graphically in the form of branching trees on the principle of parsimony, i.e., closely related individuals have more similar sequences of a particular molecule and are, therefore, more closely related. The branch points (forks) represent the most recent ancestor common to all species beyond that point.

13.6.7 Karyotype Polymorphism

Although the small size of the fungal chromosomes (between 0.2 and 10 Mb of DNA) has been a deterrent in cytological studies, their small sizes can be resolved by pulsed field gel electrophoresis (Chapter 4) and therefore provide another potentially useful marker for analysis of variation. Chromosome length polymorphism—in the form of variation in chromosome size or number—is detected among field isolates of pea root pathogen, Nectria haematococca, from diverse geographic origin. Variability was marked among minichromosomes, smaller than 2 Mb. Five isolates of the wheat-blotch pathogen Mycosphaerella graminicola had noticeably different karyotypes (McDonald and Martinez, see: Kistler and Miao, 1992).

13.6.8 Spore Killer Elements

Spore killer elements in Ascomycotina are genes that cause the death of ascospores that do not contain the killer (Skk) elements. In Neurospora, distribution of Spore killer allele in population can be made simply by crossing tester strains that are sensitive to the killer (Turner, 2001). The strains to be tested are used as a male parent and each group of octads (eight ascospores) in the sides of the cross tubes are examined under a dissecting microscope as to whether they are comprised of all black or four black and four white (aborted) ascospores. Spore killer elements are known in three other fungi: Podospora, Gibberella and Cochliobolus (Raju, 1994). In natural populations of N. intermedia, non-killer strains are frequent; killer strains are found only in samples collected from Borneo, Java and Papua New Guinea.

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