4x-2x Crosses Research Articles

Evaluation of yield and quality traits in Russet-Chipper and 4x-2x crosses in potato (Solanum tuberosum L.)

Evaluation of yield and quality traits in Russet-Chipper and 4x-2x crosses in potato (Solanum tuberosum L.)

Study on reproductive barriers in 4x–2x crosses in Dianthus caryophyllus L

Reproductive barriers often exist in 4x–2x crosses in Dianthus caryophyllus L. and seriously lead to low seed set and breeding efficiency. In this study, we inspected pre-fertilization and post-fertilization barriers in crosses between ‘butterfly’ (2n = 4x = 60) and the fertile breeding line ‘NH10’ (2n = 2x = 30) and ‘NH14’ (2n = 2x = 30) using fluorescence microscopy and paraffin section. The growth of the pollen tubes up to the ovary was observed in crosses of ‘butterfly’ × ‘NH10’ and ‘butterfly’ × ‘NH14’. However, abnormalities of pollen tube including coiled and spiky and callose deposition in incompatible tubes has also been detected. In the cross ‘butterfly’ and ‘NH10’, the number of pollen tubes at the base of the style reached the highest value, 67.2 ± 10.8 at 48 h after pollination, but the number of ovules with micropylar penetration was low, 19 ± 4.5. Similarly, in the cross between ‘butterfly’ and ‘NH14’, pollen tubes at the base of the style at 48 h after pollination was 51 ± 6.2 and the ovules with penetrated micropyles per ovary at 48 h after pollination was 11 ± 2.4. It indicated the pollen tubes difficultly penetrated into the ovules in 4x–2x crosses. In addition, it was observed that non-fused of egg and sperm or non-fused polar nucleus resulted in endosperm without embryos, and embryos without endosperm. Embryos aborted at different developmental stages (prembryo to cotyledon embryo, main globular embryo), which might be related to a deficient endosperm development in reciprocal crosses. The results suggested that pre-fertilization and post-fertilization barriers were detected in these combinations. Due to the post-fertilization barriers in 4x–2x crosses, the embryo rescue might be contributed to obtain hybrid F1 plants.

Production and Identification of Colchicine-derived Tetraploid Vaccinium darrowii and Its Use in Breeding

In Vaccinium L., most tetraploid hybrids between tetraploid cultivated highbush blueberry (V. corymbosum L.) and diploid darrow's evergreen blueberry (V. darrowii Camp) have been produced by exploiting V. darrowii's tendency to produce 2n gametes and a strong triploid block, which greatly reduces the number of triploid hybrids produced. Colchicine-derived tetraploids offer an alternative method of producing V. darrowii plants that will easily cross with tetraploid Vaccinium species. V. darrowii (2n = 2x = 24) seeds were imbibed in 0.2% aqueous colchicine solution for 24 h. The seeds were germinated and seedlings whose morphology suggested colchicine effects were selected at various stages of development. No macromorphological changes were consistently associated with chromosome doubling. However, stomatal guard cells and pollen size increased substantially as a result of chromosome doubling. Several types of plants were identified after colchicine treatment: 1) plants with a doubled LI (epidermal tissues) and LII (internal tissues) plant layers; 2) periclinal chimeras with a doubled LI layer and a normal LII; and 3) periclinal chimeras with a doubled LII layer and a normal LI. Of ≈4000 seedlings that emerged from colchicine-treated seeds, 200 were selected for further examination based on leaf and stem morphology. Of the 200, five appeared to be tetraploid in LI, LII, or both layers based on stomatal guard cell size and pollen size. Crosses between colchicine-derived V. darrowii (4x) plants and southern highbush blueberry (V. corymbosum) cultivars (4x) were successful compared with 4x–2x and 2x–4x crosses using diploid V. darrowii and tetraploid southern highbush blueberry. Stomatal guard cells and pollen screening of the colchicine-treated plants were used as indicators of doubled V. darrowii plants and periclinal chimeras. The results from the crosses between colchicine-treated V. darrowii plants and tetraploid highbush blueberry cultivars confirmed the information obtained by stomata and pollen screening.

