Analysis of the meiosis in the F1 hybrids of Longiflorum x Asiatic(LA) of lilies (Lilium) using genomic in situ hybridization

Longiflorum and Asiatic lilies of the genus Lilium of the family Liliaeeae are two important groups of modern lily eultivars. One of the main trends of lily breeding is to realize introgression between these groups. With cut style pollination and embryo rescue, distant hybrids between the two groups have been obtained. However, the F1hybrids are highly sterile or some of them could produce a small number of 2n gametes, and their BC1 progenies are usually triploids. Dutch lily breeders have selected many cultivars from these BC1 progenies based on their variation. It is presumably suggested that such variation could be caused by intergenomic recombination and abnormal meiosis during gamete formation in F1 hybrids of Longiflorum x Asiatic (LA) hybrids in Lilium. Therefore, the meiotic process of ten F1 LA hybrids was cytologically investigated using genomic in situ hybridization and traditional cytological methods in the present research.The results showed that: at metaphase I, the homoeologous chromosome pairing among different F1hybrids ranged from 2.0 to 11.4 bi-valents formed by homoeologous chromosomes per pollen mother cell (PMC), and very few multivalents, and even very few bivalents were formed by two chromosomes within one genome rather than homoeologous chromosomes in some PMCs; at anaphase I, all biva-lents were disjoined and most univalents were divided. Both the disjoined bivalents (half-bivalents) and the divided univalents (sister chromatids) moved to the opposite poles, and then formed two groups of chromosomes; because the two resulting half-bivalents retained their axes in the cell undisturbed, many crossover types, including single crossovers, three strand double crossovers, four strand double crossovers, four strand triple crossovers, and four strand multiple crossovers between the non-sister chromatids in the tetrads of bivalents,were clearly inferred by analyzing the breakpoints on the disjoined bivalents. The present investigation not only explained the reason for sterility of the F1 LA hybrids and the variation of their BC1 progenies, but also provided a new method to analyze crossover types in other F1 interspecific hybrids as well.     

Indeterminate meiotic restitution (IMR): a novel type of meiotic nuclear restitution mechanism detected in interspecific lily hybrids by GISH

A detailed analysis of microsporogenesis was carried out in three diploid lily cultivars (2n=2x=24) and three diploid interspecific hybrids (2n=2x=24) using DNA in situ hybridisation methods (GISH and FISH). In cvs. Gelria (Lilium longiflorum; L genome), Connecticut King and Mont Blanc (both Asiatic hybrids; Agenome) meiosis was regular and only haploid gametes were formed while the three interspecific hybrids between L. longiflorum×Asiatic hybrid (LA) showed a variable frequency of meiotic nuclear restitution and stainable 2n-pollen formation ranging from 3% to 30%. An analysis of meiotic chromosome behaviour of the LA hybrids through GISH and FISH revealed that: (1) the parental chromosomes could be clearly discriminated into univalents, half-bivalents and bivalents in the PMCs; (2) in some of the PMCs the entire complement was present either as univalents or half-bivalents which had the potential to divide equationally (following centromere division) during the first division leading to first division restitution (FDR) gametes; (3) more frequently, however, in one and the same PMC the univalents and half-bivalents divided equationally whereas the bivalents disjoined reductionally at the same time giving rise to 2n-gametes that could vary from the well-known FDR or SDR 2n-gametes. We indicate this novel type of restitution mechanism as Indeterminate Meiotic Restitution (IMR). In order to confirm the occurrence of IMR gametes, the chromosome constitutions of eight triploid BC₁ progenies derived from backcrossing the 2n-gamete producing the LAhybrids to the Asiatic hybrid parents were analysed through in situ hybridisation. The results indicated that there were seven BC₁ plants in which FDR 2n-gametes, with or without homoeologous recombinations, were functional, whereas in one case the 2n-gamete resulting from IMR was functional. In the latter, there was evidence for the occurrence of genetic recombination through homoeologous crossing-over as well as through the assortment of homoeologous chromosomes. A singular feature of the IMR 2n-gamete was that although it transmitted a euploid number of 24 chromosomes to the BC₁ progeny, the number of chromosomes transmitted from the two parental species was dissimilar: 9 L-genome chromosomes and 15 A-genome chromosomes instead of 12 of each. Received: 15 May 2000 / Accepted: 4 December 2000     

