In this article the term `pollinators' refers to diploid genotypes of Solanum tuberosum Group Phureja which are used to pollinate autotetraploid potatoes for the induction of autonomous growth of unfertilized egg cells . Plants obtained from such egg cells contain chromosomes of the female only and are called dihaploids . The successful selection of ideal pollinators from Group Phureja is described . A pollinator is considered to be ideal, if it has good male fertility, homozygosity for the dominant seed marker embryo-spot and the ability to induce in autotetraploid cultivars a high number of dihaploids both per berry and per 100 seeds. Embryo-spot is a deep purple or reddish coloration at the base of the cotyledons of the embryo, visible on both sides of the flat seeds . It is one manifestation of a set of complementary genes with pleiotropic action, causing a concentration of anthocyanins at the base of all plant organs that are homologous to leaves . The complementary genes are : a basic gene P for anthocyanin production (this is the seedling marker `coloured hypocotyl') and allele B°o f locus B . Thirty-three homozygous B°B°PP plants and 41 B°bPP plants were identified and a number from both classes tested as pollinator onto varieties .Among the homozygous group a number of `ideal pollinators' could be detected . Unexpectedly a few polyploids grew from spotless seeds when using homozygous pollinators on varieties ; 1 .4% showed purple hypocotyl and 2 .6 % were matroclinous. The latter group of polyploids probably may have arisen from `unreduced' egg cells of the cultivars used .In the progenies from crosses between recessive cultivars (bbbbpppp) and heterozygous pollinators (B°bPP) the ratio of spotted : spotless hybrid seeds appeared to deviate greatly from 1 :1 . This is ascribed to `unreduced' gametes, which originate during meiosis, presumably through lack of formation of the 'reductional' (= first division) cell wall . On this assumption a map distance of 30 and 24 cross-over units is estimated for locus B on the basis of the proportion of nulliplex tetraploid hybrids in two different crosses .High-and low-seed-set pollinators were found at a ratio of 8 :21 suggesting monogenic dominance of low seed-set. Seeds per berry and haploids per berry are significantly correlated using low-seedset pollinators on cultivars, but they are not when pollinators are used that produce high numbers of seeds per berry . A hypothesis is put forward to explain this phenomenon .

Solanum bulbocastanum (2n = 2x = 24) has valuable characters for potato breeding, but cannot be hybridized directly with S. tuberosum cultivars . Both S . acaule (2n = 4x) and S . phureja (2n = 2x) were used as bridging species . Triploid S. acaule x S . bulbocastanum were doubled with colchicine and the resulting fertile hexaploid F,'s crossed with S. phureja . The triple hybrids obtained were tetraploid or nearly so . The two genomes of S. acaule in these triple hybrids probably pair preferentially, which may provoke pairing and possibly crossing over between the chromosomes of S. bulbocastanum and S. phureja .More than 20000 pollinations of the triple hybrids with four potato cultivars had to be made to produce 40 quadruple hybrids . These highly vigorous hybrids varied greatly in many morphological characters, resistance to Phytophthora infestans, fertility and crossability . The chromosome numbers are 48 (24 hybrids), 49 and 46, but some higher ploidy levels (65, 66, 72 chromosomes) were found as well . Their origin is to be sought in the fusion of an unreduced egg cell from triple hybrids (either euploid or hypoploid) and a reduced male gamete from the cultivars . This view is corroborated by their extreme resistance to Phytophthora . Also some 48-chromosome hybrids are highly resistant, which may indicate introgression from S. bulbocastanum.Most quadruple hybrids are readily inter-crossable and crossable as females with cultivars ; several also as males . Two could be hybridized with S. bulbocastanum, but the few seeds dit not germinate .Studies of pachytene stage of meiosis revealed the presence of a S . bulbocastanum chromosome in at least one tetraploid hybrid, which is highly resistant to Phytophthora. At metaphase I of meiosis chromosome associations higher than quadrivalents were not found . Except in one hybrid, the frequency of quadrivalents did not exceed one per cell and the average proportion of chromosomes associated as bivalents amounted to 90% .The quadruple hybrids ('double-bridge' hybrids) appear good starting material for breeding programmes aimed at introducing genes from S . bulbocastanum into S. tuberosum cultivars .