Interspecific Crosses and Backcrosses between Diploid Vaccinium darrowii and Tetraploid Southern Highbush Blueberry

Diploid Vaccinium darrowii Camp has been used in breeding tetraploid southern highbush blueberry (Vaccinium corymbosum L.) as a source of reduced chilling requirement, adaptation to hot, wet summers, and resistance to leaf diseases. V. darrowii in Florida is quite variable, but most crosses have involved only one V. darrowii clone, Fla. 4B. The use in breeding of a wider range of V. darrowii accessions would provide beneficial diversity in the blueberry cultivated gene pool. The purpose of this research was to determine the functional 2n gamete frequency of numerous V. darrowii genotypes when crossed with tetraploid V. corymbosum, and to study the pollen fertility and backcross ability of the interspecific (V. darrowii × V. corymbosum) hybrids to V. corymbosum. Crosses between diploid V. darrowii and tetraploid highbush blueberry cultivars had low fruit set compared with the V. darrowii × V. darrowii and highbush × highbush crosses. The unusually high number of hybrids per pollinated flower (HPF) in certain 4x-2x or 2x-4x crosses was attributed to high functional 2n gamete production in certain V. darrowii genotypes. Diploid Vaccinium fuscatum Aiton and diploid V. darrowii × V. fuscatum hybrids, when crossed with southern highbush blueberry cultivars, were equally productive of hybrids whether used as male or female parents. Variation in frequency of functional 2n gametes in V. darrowii, expressed as high HPF, was present within plants (megaspores vs. microspores) and among V. darrowii plants. Of the 114 interspecific (V. darrowii × V. corymbosum) hybrids studied, 106 had pollen stainability >50%. This indicated that most of these hybrids were tetraploid, because triploid blueberries, like most triploid plants, are highly sterile. Twenty-two V. darrowii × V. corymbosum hybrids were backcrossed to tetraploid highbush blueberry cultivars. Fruit set was variable, but large populations of vigorous hybrids were obtained. Lower fruit set was associated with hybrids that had lower pollen fertility. It should be possible to obtain plants of cultivar quality in a few generations of backcrosses.

Heritability of Resistance to Common Scab in Diploid Potatoes

Common scab, caused by several species of Streptomyces, is a serious soil-borne disease of potatoes that may cause superficial, raised or pitted lesions on potato tubers. Little is known about the genetic basis of resistance to this disease. The purpose of this study was to determine if genetic resistance to common scab exists in a diploid Solanum phureja-S. stenotomum (phu-stn) population and to estimate broad-sense (H) and narrow-sense (h2) heritability for resistance to common scab. Two hundred seventy-one clones from a randomly mated phu-stn population were grown from 2003–2005 in naturally Streptomyces infested soil on a grower’s field in Northampton Co., PA, in a randomized complete block design replicated twice. After harvest, the proportion of tubers with scab lesions was determined. The variable analyzed by the general linear models procedure in SAS using the maximum likelihood estimation method was the arcsine ( √ proportion of scabby tubers). There were significant differences among clones and the year x clone interaction was significant. H for resistance was estimated as 0.18 with a 95% confidence interval of 0.15 to 0.35. The clonal source of variation was partitioned into among families and within families. All the variation occurred within families. There was no additive genetic variance for resistance in this population, hence, h2 was estimated as 0.00. Although levels of resistance to common scab cannot be improved by breeding in this diploid population, it may be feasible to transfer the high levels of resistance in this population to the tetraploid level via 4x-2x crosses.

Non-additive genetic variance governs resistance to fusarium dry rot in a diploid hybrid potato population

Fusarium dry rot, caused by severalFusarium species, is a major storage disease of potatoes for which there is no fungicidal control. Levels of resistance in commercial potato germplasm are inadequate. The purpose of this study was to determine the inheritance of resistance to Fusarium dry rot in a diploid hybridSolanum phureja-Solanum stenotomum population. Three tubers from each of four half-sibs from each of 38 diploid families were inoculated with a mixture of two isolates ofFusarium sambucinum and one isolate of Fusarium solani four times in both 2003-2004 and 2004-2005. Tubers were then incubated for 40 days at 15 C and 90% relative humidity. The surface diameter and depth of infected tissue were measured in two directions, at right angles to each other. Depth of infected tissue was determined by cutting the tuber in half through the inoculation point. Mean depth and diameter of infected tissue were analyzed. There were significant differences among clones. The experiment x clone interaction was also significant. Broad-sense heritabilities and their 95% confidence intervals for resistance to Fusarium dry rot in this population were estimated as 0.63 (0.50, 0.71) and 0.81 (0.76, 0.86) in 2003-2004 and 2004-2005, respectively, for mean diameter; and as 0.68 (0.57, 0.75) and 0.81 (0.75, 0.86) in 2003-2004 and 2004-2005, respectively, for mean depth. Narrow-sense heri-tabilities for mean diameter and depth were not significantly different from zero either year. Although there is genetic variation for resistance to Fusarium dry rot in this population, these results indicate that additive genetic variance is lacking or minimal, and therefore little or no genetic gain in resistance will be realized. A few highly resistant clones could, via 4x-2x crosses, theoretically transfer much of the dominance and epistatic variance governing resistance in this population to the tetraploid level.