ANEUPLOIDY IN CITRUS

Analyses of ploidy levels in progenies from 2x x 2x, 2x x 4x, 4x x 2x, 4x X 4x, and 4x x OP crosses during embryogenesis and seed germination indicate that the progenies include an array of aneuploids. The occurrence of some of the aneuploids was previously unreported. Most of the aneuploids were from 2x x 4x crosses, which yielded the largest number of progeny. Chromosome counts from embryos and endosperms of sectioned young seeds provide evidence that aneuploids originate in progenies of 2x x 4x crosses from the functioning of male gametes with more or fewer chromosomes than 2x = 18 in syngamy with reduced and unreduced female gametes. The origins of aneuploids in progenies from other crosses are problematical at this writing.     

Intergenomic recombination in F1 lily hybrids (Lilium) and its significance for genetic variation in the BC1 progenies as revealed by GISH and FISH.

Intergenomic recombination was assessed in a BC1 population of Oriental (O)xAsiatic (A) lilies (Lilium) backcrossed to Asiatic parents. This population consisted of 38 plants generated from the 2n gametes from 2 genotypes (951502-1 and 952400-1) of the diploid F1, Orientalx Asiatic lilies (2n=2x=24) as parents. In the majority of BC1 plants, there was evidence that first division restitution, with and without crossovers, resulted in functional gametes. However, there were 5 BC1 plants in which 2n gametes originated from indeterminate meiotic restitution (IMR). Based on the number of recombinant chromosomes for a particular homoeologous pair, 3 types of plants were identified: (i) those with both the reciprocal product of a crossover (O/A, A/O, where O represents the centromere of the O genome and A the recombinant segment of Asiatic chromosome, and vice versa); (ii) those with 1 normal chromosome of the O genome and a recombinant chromosome (O, A/O); and (iii) those with 1 normal chromosome of the A genome and a recombinant chromosome (A, O/A). An important feature of A x OA backcross progeny is the occurrence of substitutions for the segment distal in the crossover wherever the recombinant chromosome O/A was present. In the case of IMR, the substitution occurred for both proximal and distal recombinant segments. The significance of these substitutions is that they offer the potential for the phenotypic expression of recessive genes in polyploids (i.e., nulliplex genotype).     

The Application of Fluorescence In Situ Hybridization in Different Ploidy Levels Cross-Breeding of Lily

21 crossing were conducted between Asiatic Lily with different ploidy levels, the results showed that the interploidy hybridization between diploid and tetraploid lilies was not as successful as intraploidy hybridization. Regardless of male sterility, triploid lilies could be used as female parents in the hybridization which the progenies were aneuploidy. 3x×4x crosses could be cultured more successfully than 3x×2x crosses. 45S rDNA was mapped on the chromosomes of seven Lilium species and their progenies using fluorescence in situ hybridization (FISH). FISH revealed six to sixteen 45S rDNA gene loci, and normally the sites were not in pairs. The asymmetry indexes of LA (Longiflorum hybrids × Asiatic hybrids) hybrids was higher than Asiatic hybrids, the evolution degree was LA hybrids > Asiatic hybrids. 45S rDNA distributed variably on chromosome 1-10 and 12 among Asiatic hybrids. Chromosome 1 had invariable sites of 45S rDNA in all Asiatic hybrids, which could be considered as the characteristic of Asiatic hybrids. LA hybrid ‘Freya’ had two sites of 45S rDNA on one homologous chromosome 5, and also it could be found in the progenies. The karyotype and fluorescence in situ hybridization with 45S rDNA as probe were applied to identify the different genotypes of 9 hybrids. Typical chromosomes with parental signal sites could be observed in all the genotypes of hybrids, it was confirmed that all the hybrids were true.     

