Single cross alfalfa (Medicago sativa L.) hybrids produced via 2n gametes and somatic chromosome doubling: experimental and theoretical comparisons

Summary The potential breeding value of 2n gametes from diploid alfalfa (2n = 2x = 16) was tested by comparing single cross alfalfa hybrids produced via 2n = 2x gametes from diploids versus n = 2x gametes from somatic-chromosome-doubled, tetraploid counterparts. Three diploid clones, designated 2x-(rprp), homozygous for the gene rp (conditions 2n gamete formation by a first division restitution mechanism) were colchicine-doubled to produce their tetraploid counterparts, designated 4x-(SCD). These six clones were crossed as males to the same cytoplasmic male sterile clone. Yield comparisons of progeny from the six clones demonstrated a significant yield increase of the hybrid progeny from 2n = 2x gametes from the diploids over the hybrid progeny from n = 2x gametes from the chromosome doubled tetraploid counterparts. The yield gain ranged from a 12% increase to a 32% increase. Theoretical comparisons indicated the 2n = 2x gametes from diploids would have 12.5 to 50% more heterozygous loci, on average, than the n = 2x gametes derived from somatic doubling. These results confirm the importance of heterozygosity on alfalfa yield, and the results demonstrate that 2n gametes formed by first division restitution offer a unique method for producing highly heterotic alfalfa hybrids.     

The significance of 2N gametes in potato breeding

Summary Phureja-haploid Tuberosum hybrids, which produce 2n gametes in addition to n gametes, were used to obtain diploid progenies in 2x – 2x matings, and tetraploid progeny in 4x – 2x matings. Seven of these diploid clones were intermated in a modified diallel design, and also crossed to seven cultivars, to obtain 21, 16-clone diploid; and 49 (35, 2x X 4x and 14, 4x X 2x) 24-clone tetraploid families, respectively. These progenies were included together with the 14 parental clones, in a tuber yield trial conducted in two locations in Wisconsin. Heterotic responses were striking in 4x X 2x progenies. Three of these entries had mean yields of 5.0, 5.0 and 5.2 lbs/hill, for 24 unselected clones, thus outyielding not only the mid-parent but also the best tetraploid cultivars in the experiment, which are highly selected; Wis 643 (4.8 lbs/hill), and Kennebec (4.8 lbs/hill). As a group, the 14, 4x X 2x families averaged 4.4 lbs/hill, while the mean of all seven cultivars was 4.0 lbs/hill, and the mid-parent was at 3.4 lbs/hill. This unusually high mean performance of 336 unselected clones represented in the 14, 4x X 2x progenies, is interpreted as a manifestation of the capacity of 2n pollen, formed by first meiotic division restitution (FDR), to pass onto the progeny the already heterotic diploid genotype in a largely intact array. Beneficial intra and inter-locus interactions are presumably compounded upon syngamy with an unrelated n egg from the tetraploid parent. -The performance of 2x X 4x progenies was at or below that of the midparent. The failure of these families to perform as well as 4x X 2x families may be a reflection of the incapacity of 2n megasporogenesis to avoid meiotic reassortment as efficiently as FDR does, which would result in inbred 2n gametes. However, the method of 2n megasporogenesis is not known.Hereditary variances were large, both within and among families, and indicated considerable nonadditivity in the action of genes affecting tuber yield at both levels of ploidy. Inbreeding was strongly depressing at both ploidy levels.     

Segregation for sexual seed production in Paspalum as directed by male gametes of apomictic triploid plants

BACKGROUND AND AIMS: Gametophytic apomixis is regularly associated with polyploidy. It has been hypothesized that apomixis is not present in diploid plants because of a pleiotropic lethal effect associated with monoploid gametes. Rare apomictic triploid plants for Paspalum notatum and P. simplex, which usually have sexual diploid and apomictic tetraploid races, were acquired. These triploids normally produce male gametes through meiosis with a range of chromosome numbers from monoploid (n = 10) to diploid (n = 20). The patterns of apomixis transmission in Paspalum were investigated in relation to the ploidy levels of gametes. METHODS: Intraspecific crosses were made between sexual diploid, triploid and tetraploid plants as female parents and apomictic triploid plants as male parents. Apomictic progeny were identified by using molecular markers completely linked to apomixis and the analysis of mature embryo sacs. The chromosome number of the male gamete was inferred from chromosome counts of each progeny. KEY RESULTS: The chromosome numbers of the progeny indicated that the chromosome input of male gametes depended on the chromosome number of the female gamete. The apomictic trait was not transmitted through monoploid gametes, at least when the progeny was diploid. Diploid or near-diploid gametes transmitted apomixis at very low rates. CONCLUSIONS: Since male monoploid gametes usually failed to form polyploid progenies, for example triploids after 4x x 3x crosses, it was not possible to determine whether apomixis could segregate in polyploid progenies by means of monoploid gametes.     