Ploidy and Genome Segregation in Musa Breeding Populations Assessed by Flow Cytometry and Randomly Amplified Polymorphic DNA Markers

The majority of edible bananas (Musa L.) are triploid (2n = 3x = 33) cultivars known as dessert, plantain, or cooking bananas with predominant AAA, AAB, and ABB genomes, justifying the aim of breeding programs to develop improved hybrids of the different utilization classes in the triploid background. This usually involves crossing 3x landraces to 2x accessions that are donors of resistance genes, selecting 4x and 2x primary hybrids from the 3x-2x progenies, and crossing 4x-2x hybrids to produce secondary 3x hybrids. Ploidy and genome segregation routinely occurs during this process and constitutes a major hindrance for selection because this is not easily detectable at the morphological level. This study was carried out to assess the potential of early screening for discriminating ploidy and genome classes in 4x (AAAB)-2x (AA) breeding populations, based on flow cytometry and genome-specific RAPD analysis. Results indicate that progenies of 4x-2x crosses produced predominantly 3x progenies (94.1%). RAPD analysis identified seven genomic classes, dominated by AAA and AAB configurations segregating 1:1 (χ2 = 0.21, P = 0.05). Plant size and bunch weight increased with ploidy level, justifying ploidy-based grouping of progenies for homogenous field evaluation. Similarly, genome-based grouping within ploidy classes is recommended to allow proper evaluation of progenies according to targeted postharvest utilization. Thus, ploidy analysis by flow cytometry and genome determination by RAPD open prospects for enhancing breeding efficiency in the development of triploid bananas or plantains.

Ploidy variation of Musa hybrids from crosses

Plantain and banana (Musa spp) breeding involves crossing 3x (triploid) landraces to 2x (diploid) accessions as female and male parents, respectively, selecting 4x (tetraploid) and 2x primary hybrids from the 3x - 2x progenies, and crossing 4x - 2x hybrids to produce secondary 3x hybrids. In these crosses, complex ploidy and genome arrays occur frequently making it difficult to predict the products of crosses. The objective of the study was (a) to determine the ploidy of progenies of 2x – 2x, 2x – 4x and 4x – 2x crosses, and (b) to assess the agronomic characteristics of diploid and triploid progenies recovered from the same or similar crosses. The breeding population involved progenies of interploidy crosses field established in a randomized complete block design (RCBD) with three replications. Ploidy was determined using Flow cytometry analysis of nuclear DNA content and chloroplast characteristics. Results indicated that progenies of 2x–2x crosses were predominantly diploid (99.7%), those of 2x–4x crosses were mainly diploid (96.2%), while the 4x–2x crosses produced predominantly triploid progenies (94.1%). Very highly significant differences (P guard cells of progenies. Diploid hybrids recovered from crosses had shorter days to flowering, plant height and bunch weight compared to triploid hybrids

Resistance to soft rot (Erwinia carotovora subsp.atroseptica) in tetraploid potato families obtained from 4x-2x crosses

