Volume 12 Part 1 Article 17
Title: Segregatioh of Genetic Markers in Secondarily Homothallic Basidiomycetes
Authors: M.P. Challen and T.J. Elliott
Secondarily homothallic basidiomycetes have genetic Incompatibility systems and characteristically produce both self-fertile and self-sterile spores. Most of these species, Hke the cultivated mushroom Agaricus bispoius, are 2-spored. This reduction in spore number from the usual 4 to 2 means that the four nuclei resulting from meiosis are available to migrate into the two spores. Debate the presence of an incompatibility system more spores are self-fertile than are self-sterile, so a mechanism which favours the association of compatible nuclei must operate. The simplest explanation available, the random migration of nuclei to give hdnucleate spores, has been dismissed in the past because it was assumed it would give rise to a 1:1 ratio of self-fertile to self-steriie spores, a ratio not observed. However, a reconsideration of the consequences of the random migration of nuclei in 2-spored species has shown that the pairing of unlike nuclei is favoured, giving rise to a 2:1 ratio of self-fertile to self-sterile spores (Langton & EUiott 1980).
If the random migration hypothesis is correct it has predictable and testable consequences. Most of the available data for secondarily homothallic basidiomycetes are related to the segregation of mating type factors and are based on the proportions of dikaryotic and monokaryotic progeny observed. However, the random migration hypothesis is predictive in relation to any recognisable genetic difference. For a single auxotrophic marker in a 2-spored species a 5:1 ratio of prototrophs to auxotrophs would be expected in a random spore progeny and a 9:1:1:1 ratio would result from two independently segregating auxotrophs (Langton & Elliott 1980). Before 1983 the only data available for the segregation of auxotrophs was in A. bisporus, a species in which basidiospore germinatiDn is very variable and generally poor (Raper, Raper & Miller 1972). It is not therefore appropriate to test this data statistically.
To test the predictions of the random migration hypothesis the segregation of auxotrophic markers has been investigated in a 2-spored species, Coprinus bilanatus (Elliott & Challen 1983). The limited data reported for this species were supportive of the hypothesis. More recently the validity of random migration has been questioned on the basis of spore diad analyses in which much higher frequencies of dikaryotic spores were found than would be expected from random migration (Kemp 1985).
To further test the random migrairion hypothesis, new markers liave been produced. The segregation of mating-type, as well as auxotrophic and resistance markers, from different stocks of C. bilanatus have now been tested and are described in this paper.