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Abstract: A prerequisite for the controlled breeding of any organism is an understanding of its natural breeding system and this is as essential for lower organisms as it is for the most highly evolved green plant. It is paradoxical that although the mushroom has been intensively cultivated for fifty years progress is only now being made in understanding the sex-life of this interesting fungus. This deficiency in our knowledge must be rectified in view of the great commercial importance of Agaricus bisporus. The basic life-cycle of the higher fungi (with the spore as a convenient starting-point) is from spore to mycelium, to fruit-body and finally following a meiotic division back to spore again. Different fungi have contrasting mechanisms which determine their nuclear constitution at different stages in their life-cycles: the number of nuclei per cell, their ploidy and genotype may vary considerably. Basidiomycete fungi can be broadly classified either as homothallic species in which the mycelium grows and produces fruit-bodies although it contains only one type of nucleus, or as heterothallic species in which the presence of two nuclear types with different incompatibility factors is essential for the production of fruit-bodies. The two nuclear types may be brought together by the fusion of contrasting homokaryotic mycelia or they may both be present in the mycelium derived from a single heterokaryotic spore. These latter types are generally described as secondarily homothallic (Raper, 1966). Mycelia with nuclei all of one genotype are described as homokaryotic whilst those with two or more genotypes are termed heterokaryotic. Heterothallic fungi can be further differentiated into bipolar or tetrapolar types in which the compatibility of mycelia is controlled by a single gene or pair of genes respectively.
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