Diploid potato clones, interspecific hybrids ofSolanum species, having in their originS. tuberosum,S. chacoense, S. yungasense, S. phureja, S. gourlayi, andS. demissum, with resistance to soft rot, were crossed to tetraploid potato clones in 4x-2x crosses. The 24 tetraploid families obtained in a North Carolina II design were examined for tuber resistance to soft rot in a laboratory test and for basic agronomic traits in field trials conducted for two consecutive years. In addition, one family originating from a 4x-2x cross of two susceptible parents was tested. Statistical analysis of the data revealed significant general (GCA) and specific combining ability (SCA) effects, year, GCA (female) × year, GCA (male) × year, and SCA × year upon the inheritance of resistance to soft rot. About 35% of the progeny was selected as resistant to tuber soft rot, and of these 11% showed high resistance combined with good tuber yield, tuber weight, and tuber appearance. The relationships between resistance to soft rot and chosen agronomic traits were not noted or were weakly significant and sporadic. The resistance to tuber soft rot found in diploid potato hybrids can be transferred to the cultivated tetraploid pool through 4x-2x crosses, and a high frequency of offspring posses resistance.

The inheritance of resistance to soft rot (Erwinia carotovora subsp.atroseptica) in diploid potato families

Soft rot of potato tubers, caused byErwinia (Pectobacterium), is a serious disease affecting potato crops during storage. Studies on the inheritance of resistance to tuber soft rot were undertaken on diploid potato hybrids. A total of 480 clones, derived from 12 families, were examined for resistance to soft rot in laboratory tests over 3 years and for basic agronomic traits in field experiments over 2 years. General (GCA) and specific (SCA) combining abilities were significant in the inheritance of resistance to soft rot; however, GCA for female parents were significant in 1 or 2 years during 3 years of evaluation and SCA was significant for eight, two, and six families out of 10 tested in 1994, 1995, and 1996, respectively. There were also significant differences between years. Moreover, marked interactions for GCAfemales × year and SCA × year in the variation of soft rot resistance were found. Broad-sense and narrowsense heritability of resistance, measured as diameter of rotten tissue, was estimated as 0.92 and 0.89, respectively. The maternal effect, evaluated in two sets of reciprocal crosses, was not significant for the genetic determination of resistance to soft rot. No significant relationships were found between resistance to soft rot and the main agronomic traits in the tested diploid families. These results suggest that diploid potato clones with resistance to soft rot can be selected after being evaluated over a few seasons. It is also possible to select clones combining good resistance to soft rot with high yield, superior tuber characteristics, and acceptable starch content. These resistant diploids can be used as male parents in 4x-2x crosses in breeding tetraploid potato resistant to soft rot.

체세포 염색체 배가법을 이용한 백합 OA 종간 잡종의 불임극복과 후대획득

This study was carried out to develop the efficient methods for the subsequent progeny of intersectional hybrid between Oriental and Asiatic lily hybrids. The pollen fertility and germination ability of 3 different allotetraploids (OAOA) after somatic chromosome doubling was ranged from 0 to 80 percent on artificial pollen germination medium. The number of BC progeny using allotetraploid of F OA-hybrid as male and female parent was different. The efficiency of BC progeny production was increased when FOA-hybrids was used as male rather than as female parent. And in back crosses of F OA-hybrids with the Asiatic and Oriental hybrids, Asiatic hybrids showed higher efficiency on BC progeny production. The ploidy level between 2x or 4x Asiatic hybrid and allotetraploid F OA-hybrid was determined and showed higher progeny production in 2x-4x crosses rather than 4x-4x.

GENOTYPE BY ENVIRONMENT INTERACTION AND SELECTION IN TRUE POTATO SEED BEEDING

The aim of this work was to determine the importance of the genotype-by-environment interaction in true potato seed breeding. In order to establish the importance of this type of interaction, experiments were sited across three Peruvian locations, San Ramon (warm mid-altitude slopes), Huancayo (cool Andean highlands), and La Molina (coastal desert). The genotype by environment interaction significantly affected tuber yield in all breeding materials (4x-4x from random or selected parents, and 4x-2x crosses). For example, the 10 most outstanding 4x-2x crosses for tuber yield were different when tested in warm and cool locations. Similarly, two clones derived from 4x-4x crosses were among the best 15 hybrids assessed in the two lower altitude locations. These two offspring provide examples of crossover genotype-by-environment interaction for tuber set, non-cross order genotype-by-environment interaction for tuber yield and lack of genotype-by-environment interaction for days to flowering. Because top yielding true potato seed offspring for each location were locally adapted, cultivars or advanced selections from the target area should be the parental sources of true potato seed offspring.

