Unilateral Sexual Polyploidization Research Articles

DINÁMICA EVOLUTIVA DE AUTOPOLIPLOIDES EN POBLACIONES NATURALES: EL CASO DEL COMPLEJO TURNERA SIDOIDES

Turnera sidoides (x=7) is one of the few well-studied South American autopolyploid complexes. Since polyploidy has played a prominent role within this complex, ongoing studies in T. sidoides focus on understanding the mechanisms involved in the origin and the establishment of polyploids using integrative approaches. This paper synthesises the results of more than 20 years of research on this topic. Cytogenetics analysis provided evidences for the production of unreduced male and female gametes, supporting the hypothesis of bilateral sexual polyploidization as the mechanism of origin of polyploids in T. sidoides. The finding of viable triploids suggested that unilateral sexual polyploidization could also be an important mechanism for the origin of tetraploids in T. sidoides. The occurrence of plants continuously forming many unreduced gametes would play a key role in the establishment of neopolyploids in natural populations. Also, the higher number of propagules that tetraploids contribute to subsequent generations, the ability to multiply asexually by rhizomes, and the occurrence of occasional cases of self-compatibility and successful illegitimate crosses in polyploids increase the likelihood that a low frequency of neopolyploids can be maintained in natural populations of T. sidoides. In addition, integration of cytogeographic and genetic divergence data together with past niche modelling provided further insights supporting the hypothesis that historical climatic and geomorphological events provided favourable conditions for the establishment of autopolyploids, with the wider distribution of tetraploids of T. sidoides being the result of their range expansion. Key words: cytogeography, establishment, origin, polyploidy, unreduced gametes

Disaggregation of Ploidy, Gender, and Genotype Effects on Wood and Fiber Traits in a Diploid and Triploid Hybrid Poplar Family.

Triploid breeding based on unilateral sexual polyploidization is an effective approach for genetic improvement of Populus, which can integrate heterosis and ploidy vigor in an elite variety. However, the phenotypic divergence of unselected allotriploids with the same cross-combination remains poorly understood, and the contributions of ploidy, gender, and genotype effects on phenotypic variation are still unclear. In this study, wood and fiber traits, including basic density (BD), lignin content (LC), fiber length (FL), fiber width (FW), and fiber length/width (FL/W), were measured based on a 10-year-old clonal trial, including full-sib diploid and triploid hybrids of (Populus pseudo-simonii × P. nigra ‘Zheyin3#’) × P. × beijingensis, and contributions of ploidy, gender, and genotype effects on the variation of these traits, were disaggregated to enhance our understanding of triploid breeding. We found a significant phenotypic variation for all measured traits among genotypes. All the wood and fiber traits studied here underwent strong clonal responses with high repeatabilities (0.55–0.76). The Pearson’s correlation analyses based on the best linear unbiased predictors (BLUPs) revealed that BD was significantly positively correlated with FL (r = 0.65, p = 0.030), suggesting that BD could be improved together with FL during triploid breeding. The FL of the triploids was significantly larger than that of the diploids (p < 0.001), suggesting that ploidy strongly affected the variation of FL traits. The difference between females and males was not significant for any measured trait, implying that gender might not be a major factor for variation in these traits. Further analyses of variance components showed that genotype dominantly contributed to the variation of BD, LC, and FW traits (with 54, 62, and 53% contributions, respectively) and ploidy contributed strongly to variation in FL and FL/W (77 and 50%, respectively). The genetic coefficient of variation (CVG) of triploids for each trait was low, suggesting that it is necessary to produce many triploids for selection or to use different Populus species as parents. Our findings provide new insights into the genetic effects of ploidy, gender, and genotype on wood and fiber traits within a full-sib poplar family, enhancing the understanding of the triploid breeding program of Populus.

Cytological Study of 2n Pollen Formation in Musa

Current Musa breeding strategies are complex and time consuming involving the selection of tetraploids from 3x - 2x crosses. Secondary triploids are then obtained by crossing these tetraploids with diploids. Considering the very low hybrid seed set, routine embryo rescue procedures of hybrid seeds and the long growth cycle of banana, it takes approximately 10 - 12 years to produce an acceptable banana hybrid. The banana breeding process could benefit tremendously if triploid bananas could be obtained directly from 2x - 2x crosses through the process of unilateral sexual polyploidization. There are few reports on the mechanisms through which Musa species produce 2n pollen. This study investigated the type of meiotic irregularities that lead to 2n pollen formation in diploid, triploid and tetraploid Musa accessions using cytological analyses. The results showed that aberrations in cytokinesis and karyokinesis during microsporogenesis are possible mechanisms for 2n pollen formation in Musa. The meiotic aberrations described in this study have implications for Musa breeding. It appears that 2n pollen formation in Musa occurs via both first division restitution (FDR) and second division restitution (SDR). FDR is said to be more promising in transferring more heterozygosity from parents to offspring.