Chromosome inheritance in triploid Pacific oyster Crassostrea gigas Thunberg

Reproduction and chromosome inheritance in triploid Pacific oyster (Crassostrea gigas Thunberg) were studied in diploid female x triploid male (DT) and reciprocal (TD) crosses. Relative fecundity of triploid females was 13.4% of normal diploids. Cumulative survival from fertilized eggs to spat stage was 0.007% for DT crosses and 0.314% for TD crosses. Chromosome number analysis was conducted on surviving progeny from DT and TD crosses at 1 and 4 years of age. At Year 1, oysters from DT crosses consisted of 15% diploids (2n=20) and 85% aneuploids. In contrast, oysters from TD crosses consisted of 57.2% diploids, 30.9% triploids (3n=30) and only 11.9% aneuploids, suggesting that triploid females produced more euploid gametes and viable progeny than triploid males. Viable aneuploid chromosome numbers included 2n+1, 2n+2, 2n+3, 3n-2 and 3n-1. There was little change over time in the overall frequency of diploids, triploids and aneuploids. Among aneuploids, oysters with 2n+3 and 3n-2 chromosomes were observed at Year 1, but absent at Year 4. Triploid progeny were significantly larger than diploids by 79% in whole body weight and 98% in meat weight at 4 years of age. Aneuploids were significantly smaller than normal diploids. This study suggests that triploid Pacific oyster is not completely sterile and cannot offer complete containment of cultured populations.     

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.     

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.     

Quantitative Genetics of 4x-2x Hybrid Populations with First-Division Restitution and Second-Division Restitution 2n Gametes Produced by Diploid Parents

A model is proposed to describe the genetic value of 4x-2x hybrids resulting from crosses between tetraploid genitors and diploid genitors that produce 2n gametes. The model takes into account the genetic consequences of the First Division Restitution (FDR) and Second Division Restitution (SDR) meiosis, particularly on the homozygosity level that 2n gametes contribute to 4x-2x hybrids. As genes can be identical by descent, numerous parameters are needed in the classical approach to describe the inbreeding effects on the mean and variance of 4x-2x hybrids. Using the concept of test value, the model allows a large decrease in the number of required parameters. The model gives the components of genetic variance and usual covariances between relatives using these synthetic parameters. The model is then used to study the efficiency of a recurrent breeding scheme to improve diploid genitors for their combining ability with tetraploid genitors. It appears that, in presence of dominance, ignoring the meiosis pattern will lead to an overestimation of additive variance and then of genetic advance. Some genetic considerations on the differences between FDR and SDR mechanisms lead us to suggest an experimental comparison of their respective advantages and disadvantages for the type of considered recurrent selection. An experimental crossing design is proposed to obtain estimates of the genetic parameters needed for this comparison.     

The use of Endosperm Balance Number and 2n gametes to transfer exotic germplasm in potato