Evaluation for chipping and tuber soft rot (Erwinia carotovora) resistance in potato clones from unilateral sexual polyploidization (2x × 4x)

The aim of this work is the evaluation of tetraploid potato clones with germplasm fromS. chacoense andS. phureja for chipping and resistance to tuber soft rot (Erwinia carotovora subsp.carotovora). Tetraploid potato clones were obtained via 2n eggs through 2x–4x crosses. Cytological mechanisms were investigated in the diploid hybrid 2n-egg producer used as parent in the unilateral sexual polyploidization scheme; the omission of the second meiotic division and cytokinesis failure at the end of meiosis were identified with a genetic consequence equivalent to second division restitution (SDR). Tuber marketable yield, specific gravity and chip-processing ability evaluated at harvest and after cold storage with and without reconditioning were higher in some clones as compared to theS. tuberosum tetraploid parent. Tuber soft rot resistance, contributed byS. chacoense, was successfully transferred to the4x level. After inoculation withErwinia carotovora subsp.carotovora and estimation of the rotted area, one tetraploid clone proved to be resistant and three partially resistant. The genetic value of exotic germplasm and sexual polyploidization strategy are discussed in relation to potato improvement.

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.

Multitrait selection system using populations with a small number of interploid (4x-2x) hybrid seedlings in potato: Degree of high-parent heterosis for yield and frequency of clones combining quantitative agronomic traits

One of the most attractive features of the 4x-2x breeding scheme via unilateral sexual polyploidization (USP) is the possibility of obtaining heterotic clones with a combination of desirable traits based upon selection in a small number of hybrid seedlings. A set of experiments was carried out to verify this characteristic of the USP scheme using 42 families (with 20 plants each) derived from 2x Phureja-haploid Tuberosum or haploid Tuberosum-Solanum chacoense hybrids as male parents. These clones were 2n-pol-len-producers by either first-division restitution with crossing over (FDR-CO) or without crossing over (FDR-NCO). The 4x parents were eight cultivars of USA and European origin. A total of 168 out 840 clones was initially selected (four clones/family) based on general tuber appearance (GTA). An additional round of selection (not taking into account the parentage of the clone) for total tuber yield (TTY) reduced the sample to 96 clones. These selected clones and the original 4x parents were evaluated in two locations at Wisconsin (Hancock – E?1, and Rhinelander – E?2). The average high-parent heterosis values for TTY ranged from 27.8% (E?2) to 48% (E?1). The population of experimental clones also had a TTY range wider than that of the 4x parents. The best clone gave a yield of 101.6% and 63.2% over the best 4x parent at E?1 and E?2, respectively. For TTY, a significant genotype×environment (G×E) interaction for the experimental clones was found when a combined analysis of variance for both locations was carried out. However, the G×E interaction was not significant when only the 4x-parent group was considered. Clones derived from 4x European cultivars had higher GTA scores than clones derived from 4x USA cultivars. With two culling levels being set on the TTY≥4x parent group mean and the GTA≥three, about 56% (E?1) and 48% (E?2) of the clones would be retained for further evaluation. These percentages of selected clones are much higher than those reported using conventional 4x-4x crosses. Our results indicate that a USP strategy with 4x-2x (FDR) crosses would be more effective than intra-Tuberosum crosses in generating heterotic clones with a combination of desirable quantitative traits using populations with a small number of hybrid seedlings.

Chromosome regions between centromeres and proximal crossovers are the physical sites of major effect loci for yield in potato: genetic analysis employing meiotic mutants.

Meiotic mutant (2n) gametes formed by first-division restitution without crossover (FDR-NCO) are expected to be superior to FDR with crossover (FDR-CO) because they transmit to the progeny, without disruption by recombination, almost 100% of the parental genotype. FDR-CO transfers approximately 80% of the parental heterozygosity and a large fraction of the epistatic interactions. Another genetic expectation associated with both FDR gametes is their equivalence for the phenotypic expression of traits controlled by genes residing between centromeres and proximal crossover sites. This set of unique cytogenetic features of FDR mutants was employed here as a tool to infer physical location of quantitative trait loci controlling total tuber yield (TTY) in potato. Two assays were conducted to verify the superiority of FDR-NCO over FDR-CO gametes for TTY by using progenies from 4x-2x factorial crosses. Male clones were 2n-pollen producers by either FDR-CO or FDR-NCO mechanisms. Compared with the 4x parents, TTY of the progenies ranged from 41% to 175% (i.e., high-parent heterosis). However, no significant TTY differences were observed between FDR-CO and FDR-NCO families. In addition, the size of variance components of males was smaller than females and near zero. Our results reinforce the hypothesis that genes controlling yielding ability have a predominant physical location between centromeres and proximal chiasmata. Quantitative trait loci in chromosome regions with reduced levels of recombination may provide a partial explanation for the slow progress in increasing TTY through conventional 4x-4x crosses and for the often high degree of heterosis obtained by introgressing genetic diversity via 4x-2x crosses in potato.