Cytological,morphological and molecular analyses of controlled progenies from meiotic mutants of alfalfa producing unreduced gametes

A program of sexual polyploidization was carried out in alfalfa using plants from wild diploid species that produced male or female unreduced gametes. Sixteen progenies from 2x-4x and 2x-2x crosses were examined with a combination of morphological, cytological and molecular analyses. The chromosome counts revealed diploid, tetraploid and aneuploid plants. Plants with B chromosomes were also detected. The leaf area of the plants was a useful characteristic for distinguishing tetraploid from diploid plants obtained by unilateral or bilateral sexual polyploidization. Leaf shape and leaf margin were not correlated with the ploidy levels. Plants with supernumerary chromosomes displayed obovate or elliptic leaves which differed markedly from the range of forms typical of diploid and tetraploid alfalfa plants. RAPD markers were investigated in all progeny plants to determine maternal and paternal amplification products. Three alfalfa-specific primers proved to be effective in revealing the hybrid origin of the plants. A combination of cytological, morphological and molecular analyses is essential for a detailed genetic characterization of progenies in programs of sexual polyploidization.     

Pollen markers for gene-centromere mapping in diploid potato

The utility of two pollen genetic markers for estimating the extent of meiotic recombination between the centromere and a marker gene was tested in 2n pollen of diploid potato clones. One of these markers was the distal locus amylose-free (amf) on chromosome 8 and the other was the isozyme locus alcohol dehydrogenase (Adh-1) on chromosome 4. In the case of the amf locus, the gene-centromere distance was estimated in a normal synaptic and a desynaptic genotype. In both cases the genetic analysis was confined to: (1) a direct estimation of the phenotypic (blue vs red) segregation ratios in FDR (first-division restitution) 2n pollen and (2) a classification of the 4 x progeny from 4x (nulliplex amf) x 2x (Amf/amf) crosses into duplex, simplex and nulliplex classes. The recombination frequency between the centromere and the amf locus in the normal synaptic genotype B92-7015-4 corresponded to a gene-centromere distance of 48.8 cM, whereas this distance amounted to 13.3 cM in the desynaptic genotype RS93-8025-1. Hence desynapsis reduced crossing-over by 73%. The observed genetic distance of 48.8 cM in the normal synaptic clone, B92-7015-4, is the highest gene-centromere distance reported so far in potato and this could be explained on the assumption of absolute chiasma interference. For the Adh-1 locus, it was found that heterozygous 2n pollen grains could be detected in pollen samples of the diploid clones, because of the occurrence of a heterodimeric band of the isozyme. Unlike the amf locus, the genecentromere distance for the Adh-1 locus was estimated only on the basis of the duplex, simplex and nulliplex classes in the progenies from 4x (nulliplex Adh-1 ² )x B92-7015-4 (Adh-1 ¹ /Adh-1 ² )crosses and was found to be 19.4 cM. Because the accurate positions of centromeres in relation to other loci are not available in the existing genetic maps of potato, which are saturated with molecular markers, halftetrad analysis is a promising additional approach to the basic genetics of this crop.     

Heterozygosity in 2n gametes of potato evaluated by RFLP markers

The heterozygosity transmitted through 2n gametes in potato was evaluated by following the segregation of RFLP markers in tetraploid progeny from bilateral sexual polyploidization in a cross between two diploid (2x) interspecific hybrids which produce 2n SDR eggs or 2n FDR pollen. Out of 84 probe/enzyme combinations tested, 23 revealed polymorphism between the parents and were heterozygous in at least one of the parents. These probes characterized 13 loci distributed on five different chromosomes of the potato RFLP map. The heterozygosity transmitted through SDR and FDR gametes was estimated to be 31.8% and 71.4%, respectively. Two different indices (LH and RHI) were used to select plants showing a high level of heterozygosity in the tetraploid progeny. The recombination events and the centromere positions were estimated for chromosomes I, VI and VII, following the segregation ratios of SDR or FDR gametes produced by the parents. A different recombination rate was observed between the two interspecific hybrids.Contribution no. 119 from Research Centre for Vegetable Breeding, C.N.R., Portici, Italy. Research supported by National Research Council of Italy, special Project RAISA, Sub-project No. 2, Paper No. 2090     

