Chromosome visualisation in filamentous fungi.

Many attempts have been made to study the chromosomes of fungi, but a major problem is that fungal nuclei are so small. Fungal chromosomes are at the lowest resolution of light microscopy; thus few attempts to visualise fungal chromosomes have been successful. Fungi examined have been mainly Ascomycotina. The number of chromosomes per nucleus, estimated by conventional light visualisation and stained with standard dyes like Giemsa or aceto-orceine, usually does not exceed ten. A method developed in the late 1980s called 'germ tube burst' enables the discharge of condensed chromosomes from the hyphal cell and their spread on the surface of a slide. This more accurate method, usually gives better resolution of chromosomes. It was used with conventional light microscopy dyes as well as in fluorescent microscopy or for in situ hybridisation. A breakthrough has been made in fungal genetics by using pulse field gel electrophoresis (PFGE). Separation of the chromosomes on the gel enables the determination of their number and estimation of genome size. It also reveals chromosome length polymorphism and the presence of supernumerary chromosomes, which are usually too small to visualise in nuclei. A combination of two methods, cytological analysis by light microscopy and PFGE, should give a tool allowing the complex analysis of fungal genomes.