Summary A triploid hybrid (2n=3x=36) between a colchicine-induced 4x(2EBN) Solanum brevidens (a non-tuber-bearing species) and 2x(2EBN) S. chacoense (a tuber-bearing species) was used as a vehicle for germplasm transfer to S. tuberosum Group Tuberosum. The use of 2n gametes from the triploid allowed the unique opportunity for transferring exotic germplasm from Series Etuberosa to Gp. Tuberosum material. The triploid hybrid used had a pollen stainability of less than 0.1%. Observations of microsporogenesis revealed that metaphase I pairing configurations were primarily 12 bivalents and 12 univalents with occasional trivalents. Anaphase I separations were irregular, often with lagging univalents. Meiotic observations and pollen morphology suggest that the stainable pollen produced by the hybrid was 2n=3x=36. A single pentaploid hybrid (2n=5x=60) was produced by the fertilization of a rare 2n egg from the triploid with a normal male gamete from the clone Wis AG 231 (2n=4x=48). Limited crosses to other 1, 2 and 4EBN species and cultivars were unsuccessful. The pentaploid hybrid had a more regular meiosis than the triploid and dramatically improved pollen stainability (37% stainable pollen). Stylar blocks prevented estimates of male fertility in crosses. Female fertility in 47 crosses with nine cultivars averaged 19 seeds per fruit. Although S. brevidens is non-tuber-bearing, and the triploid produced only stolons, the pentaploid hybrid tuberized well under field conditions, despite being very late. Results suggest that the tuberization response is a dosage and/or threshold effect. This approach to the incorporation of 1EBN germplasm indicates the utility of the EBN concept coupled with 2n gametes. Further, it demonstrates a means for the introgression of 1EBN species genes into Gp. Tuberosum material.     

A comparison of performance of tetraploid progenies produced by diploid and their vegetatively doubled (tetraploid) counterpart parents

 Two diploid potato parents, W5295.7 and W5337.3, and their colchicine-doubled tetraploids were used as male to cross with three cultivars: ‘Raritan’, ‘Shepody’ and ‘Superior’. Both diploids produced 2n gametes via first division restitution (FDR) during meiosis. Field experiments were carried out on seedlings of the 12 hybrid progenies in 1991 and 1992. Six of the progenies represented the tetraploid-diploid (4x-2x) hybrids and the other six tetraploid-tetraploid (4x-4x) progenies. The genetic consequence of FDR gametes was examined by comparing the mean and genotypic variance of six tuber traits of 4x-2x and 4x-4x progenies. Tuber appearance, eye depth, specific gravity and tuber size showed significant differences between the means of 4x-2x and 4x-4x progenies from either both or one of the diploid parents. The genotypic variances of these traits in 4x-2x progenies were reduced in various degrees from those observed in 4x-4x progenies. Major genes responsible for the inheritance of these traits are probably located close to the centromere. The two diploid parents may also carry alleles of different size and direction of non-additive genic effects in their 2n gametes. The 4x-2x progenies also outperformed the 4x-4x progenies with respect to the means of total and marketable yield. There were thus heterotic gene loci for high yield located close to the centromere. No marked reduction of genotypic variance, however, was observed. The results suggest that genes for yield would be scattered between the centromere and the site for maximum recombination. Received: 24 April 1996 / Accepted: 26 July 1996     

Fertilization fitness and offspring ploidy in 3x × 2x matings in potato

Abstract

The main objective of the current research was to study the reproductive behaviour of artificial triploid potato hybrids between wild Solanum commersonii and the cultivated potato Solanum tuberosum. When used in 3x × 2x crosses, triploids gave aneuploid progenies with somatic chromosome number ranging from 29 to 36. Fertilization fitness data suggested that the survival rate of gametes produced by the triploid parents may be related to their chromosome number. In addition, consistent with molecular data, our results indicated that fitness of gametes and chromosome number of progenies are influenced by the genome dosage of interspecific triploids. Since a main route to polyploidy formation is via 2n gametes and triploids, our study may contribute to a better understanding of polyploid plant reproduction, evolution and breeding.     

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.     

Inheritance and mapping of 2n-egg production in diploid alfalfa.