Two highly fertile and self-compatible dihaploids (2n 2x 24) from Solanum tuberosum L. (2n 4x 48) were investigated to elucidate the genetic basis of their self-compatibility. To this end the two dihaploids were selfed and reciprocally intercrossed and the resulting I~ and F~ plants tested for self-compatibility. Reciprocal backcrosses of I~-plants and Fl-plants were made. Complete diallels both within self-compatible and within self-incompatible Fl-plants were carried out as well as reciprocal matings between self-compatible and self-incompatible Fl-plants. From the wealth of data it could be concluded, that the dihaploids have two intact S-alleles, one being common to both. Six hypotheses were tested for explaining self-compatibility in these particular dihaploids. All but one had to be discarded. It is concluded that the self-compatibility most likely is brought about by the presence of an S-bearing translocation, which is not linked to the S-locus. The ratio sc :si in the Fl'S point either to certative disadvantage of translocation-bearing pollen or to lethality of translocation homozygotes. The importance of this self-compatibility mechanism for genetic and breeding research in potato is discussed.

Numerically unreduced (2n) gametes from first division restitution (FDR) are considered to be superior to 2n-gametes from second division restitution (SDR) because they transfer a larger proportion of the total parental heterozygosity and epistasis intact to the tetraploid progeny. This supposed superiority was investigated by comparing 12 sets of reciprocal 4x-2x crosses. Each diploid parent used in a reciprocal set produced 2n-pollen by FDR and 2n-eggs by SDR. Six agronomic characters were investigated. FDR progenies (from 4x.2x) were found to have higher mean yields due to more and bigger tubers. With respect to underwater weight, the overall progeny mean of FDR progenies was significantly higher than that of SDR progenies (from 2x.4x). However, the absolute difference found between both overall progeny means was too small to be of practical significance. No differences between FDR and SDR progeny means were found for vine maturity and chip colour. In addition to the progeny mean, within-progeny variation is important in potato breeding. For vine maturity a higher within-progeny variation was detected in SDR progenies, whereas within-progeny variations for yield, underwater weight and chip colour were not different in FDR and SDR progenies. With regard to vine maturity, we conclude that SDR 2n-gametes are superior to FDR 2n-gametes because, with the same progeny means of FDR and SDR progenies, the within-progeny variation was higher in SDR progenies. Therefore the assumed superiority of FDR 2n-gametes was confirmed for yield but was not observed for vine maturity, underwater weight and chip colour.

Hybrid dwarfness is the phenomenon that after crossing of normal genotypes dwarfs are obtained in the Fl or not before the F2 generation . The literature on hybrid dwarfness in wheat is critically discussed . A new hypothesis on its genetic basis is given, taking MCMILLAN'S (1937) as a starting point . Dwarfness is assumed to be determined by the additive interaction of three genes DI , D2 and D3 , differing in dominance relations and in quantitative contribution to the dwarf phenotype .Three dwarf types are described. Type 1-dwarfs are dwarf during their whole life cycle and normally do not produce seeds . Type 2-dwarfs start as normal seedlings, become dwarfs while tillering and die dwarfs ; some produce seeds, others do not . Type 3-dwarfs emerge as normal seedlings, become dwarfs during the tillering stage, but after some time they start to shoot and develop into nearly or even completely normal plants ; in the F2 the proportion of dwarfs decreases during the growing season . The occurrence and genetic basis of the three dwarf types is discussed .On the basis of their genotype 315 varieties and lines are divided into six genotypeclasses . Over 1000 intra-and inter-class crosses were made and F,, F 2 , F3 , and BC studied . Also some triple crosses and crosses with pure breeding dwarfs were investigated . In general the results obtained fit the hypothesis . Unstable ratios involving type 3-dwarfs are discussed separately .Linkage of the dwarf gene D2 and the necrosis gene Nee (both on chromosome 2B (XIII) was apparent from F 2-data and from results of a triple cross in which both forms of hybrid weakness occurred . Crossing-over between D2 and Ne e is calculated to be 34% .Methods are outlined to use hybrid dwarfness in a wheat breeding programme . The possible incompleteness of the three-gene hypothesis and the variability of dwarfness are discussed and finally some suggestions are made for future research .