable and provide insufficient taxonomic resolution. The development of molecular methods has opened up new paths to study the fungi as these techniques are universally applicable.
Among the electrophoretic methods, restriction length polymorphism (RFLP) and Random primer polymorphic DNA (RAPDS) are particularly useful for taxonomy. RFLP-based typing methods have been used to reveal anamorph-teleomorph connections. Random primed methods are particularly useful to determine relationships below the level of species, but depending on the length of the primers and the recognized taxonomic diversity of the group under study, the method may help to discriminate species.
More recently, a new electrophoretic technique had allows the separation of stable low molecular weight (LMW) RNA that in fungi includes 5.8s rRNA, 5S rRNA and class 1 and class 2 tRNA (Velázquez et al., 2000, Velázquez et al., 2001). The LMW RNA profiles are molecular signatures of each species of both prokaryotes and eukaryotes. In fungi, the 5.8S-5S rRNA zone are different and characteristic for each genus and the tRNA profiles are different and characteristic for each species of the same or different genera.
Among filamentous fungi that solubilise phosphate, the genera Aspergillus and Penicillium are the most representative (Abd-Alla & Omar, 2001; Vázquez et al., 2000; Whitelaw et al. 1999). Strains of Trichoderma have also been reported to contain this ability (Altomare et al. 1999).As more studies are conducted a wider diversity of phosphate-solubilising filamentous fungi are expected to be described. Among the yeasts, only a few studies have been conducted to assess their ability to solubilise phosphate; these include Yarrowia lipolytica (Vassilev et al., 2001) Schizosaccharomyces pombe and Pichia fermentans (Velázquez et al., unpublished results).
Therefore, currently there are numerous techniques that can be applied to fungal taxonomy, but it is necessary an exhaustive revision of the classification and nomenclature of filamentous fungi and yeast as well as the criteria for taxa delimitation and description of new species.