Comparison of haploid Tuberosum- Solanum chacoense versus Solanum phureja -haploid Tuberosum hybrids as staminate parents of 4x–2x progenies evaluated under distinct crop management systems

The relative value of haploid Tuberosum-Solanum chacoense (TCH) vs. Solanum phureja-haploid Tuberosum (PTH) hybrids as male parents in 4x–2x crosses was estimated under two distinct crop management systems. The first experiment (E#1) was carried out with supplementary irrigation (three times a week) at Hancock-Wisconsin (WI) with 27 families derived from a sub-set of crosses involving 11 tetraploid (4x) cultivars and 10 diploid (2x) clones (3 TCH; 6 PTH; and one TCH-PTH hybrid clone). A second experiment (E#2) was conducted at Rhinelander-WI with the same group of families but it was only rainfed with no supplementary irrigation being provided during the entire course of the assay. For comparison, a complete set of 4x and 2x clones (used as parents) was also planted in each location. Three traits were evaluated in both locations: total tuber yield (TTY), haulm maturity (HM), and plant vigor (PV). Altogether, 18 out of 27 and 10 out of 27 families outyielded the corresponding 4x parents at E#1 and E#2, respectively. Significant differences among all 4x–2x families and among families of the TCH group were observed for TTY at both locations. No difference was found for TTY among families of PTH and [TCH–PTH] groups at each location. A total of 75% and 50% of the TCH families outyielded their 4x parents at E#1 and E#2, respectively. The percentage of families from the PTH group that outyielded their 4x parents at E#1 and E#2 was 83.3% and 25%, respectively. The TCH group had on average lower HM scores than PTH (at both locations) indicating some segregation for earliness. However, the HM values of the 4x–2x families were, in general, higher than those observed for the 4x parents. For PV, the PTH group mean was significantly higher than TCH group only in E#1. The PV values of the 4x–2x families were higher than the 4x-parent group at both locations. Our results indicated that TCH hybrids might be as good parents as PTH to increase the TTY of 4x–2x progenies. In addition, the TCH families displayed a slight higher level of performance under more stressful growing conditions than PTH hybrids as indicated by the assay at Rhinelander. Therefore, selection of genetic materials with potential broad range of adaptation seems to be feasible with both hybrids via the 4x–2x breeding scheme.

Numerically unreduced (2n) gametes and sexual polyploidization in Actinidia

This is the first report on the production of numerically unreduced (2n) gametes in Actinidia species. Their occurrence was apparent from direct observations of microporogenesis in 2x, 3x, 4x and 6x plants and was inferred from ploidy analysis of progenies from 4x-2x and 2x-4x crosses. Both male and female plants were found to produce 2n gametes. 2n gamete producers were detected only in interploidy crosses. The observed frequency of 2n gametes was either similar to or towards the lower end of the ranges reported for other genera. The role of sexual polyploidization in the evolution of the genus is discussed.

Transmission of resistance to common scab from the diploid to the tetraploid level via 4x-2x crosses in potatoes

Diploid potato clones selected for their reaction to common scab and their ability to produce 2n male gametes were used in a series of crosses to a susceptible tetraploid female parent (cv. Shepody). In addition, two tetraploid clones were also selected for their reaction to common scab and crossed with Shepody as a female parent. Results indicated that resistance to common scab can be effectively transmitted from the diploid to the tetraploid level via 4x-2x crosses. Diploid parents producing 2n pollen via either first division or second division restitution can be used to transmit scab resistance. A relatively small proportion of resistant individuals could be recovered from susceptible x susceptible crosses in both 4x-2x and 4x-4x combinations. The data support a previously developed hypothesis that scab resistance is relatively simply inherited.

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.