Mating interactions between coexisting dipoloid, triploid and tetraploid cytotypes ofHieracium Echioides (Asteraceae)

Experimental crosses between diploids, triploids and tetraploids ofHieracium echioides were made to examine mating interactions. Specifically, cytotype diversity in progeny from experimental crosses, intercytotype pollen competition as a reproductive barrier between diploids and tetraploids, and differences in seed set between intra- and intercytotype crosses were studied. Only diploids were found in progeny from 2x × 2x crosses. The other types of crosses yielded more than one cytotype in progeny, but one cytotype predominated in each cross type: diploids (92%) in 2x × 3x crosses, tetraploids (88%) in 3x × 2x crosses, triploids (96%) in 2x × 4x crosses, triploids (90%) in 4x × 2x crosses, tetraploids (60%) in 3x × 3x crosses, pentaploids (56%) in 3x × 4x crosses, triploids (80%) in 4x × 3x crosses and tetraploids (88%) in 4x × 4x crosses. No aneuploids have been detected among karyologically analyzed plants. Unreduced egg cell production was detected in triploids and tetraploids, but formation of unreduced pollen was recorded only in two cases in triploids. Triploid plants produced x, 2x and 3x gametes: in male gametes x (92%) gametes predominated whereas in female gametes 3x (88%) gametes predominated. Cytotype diversity in progeny from crosses where diploids and tetraploids were pollinated by mixture of pollen from diploid and tetraploid plants suggested intercytotype pollen competition to serve as a prezygotic reproductive barrier. No statistically significant difference in seed set obtained from intra- and intercytotype crosses between diploids and tetraploids was observed, suggesting the absence of postzygotic reproductive barriers among cytotypes.     

Estimation of the coefficient of double reduction in the cultivated tetraploid potato

Summary Theoretical models to estimate the coefficient of double reduction in tetraploid organisms and the standard error of this estimate are derived. Using these models, we were able to estimate the coefficient of double reduction for several loci in tetraploid potatoes, Solanum tuberosum L., through examination of segregating isozyme loci in a series of 4x-2x crosses and in haploid progeny derived from six cultivated tetraploid potatoes. Tetraploid x diploid crosses are useful for estimating the frequency of double reduction because of the availability of homozygous diploid tester lines and the large number of tetraploid progeny generated via the functioning of 2n pollen. The strength of haploid analysis is the examination of diploid progeny. However, it is frequently difficult to obtain large numbers of progeny for testing. Based on our results, we conclude that double reduction occurs sporadically in tetraploid potatoes.     

Variation patterns of parthenogenetic plants derived from “unreduced” embryo-sacs of Solanum tuberosum subspecies andigena (Juz. et Buk.) Hawkes

Summary Parthenogenetic seed induction was performed on one clone of Solanum tuberosum subspecies andigena (2n=4x=48) using S. phureja (2n=2x=24) marker inducer clones. The parthenogenetic population when grown was found to contain both diploid and tetraploid individuals presumably arising from reduced and unreduced gametes, respectively. Variation patterns in the diploid and tetraploid sub-populations, as well as a population obtained by selfing the parental clone, were compared to try and elucidate the origin of the tetraploid parthenotes. From the results of this one generation it appeared that the tetraploid parthenogenetic plants had been produced by a mechanism equivalent to second division restitution (SDR).     

