B S

Figure 13.5 Basic organization of one unit of ribosomal DNA.

The fruiting bodies of the split-gill mushroom fungus Schizophyllum commune are readily recognized on natural woody substrates and this tissue collected for isolation and analyses of genomic DNA (James et al., 2001). Using the primer sequences annealing to the 5S and 18S rDNA genes, the intragenic spacer regions were amplified by PCR technique and analyzed for molecular variation. Of the 195 strains collected from different geographical regions, 145 haplotypes (distinct DNA fingerprints) showed unique IGS1. The sequence data showed that populations from the Eastern hemisphere, North America, South America and the Caribbean are genetically discrete.

13.6.5 Mitochondrial DNA and Mitochondrial Plasmids

Not all variations are due to changes in nuclear DNA. Within a fungal species, mitochondrial DNA shows a high level of variation at the population level, detected as restriction length polymorphisms (RFLPs). In addition, natural populations of fungi have a variety of linear and circular plasmids in mitochondria in mitochondria (Table 13.4). Some of these plasmids integrate into mitochondrial DNA and cause the death of strains that harbor these plasmids (Griffiths, 1995). By simple subculture tests, natural populations of Neurospora have been broadly classified into senescence-prone and immortal strains (Chapter 14).

13.6.6 DNA Sequence

The DNA sequence approach basically involves comparing nucleotide sequences of specific segments of DNA. The sequences are aligned to determine the numbers of differences between comparable molecules. This allows the measurement of similarities and differences between individuals, to determine how they may have evolved and in how much time their genomes diverged from another from a common ancestor. Nucleic acid sequence data is the ultimate level for determining homology. Computer programs fit the data

Table 13.4 Mitochondrial Plasmids in Naturally Occurring Strains of Neurospora intermedia1

Plasmid

Structure

Size (bp)

pVARKUD

Circular

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