Internuclear recognition: A possible connection between euascomycetes and homobasidiomycetes.

In the heterothallic Euascomycete Podospora anserina, fertilization is followed by mitotic divisions of parental nuclei, resulting in a plurinucleate stage. Nuclei of opposite mating type then recognize one another and form dikaryons which undergo karyogamy and meioisis. The internuclear recognition is a characteristic feature of the sexual cycle of filamentous Euascomycetes and is controlled by mating-type genes. These genes encode transcription factors which have nucleus-limited expression. It is assumed that this characteristic allows nuclei of different mating type to express a pattern of specific proteins directly involved in internuclear recognition. As the molecular nature of these proteins is unknown, the exact mechanism of internuclear recognition remains elusive. Schuurs et al. (1998) have proposed that internuclear distance affects gene expression through a pheromone/receptor system in the Homobasidiomycete Schizophyllum commune. This model can be applied to internuclear recognition in P. anserina and can account for all data resulting from genetic analyses.