Inter-embryo competition in the progeny of autotriploid Aloineae (Liliaceae)

In reciprocal crosses between diploid and triploid Aloineae the progeny are largely diploid or diploid plus one or two chromosomes, but in reciprocal crosses between triploids and tetraploids they are tetraploid or nearly so. Thus the triploids contribute circa haploid gametes to the progeny when crossed with diploids but circa diploid gametes when crossed with tetraploids. These results are compared with those of a number of earlier workers. It is concluded that the bias in the frequency of progeny types towards diploidy or tetraploidy, depending on the ploidy level of the plant which is crossed with the triploid, is caused by inter-embryo competition. Those embryos with an endosperm/embryo factor of 1.5, the value found in normal diploid/diploid crosses having triploid endosperms, are selected in preference to those with factors higher or lower than 1.5.Inter-gamete competition also occurs among the euploid and aneuploid gametes produced by the triploids. This is more pronounced on the male side, because the degree of survival of aneuploid pollen from the triploids into the next generation is much lower than that of aneuploid egg nuclei.Non-reduction in the triploids gives rise to occasional pentaploid progeny in crosses with tetraploids, but it is more probable that in diploid/triploid crosses tetraploid progeny are the products of non-reduction in the diploid.     

Using disomic 4x(2EBN) potato species' germplasm via bridge species Solanum commersonii.

The cultivated potato Solanum tuberosum Dunal has many wild related species with desirable traits. Some of these wild tetraploids have disomic chromosome pairing, ready selfing with little inbreeding depression, but have strong crossing barriers with cultivars. They hybridize most easily with 2EBN forms (which include most diploid species). Chromosome doubling to the 8x level, use of 2n gametes, use of 2n gametes of 4x-2x triploid hybrids, and embryo rescue have been proposed to overcome the crossability barrier of these species with S. tuberosum. In this study, 2x S. commersonii (cmm) was used as a bridge species with S. acaule and series Longipedicellata species. Synthetic tetraploid 4x-cmm crossed readily to disomic 4x species, resulting in fertile F1 and F2 hybrids. Some of these had 2n gametes, which enabled direct crossing to tuberosum, resulting in 6x hybrids. The benefits of this scheme are (i) hybrids are relatively fertile, so many progeny may be produced for selection at each step, (ii) hybridization with cmm results in 2n gametes needed for crossing to tuberosum, and breaks up restricted recombination within disomic genomes, and (iii) simple techniques and tools are employed.     

Transmission genetics of chromatin from a synthetic amphidiploid to cultivated peanut (Arachis hypogaea L.). broadening the gene pool of a monophyletic polyploid species

Polyploidy creates severe genetic bottlenecks, contributing to the genetic vulnerability of leading crops. Cultivated peanut is thought to be of monophyletic origin, harboring relatively little genetic diversity. To introduce variability from diploid wild species into tetraploid cultivated Arachis hypogaea, a synthetic amphidiploid [[A. batizocoi K9484 x (A. cardenasii GKP10017 x A. diogoi GKP10602)](4x)] was used as donor parent to generate a backcross population of 78 progeny. Three hundred seventy RFLP loci were mapped onto 23 linkage groups, spanning 2210 cM. Chromatin derived from the two A-genome diploid ancestors (A. cardenasii and A. diogoi) comprised mosaic chromosomes, reflecting crossing over in the diploid A-genome interspecific F(1) hybrid. Recombination between chromosomes in the tetraploid progeny was similar to chromosome pairing reported for A. hypogaea, with recombination generally between chromosomes of the same subgenomic affinity. Segregation distortion was observed for 25% of the markers, distributed over 20 linkage groups. Unexpectedly, 68% of the markers deviating from expected segregation showed an excess of the synthetic parent allele. Genetic consequences, relationship to species origins, and significance for comparative genetics are discussed.     

Evidence for non-disomic inheritance in a Citrus interspecific tetraploid somatic hybrid between C. reticulata and C. limon using SSR markers and cytogenetic analysis

Artificial tetraploid somatic hybrids have been developed for sterile triploid citrus breeding by sexual hybridization between diploid and tetraploid somatic hybrids. The genetic structure of diploid gametes produced by tetraploid genotypes depends on the mode of chromosome association at meiosis. In order to evaluate tetraploid inheritance in a tetraploid interspecific somatic hybrid between mandarin and lemon, we performed segregation studies using cytogenetic and single sequence repeat molecular markers. Cytogenetic analysis of meiosis in the somatic hybrid revealed 11% tetravalents and 76% bivalents. Inheritance of the tetraploid hybrid was analyzed by genotyping the triploid progeny derived from a cross between a diploid pummelo and the tetraploid somatic hybrid, in order to derive genotypes of the meiospores produced by the tetraploid. A likelihood-based approach was used to distinguish between disomic, tetrasomic, and intermediate inheritance models and to estimate the double reduction rate. In agreement with expectations based the cytogenetic data, marker segregation was largely compatible with tetrasomic and inheritance intermediate between disomic and tetrasomic, with some evidence for preferential pairing of homoeologous chromosomes. This has important implications for the design of breeding programs that involve tetraploid hybrids, and underscores the need to consider inheritance models that are intermediate between disomic and tetrasomic.     

