Molecular Maps in Yeast and Other Fungi
Since E. colican not serve the purpose of understanding eukaryotic genomes, yeast has been selected as the simplest eukaryote to understand the eukaryotic genome. Following are the reasons for the choice: (i) yeast has haploid DNA, which is only 3.5 times that in E. coli;Drosophilacontains 45 times the DNA of E. coli,and higher vertebrates have ~1000 times this DNA; (ii) work with Yeast is not as expensive as with higher vertebrate cells, due to high cost of media required to grow them in culture; (iii) yeast cells grow and divide like bacteria, doubling their mass every 90 minutes; (iv) yeast genetic system has been thoroughly understood, where large number of genes are already mapped, (v) DNA molecules, as long as 100kb can be routinely handled through PFGE. These can be produced by enzymes like DpnIor NotI,which recognize rare sites.

Using the technique of PFGE and its refinement known as contour clamped homogeneous electric field (CHEF) gel electrophoresis, genomes of several fungi could be resolved in chromosome bands, popularly described as electrophoretic karyotypes. These include Saccharomyces cerevisae(14 Mbp), Schizosaccharomyces pombe(15 Mbp), Neurospora crassa(47 Mbp), Aspergillus nidulans(31 Mbp) and Dictyostelium discoideum(52-56 Mbp). For electrophoretic karyotypes, inact chromosomal DNA has to be prepared in agarose plugs, which can also be purchased from commercial firms for organisms like yeast.

The sizes of bands are measured by comparing with bands of known size obtained from agarose plugs having yeast intact chromosomes. The bands are assigned to specific linkage groups by using one of the several possible methods. For instance, in Aspergillus,this was achieved by combination of two methods: (i) hybridization with labelled DNA probes containing genes that are already mapped to their respective linkage groups; (ii) examination of CHEF gel band patterns of genomic DNAs from strains carrying known translocations. Utilizing these approaches, eight different linkage groups (LGs) in Aspergillus could be assigned to six electrophoretic DNA bands. The CHEF and genetic linkage data agreed that LGs VIII (5.0 Mb), VII (4.5 Mb) and II (4.2 Mb) are the largest, with LGs V/I (3.8 Mb) and VI/III (3.5 Mb) of intermediate size and LG IV (2.0 Mb) as the smallest.