Persistent meiotic arrest in basidia of Agaricus bisporus

The pattern of distribution of genetic markers among progeny of Agaricw bisporus, whether conventional mutants, electrophoretic isozyme variants or restriction fragment length polymorphisms, implies a combination of reduced recombination rates and preferential segregation of centromeres during meiosis. We have considered an alternative explanation, namely that meiosis may be an infrequent event, the majority of basidiospores being produced by mitosis. By examining the dynamic population structure of nuclear division in the A. bisporus hymenium over the whole of the development of the mushroom fruit body, we find that meiosis is common. Fusion nuclei were readily observable and were in a large majority. However, basidioles with single fusion nuclei remained the majority class throughout the life of the fruit body, right through to senescence. This suggests that release from meiotic arrest is a rare event and some of the observed peculiarities in segregation patterns might be a consequence.