Genetic diversity in sexual diploid and apomictic tetraploid populations of Paspalum notatum situated in sympatry or allopatry

Paspalum notatum is a subtropical grass widely distributed in the temperate areas of America. Diploids are sexual while polyploids give rise to clonal seeds through aposporous apomixis. RAPD markers were used to analyze the genetic structure of three natural populations: i) diploids reproducing sexually (R2X); ii) sympatric apomictic tetraploids collected in the vicinity of the diploids (R4X); iii) allopatric apomictic tetraploids growing in isolation (C4X). The apomictic reproduction rate was evaluated by the use of molecular markers in progeny tests, while chromosome-counting allowed the verification of ploidy levels. Data revealed that the R4X group presented a variation considerably higher than that observed for C4X. Jaccards coefficients were used to produce a cluster diagram using the UPGMA method. All but one tetraploid genotypes grouped together and were associated to diploid genotype A21. The possibility of occasional generation of novel tetraploid clones from the interaction between tetraploid and diploid individuals is discussed.     

Estimation of preferential pairing in tetraploid x diploid hybridizations

Summary Crosses made between tetraploid and diploid, 2n pollen-producing species directly transfer from one-half to the entire diploid genome from the diploid to the tetraploid level, depending on the mechanism of 2n pollen formation and the amount of crossing-over that occurs. Tetraploid plants that result from tetraploid x diploid hybridizations can be further utilized in a breeding program. It is postulated that preferential pairing between homologous chromosomes derived from the original tetraploid or diploid parent occurs in the tetraploid x diploid hybrid. Depending on the genetic divergence of the species involved, preferential pairing of homologous chromosomes may range from zero to one. Theoretical estimates of the amount of preferential pairing and the standard errors of these estimates are derived for cases where the diploid parent produces 2n gametes by either a first division or a second division restitution mechanism.     

Inheritance of seed colour in turnip rape (Brassica campestris L.)

Summary A high gene frequency for ps (parallel spindles) is expected in cultivated tetraploid potatoes, S. tuberosum Group Tuberosum, if 2n pollen produced by ancestral diploid plants which were psps was involved in the origin and evolution of the potato. Fifty-six North American cultivars (varieties and advanced selections) were pollinated by diploid clones, either W 5295.7 or W 5337.3 which are homozygous recessive for ps. The segregation ratios in regard to 2n pollen production in derived tetraploid progenies, from 4x×2x crosses, reveal the genotype of ps in the cultivars. Microsporogenesis of 2n pollen producing 4x progeny was observed to avoid an overestimation of the frequency of 2n pollen producing plants due to mechanisms other than parallel spindles. More than 50% of the 56 cultivars are simplex (Pspspsps), since in each of these cultivars about 50% of their progeny produced 2n pollen. The ps gene frequency in the 56 cultivars was estimated as high as 0.69. The high frequency of ps in the tetraploid cultivars clearly supports the hypothesis that 2n pollen produced by plants homozygous recessive for ps have been involved in the origin of cultivated tetraploid potatoes, since a higher frequency of ps in the tetraploid than in the ancestral diploid population can be expected from sexual polyploidization but not from somatic doubling. The importance of meiotic mutants such as ps for the successful evolution of polysomic polyploids is emphasized.     

Use of 2n pollen-producing triploid hybrids to introduce tetraploid Mexican wild species germ plasm to cultivated tetraploid potato gene pool

