Chromosome Numbers

The majority of fungi are haploid. The genome size of fungi varies from 15 to 45 megabases (Mb or millions of nucleotide pairs of DNA). This genome is smaller than that of other eukaryotes and consequently fungal chromosomes in the vegetative phase are at the limit of resolution of a light microscope. However, in the exceptionally large meiotic cells of certain species belonging to Ascomycotina (e.g., Neurospora), the chromosomes are duplicated and condensed. In such cells, the pachytene stage is often used for microscopic determination of chromosome number after staining with aceto-orcein, iron-haematoxylin, Giemsa or acriflavine. Some examples of chromosome numbers (where n is one chromosome complement) are: Aspergillus nidulans (8), Alternaria spp. (9-11), Cochliobolus heterostrophus (15), Magnaporthe grisea (7), Nectria haematococca (Fusarium solani) (10-14), Neurosora crassa (7) and Podospora anserina (7). The unicellular yeast Saccha-romyces cerevisiae has 16 chromosomes.

The difficulty of resolving intact, tangled chromosomes for microscopic count of chromosome numbers can be overcome by a new procedure (Tsuchiya and Taga, 2001). Protoplasts, generated from germinated conidia using the cell wall-lytic enzyme preparations, are lysed on a microscopic slide to discharge intact chromosomal DNA. The DNA molecules are then separated by pulsed field gel electrophoresis (PFGE). In this technique, the orientation of the applied electrical field on agarose gel is periodically changed, thereby reorienting and separating chromosomes. After staining with ethidium bromide, the chromosomes are resolved as bands and counted under UV light. By this technique chromosome numbers of fungi which lack sexual reproduction—and for this reason the meiotic stage is not available—may be determined. For a PFGE showing separation of chromosomes, see Alexopoulos et al. (1996).

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