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Viable monohaploids with 12 chromosomes in their somatic cells were obtained from an autotetraploid cultivar of Solanum tuberosum (2n = 4x = 48) by inducing female parthenogenesis successively in the tetraploid cultivar and in dihaploids from that cultivar. Both dihaploids and monohaploids were induced using the diploid S. phureja clones IvP 35 and IvP 48 as pollinators. The average frequencies of dihaploids and monohaploids in 1973 were 2.0 and 0.14 per berry respectively.Non-homologous chromosome associations (bivalents, trivalents and even quadrivalents) were observed at metaphase I of meiosis in pollen mother cells of the two monohaploids studied.The occurrence of non-homologous associations of chromosomes during meiosis is discussed. In addition the potential significance of monohaploids for basic research and for breeding of potatoes is considered.

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Potato. dihaploid, marker genes . SUMMARY In the first inbred generation (I,) of cv . Black 4495 a dark green, slowly growing mutant (coded ds) was found, whereas the 1, of its self-compatible dihaploid, B16, comprised this ds mutant and in addition the mutants virescens (v) and yellow margin (ym). The occurrence of ds and ym might trace back to diploid S . phureja, one of the ancestors of Black 4495 . No lethal mutants were observed in 1, of B16 . Analysis of I, of cv . Gineke revealed a simplex condition for virescence and either duplex or triplex heterozygosity for one lethal gene . On the other hand, the I, of its dihaploid, G254, segregated for virescence and for three different lethal genes . It is shown that both in B 16 and in G254 homozygosity of an S-bearing translocation causes early death of embryo and endosperm, thus preventing seed development . From this study it appeared that the three lethal genes from G254 affect germination rate of the seeds . The genotypes at 11 loci of B 16 and G254 are presented .

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