The production of eggs with the sporophytic chromosome number (2n eggs) in diploid alfalfa (Medicago spp.) is mainly associated with the absence of cytokinesis after restitutional meiosis. The formation of 2n eggs through diplosporic apomeiosis has also been documented in a diploid mutant of M. sativa subsp. falcata (L.) Arcang. (2n = 2x = 16), named PG-F9. Molecular tagging of 2n-egg formation appears to be an essential step towards marker-assisted breeding and map-based cloning strategies aimed at investigating and manipulating reproductive mutants of the M. sativa complex. We made controlled crosses between PG-F9 and three wild type plants of M. sativa subsp. coerulea (Less.) Schm. (2n = 2x = 16) and then hand-pollinated the F1 progenies with tetraploid plants of M. sativa subsp. sativa L. (2n = 4x = 32). As a triploid embryo block prevents the formation of 3x progenies in alfalfa because of endosperm imbalance, and owing to the negligible selfing rate, seed set in 2x-4x crosses was used to discriminate the genetic capacity for 2n-egg production. F1 plants that exhibited null or very low seed sets were classified as normal egg producers and plants with high seed sets as 2n-egg producers. A bulked segregant analysis (BSA) with RAPD (random amplified polymorphic DNA), ISSR (inter-simple sequence repeat), and AFLP (amplified fragment length polymorphism) markers was employed to identify a genetic linkage group related to the 2n-egg trait using one of the three F1 progenies. This approach enabled us to detect a paternal ISSR marker of 610 bp, generated by primer (CA)8-GC, located 9.8 cM from a putative gene (termed Tne1, two-n-eggs) that in its recessive form determines 2n eggs and a 30% recombination genomic window surrounding the target locus. Eight additional RAPD and AFLP markers, seven of maternal, and one of paternal origin, significantly co-segregated with the trait under investigation. The minimum number of quantitative trait loci (QTLs) controlling seed set in 2x-4x crosses was estimated by ANOVA and regression analysis. Four maternal and three paternal independent molecular markers significantly affected the trait. A paternal RAPD marker allele, mapped in the same linkage group of Tne1, explained 43% of the variation for seed set in 2x-4x crosses indicating the presence of a major QTL. A map of the PG-F9 chromosome regions carrying the minor genes that determine the expression level of 2n eggs was constructed using selected RAPD and AFLP markers. Two of these genes were linked to previously mapped RFLP loci belonging to groups 1 and 8. Molecular and genetic evidence support the involvement of at least five genes.     

Development of a ploidy series from a single interspecific Trifolium repens L.Ā . nigrescens Viv. F1 hybrid

 The objective of the current research was to generate a ploidy series of backcross progenies from a single triploid (2n=3x=24) Trifolium repens×T. nigrescens F₁ hybrid (3x H-6909-5). The 3x H-6909-5 plant was highly sterile and produced no seeds from approximately 3000 reciprocal backcrosses to both parental species. Chromosome doubling by an in vitro colchicine method resulted in a marked increase in fertility. Pollen stainability was increased from 9.9% in 3x H-6909-5 to an average of 89.2% (range 87.7–90.9%) in the three chromosome-doubled 6x H-6909-5 plants. Subsequent backcrosses of 6x H-6909-5 and interbreeding of backcross derivatives resulted in an array of fertile hybrids at 4x, 5x and 7x levels and some aneuploids. The occurrence of 7x BC₁F₁ progeny from the T. repens×6x H-6909-5 (4x×6x) cross is the first unequivocal evidence of functional female 2n gametes in white clover. Meiotic pairing in F₁ and BC₁F₁ progeny indicated the presence of allosyndetic pairing, suggesting that genetic exchange between the two species is possible. Received: 17 October 1996 / Accepted: 8 November 1996     

Analysis of the meiosis in the F_1 hybrids of Longiflorum × Asiatic(LA) of lilies(Lilium) using genomic in situ hybridization

Longiflorum and Asiatic lilies of the genus Lilium of the family Liliaceae are two important groups of modern lily cultivars.One of the main trends of lily breeding is to realize introgression between these groups.With cut style pollination and embryo rescue,distant hybrids between the two groups have been obtained.However,the F1 hybrids are highly sterile or some of them could produce a small number of 2n gametes,and their BC1 progenies are usually triploids.Dutch lily breeders have selected many cultivars...     

