1. The four possible kinds of heterokaryon of Coprinus lagopus with no, one or both mating-type factors in common (dikaryon, common A, common B and common AB) were produced. Analysis of hyphal tips of common A and common AB heterokaryons has shown that both nuclei may be present in the same hypha.2. All four heterokaryons are prototrophic when synthesized from two auxotrophic components with different requirements.3. When synthesized in this way compatible heterokaryons were stable in all tests, but the other heterokaryons showed different degrees of stability. Common B heterokaryons were the most stable and rarely gave rise to monokaryotic mycelia. Dissociation of the common A and the common AB heterokaryon into either component took place much more easily.4. Comparisons of the growth-rates of wild-type heterokaryons on complete medium show that common A heterokaryons are less vigorous, and dikaryons more vigorous than their monokaryon components. On minimal medium both compatible and common A heterokaryons are less vigorous than their wild-type monokaryon components. The possible reasons for this are discussed.5. Fruit-bodies have been obtained from both common A and common B heterokaryons. Both types showed normal segregation at the heterozygous locus (B or A), but showed in addition the segregation of new reactions at the ‘homozygous’ locus.

1. Four main types of interaction between paired mycelia of Coprinus lagopus have been defined in terms of the extent of nuclear migration.2. Nuclear migration was demonstrated in matings between monokaryotic mycelia with common A alleles. No extensive migration of nuclei was found in common B or common AB matings.3. The speed of nuclear migration in common A matings was slower than in compatible matings. Migration occurred to approximately the same extent in both kinds of matings.4. In heterokaryon-monokaryon matings compatible and common B heterokaryons acted only as donors. Common AB heterokaryons acted as donors and as acceptors of compatible or common A nuclei. Common A heterokaryons always acted as donors and frequently acted as acceptors of compatible or common A nuclei.5. A simple explanation is suggested for the frequently observed fact that in an incompatible di-mon mating both nuclei of the dikaryon may migrate and eventually eliminate the nuclei of the established monokaryon.

Available information has been reviewed on tuber resistance to P. infestans, its inheritance and breeding procedures used to obtain potato cultivars superior in this character.P. infestans is worldwide the most destructive potato pathogen. Tuber resistance is an essential component of potato resistance as this pathogen is often responsible for tuber rot in storage, and infected tubers of susceptible cultivars provide an important way for its overwintering.In Europe many cultivars and advanced breeders selections have been obtained with tuber resistance to P. infestans, but in the last decades no progress is noted in the mean level of this resistance in cultivars. The expression of tuber resistance depends to a large extent on testing conditions and therefore it can be difficult to evaluate. Consequently making progress in breeding is not easy, and also cultivar assessment data from various countries sometimes differ considerably. It is concluded from published data that it should be easier to make progress in breeding potato cultivars with resistant tubers if more attention is paid to combining the various types of resistance which are already known, and if the genetic determination of durable resistance to P.infestans is better understood.

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A high level of PLRV resistance has been found in four diploid genotypes originating from resistant ancestors widely utilized in European potato breeding. Plants of these genotypes were difficult to infect not only with aphids, but also with graft inoculation. Their resistance is associated with limited virus spread, but not with intolerance.