Summary Tetraploid (2n=4x=48) 2EBN Mexican wild species in the series Longipedicellata, which consists of Solanum fendleri, S. hjertingii, S. papita, S. polytrichon, and S. stoloniferum, were crossed with two 2EBN cultivated diploid (2n=2x=24) clones. The resulting triploid hybrids (2n=3x=36) produced 2n pollen (triplandroids) by the mechanism of parallel orientation of anaphase II spindles. The percentage of stainable pollen in 520 triploids ranged between 0 and 23.5%, with a mean of 2.7%. Triploids producing between 13.0 and 23.5% stainable pollen were crossed as staminate parents to the tetraploid cultivars, resulting in abundant pentaploid (2n=5x=60) and near-pentaploid hybrid progeny. Crosses of triploids with lower percentage of stainable pollen as pollen parent to the tetraploid cultivars did not yield fruit, unless rescue pollen from a tetraploid cultivar was added 2 days later. Pentaploid hybrids were selected among selfed tetraploid progenies using morphological and isoxyme markers transmitted from their cultivated diploid parents. These pentaploid hybrids were vigorous and had uniformly sterile pollen. They were female fertile and were crossed with tetraploid cultivars, yielding an average of 19 seeds per fruit. Triplandroids provide the opportunity of transferring 2EBN tetraploid Mexican wild species in the series Longipedicellata germ plasm into the 4EBN cultivated potatoes.Cooperative investigations of the ARS, USDA, and the Washington State University Agricultural Research Center, Prosser, WA 99350, USA. H/LA Paper No. 90-03, College of Agriculture and Home Economies Research Center, Washington State University, Pullman, WA 99164, USA     

Comparisons of alfalfa somaclonal and sexual derivatives from the same genetic source

Summary Tetraploid alfalfa (Medicago sativa L.) clones were derived from the same diploid genetic background by four different methods. A phenotypically superior clone was selected from each method and compared for herbage yield and fertility. The four methods and their best clones were: a) In vitro somatic chromosome doubling of one diploid hybrid (HG2-4x); b) selection within a two allele tetraploid synthetic population derived from HG2-4x (HAG); c) somaclonal variant selection from cell suspension culture of the diploid hybrid (NS1); and d) sexual polyploidization of a sibling hybrid (HXG). Clones HG2-4x, HAG, and NS1 were likely diallelic or monoallelic at all loci. Clone HXG was probably tetrallelic or triallelic at most loci. Experiments measured fertility, clonal herbage yield, and herbage yield of test cross progeny for each selected clone. Fertility rankings were HXG = HAG > NS1 > HG2-4x. Clonal herbage yield rankings were HXG = HAG > NS1 > HG2-4x. Test cross progeny herbage yield rankings varied depending on the tester, but, in general, HXG HAG NS1 HG2-4x. Overall the best clones from the sexual methods exceeded the best somaclonal variant which, in turn, was better than the chromosome doubled clone.     

RFLP Analysis of Chromosomal Segregation in Progeny from an Interspecific Hexaploid Somatic Hybrid between Solanum Brevidens and Solanum Tuberosum

Segregation of restriction fragment length polymorphism (RFLP) loci was monitored to determine the degree of homeologous pairing and recombination in a hexaploid somatic hybrid, A206, the result of protoplast fusion between Solanum tuberosum (PI 203900, a tetraploid cultivated potato) and Solanum brevidens (PI 218228), a diploid, sexually incompatible, distant relative harboring several traits for disease resistance. Somatic hybrid A206 was crossed to Katahdin, a tetraploid potato cultivar, to generate a segregating population of pentaploid progeny. Although the clones of the tetraploid S. tuberosum lines PI 203900 and Katahdin were highly polymorphic, the diploid S. brevidens clone was homozygous at all but two of the tested RFLP loci. Thus, homeologous recombination could be detected only when S. tuberosum and S. brevidens chromosomes paired and the S. brevidens homologs then segregated into separate gametes. A bias toward homologous pairing was observed for all 12 chromosomes. At least four and perhaps six chromosomes participated in homologous pairing only; each of 24 progeny contained all S. brevidens-derived RFLP markers for chromosomes 4, 8, 9 and 10. The remaining six chromosomes paired with their homolog(s) about twice as often as expected if hexaploid pairings were completely random. Where detectable with RFLPs, homeologous recombinations (both single and double) occurred at a frequency of 1.31 per chromosome. Cytological observations of meiosis I in the somatic hybrid indicated that homeologous pairing had occurred. Enhanced recombinational activity was observed for chromosome 2. A specific small deletion from chromosome 4 was detected in A206 and 11 other somatic hybrids out of 14 screened. These hybrids represent 13 independent fusion events between the same clones of S. brevidens and S. tuberosum. In one instance, this deletion occurred in one of two plants resulting from the same callus, indicating that the loss occurred in culture after fusion had taken place. It is possible that this deletion contributes to somaclonal variation.     

Comparative performance of FDR and SDR progenies from reciprocal 4x-2x crosses in potato

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.