Genetic-cytoplasmic male sterility in progeny of 4x-2x crosses in cultivated potatoes

Summary Forty-three percent of the progeny from 4x × 2x crosses [Group Tuberosum cultivar x diploid hybrid (Groups Phureja or Stenotomum x Group Tuberosum haploid)] were male sterile. In contrast only four percent of the progeny from the reciprocal crosses were male sterile. Male sterility among the former progeny is presumed to result from the interaction of Tuberosum cytoplasm with dominant genes from the cultivated diploids, Groups Phureja and Stenotomum, an interaction known to occur in crosses of Tuberosum haploid x cultivated diploid species (2n = 2x = 24). The frequency of fertile progeny from the 4x × 2x crosses (57%) was significantly higher than that from the 2x × 2x crosses (Tuberosum haploid x cultivated diploid), (28%). The frequency of male fertility among progeny from different cultivars in 4x × 2x crosses varied from 31–82 percent. The difference between cultivars strongly suggests that some cultivars may have dominant male fertility restorer genes.     

Gene-centromere mapping by 4x-2x matings in potatoes

Summary Two of the four strands of a bivalent are recovered together in tetraploid progeny arising from 4x-2x matings. This provides a method for gene and centromere mapping. The cross pppp x Pp produced 62 nulliplex tetraploid individuals in a total of 951, i.e., 6.5%. The diploid clone was found to produce diplandrous gametes through first division restitution (FDR). The map distance P-centromere was estimated as 13.0 units, the limiting values at the 95% binomial confidence interval being 10.1 and 16.3. The mapping method is explained in detail and a formula is deduced to obtain genotypic series of 2n gametes under particular assumptions. The direction 4x×2x is advantageous, since high seed set diploid clones which give 99% tetraploid progeny, and whose method of diplandroid formation is known are already available. Diploid clones heterozygous for many markers and tetraploids nulliplex for the same markers are needed to fully exploit this method.     

Effects of 2n-pollen formation by first meiotic division restitution with and without crossover on eight quantitative traits in 4x-2x potato progenies

 The 2n-pollen grains formed by first-division restitution without crossover (FDR-NCO) are unique breeding tools, since they can transmit almost 100% of non-additive genetic effects from the parent to the progeny. FDR-NCO gametes are considered superior to those formed by FDR with crossing over (FDR-CO), which can pass on to the progenies approximately 80% of the heterozygosity and a large fraction of the epistasis. However, 2n-pollen formation by FDR-NCO mechanism requires the incorporation (in homozygous condition) of at least two recessive alleles. In the present work, 40 tetraploid families derived from complete 4x-2x factorial crosses were evaluated under short-day conditions to verify whether or not the postulated genetic superiority of FDR-NCO over FDR-CO gametes holds true for eight quantitative traits in potato. Families were derived from crosses between four 4x commercial cultivars, and a random sample of ten diploid Solanum phureja-haploid S. tuberosum hybrids producing 2n-pollen by either FDR-CO or FDR-NCO. The results indicated no significant superiority of FDR-NCO over FDR-CO families for total tuber yield (TTY) and six other traits (haulm maturity – HM; plant vigor – PV; plant uniformity – PU; eye depth – ED; number of tubers per hill – NTH; and commercial over total yield index – CTI). Based upon cytological observations, the FDR-CO and FDR-NCO gametes are expected to be genetically equivalent for all loci between the centromeres and the chromosomal site of maximum recombination. In our experiment, differences between FDR-CO- and FDR-NCO-derived progenies were not observed for TTY. Therefore, our results can be interpreted as additional evidence for the hypothesis that genes with major effect on TTY expression might have a physical location between centromeres and proximal crossovers in the potato chromosomes. In addition, a similar trend was observed for HM, PV, and ED but apparently not for commercial yield – CY (i.e., tubers with more than 33 mm in diameter). Received: 21 September 1998 / Accepted: 26 October 1998     

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.