Comparison of Two Strategies to Introgress Genes for Resistance to Common Scab from Diploid Solanum chacoense into Tetraploid Cultivated Potato

Wild potato relatives are important sources of novel genes useful for potato improvement. Since most wild potato species are diploid and most cultivars are tetraploid, it is typically necessary to employ ploidy manipulations to access wild potato germplasm. A common strategy is unilateral sexual polyploidization (USP), in which a tetraploid cultivated clone is crossed with a diploid that produces 2n gametes. The diploid parent may be a wild plant selected for a trait of interest. Alternatively, the wild plant may be crossed to a cultivated diploid and, after selection, the diploid hybrid is then crossed to the tetraploid parent. This study was designed to compare these two strategies for the introgression of common scab resistance from wild diploid Solanum chacoense (chc) into cultivated potato. Progeny were evaluated in naturally scab-infested soil in Maine and Michigan. There were no differences between introgression methods for tuber surface area covered by scab. Scab lesions were significantly more severe when a hybrid was used as the diploid parent, but this difference is likely not large enough to be important in a breeding program. Reciprocal cross differences were not significant. However, location x clone interactions were detected for both surface area and lesion type. This study revealed that, while hybridization strategy did not play a major role in developing this scab resistant germplasm, it is critical to consider production environment when screening for resistance.

Germplasm Release: Three Tetraploid Potato Clones with Resistance to Common Scab

Common scab caused by the soil-borne bacterium Streptomyces scabies is a serious disease for the potato industry. We have identified a strong source of resistance in the diploid wild relative Solanum chacoense. This resistance has been introgressed into tetraploid cultivated potato via unilateral sexual polyploidization. This paper describes three hybrid clones (M8, M16, M17) for use by potato breeding programs to enhance resistance to common scab. They were created by crossing a diploid clone (50% S. tuberosum, 50% S. chacoense) to tetraploid cultivars, producing hybrids with 75% cultivated and 25% wild germplasm. The clones are male and female fertile, and are adapted to temperate zone production environments.

Ecological distributions, phenological isolation, and genetic structure in sympatric and parapatric populations of the Larrea tridentata polyploid complex.

Polyploidy is widely recognized as a mechanism of diversification. Contributions of polyploidy to specific pre- and postzygotic barriers-and classifications of polyploid speciation as "ecological" vs. "non-ecological"-are more contentious. Evaluation of these issues requires comprehensive studies that test ecological characteristics of cytotypes as well as the coincidence of genetic structure with cytotype distributions. We investigated a classical example of autopolyploid speciation, Larrea tridentata, at multiple areas of cytotype co-occurrence. Habitat and phenological differences were compared between diploid, tetraploid, and hexaploid populations on the basis of edaphic, community composition, and flowering time surveys. Frequency of hybridization between diploids and tetraploids was investigated using a diploid-specific chloroplast DNA (cpDNA) marker; genetic structure for all cytotypes was assessed using amplified fragment length polymorphisms (AFLPs). Across contact zones, we found cytotypes in habitats distinguished by soil and vegetation. We observed modest differences in timing and production of flowers, indicating a degree of assortative mating that was asymmetric between cytotypes. Nonetheless, cpDNA analyses in diploid-tetraploid contact zones suggested that ∼5% of tetraploid plants had hybrid origins involving unilateral sexual polyploidization. Genetic structure of AFLPs largely coincided with cytotype distributions in diploid-tetraploid contact zones. In contrast, there was little structure in areas of contact between tetraploids and hexaploids, suggesting intercytotype gene flow or recurrent hexaploid formation. Diploid, tetraploid, and hexaploid cytotypes of L. tridentata are segregated by environmental distributions and flowering phenology in contact zones, with diploid and tetraploid populations having corresponding differences in genetic structure.

Variation of DNA methylation and phenotypic traits following unilateral sexual polyploidization in Medicago

Sexual hybridization is an important generator of biodiversity and a powerful breeding tool. Hybridization can also overcome ploidy barriers when it involves 2n gametes, as in the case of unilateral sexual polyploidization (USP) that has been utilized in several crops, among which alfalfa. This research was aimed at gaining insights into the effects of USP on genome methylation and on phenotypic traits in alfalfa, an important forage species. The Methylation-Sensitive Amplified Polymorphism technique was used to estimate the cytosine methylation changes occurring in a tetraploid (2n = 4x = 32) USP progeny from crosses between a diploid Medicago sativa subsp. falcata genotype that produces 2n eggs and a cultivated tetraploid Medicago sativa subsp. sativa variety. De novo methylation or demethylation in the USP progeny were observed for 13% of the detected genomic sites, indicating that methylation changes can be relevant. USP plants showed larger surface area of the leaf epidermis cells than both parents, but this did not result in larger leaf size or higher plant biomass. They displayed significant higher ovule sterility than the tetraploid parent, but normal fertility was observed in crosses with unrelated male testers. We conclude that hybridization and sexual polyploidization resulted in novel variation in terms of remodeling of the methylation landscape as well as changes in phenotypic traits in alfalfa.

Origin of triploid Arachis pintoi (Leguminosae) by autopolyploidy evidenced by FISH and meiotic behaviour.

Polyploidy is a dominant feature of flowering-plant genomes, including those of many important crop species. Arachis is a largely diploid genus with just four polyploid species. Two of them are economically important: the cultivated peanut and A. glabrata, a tropical forage crop. Even though it is usually accepted that polyploids within papilionoid legumes have arisen via hybridization and further chromosome doubling, it has been recently suggested that peanut arose through bilateral sexual polyploidization. In this paper, the polyploid nature of the recent, spontaneously originated triploid cytotype of the tropical lucerne, A. pintoi, was analysed, and thereby the mechanism by which polyploids may arise in the genus. Chromosome morphology of 2x and 3x A. pintoi was determined by the Feulgeńs technique and the rDNA sites were mapped by FISH. To investigate whether polyploidization occurred by means of unreduced gametes, a detailed analysis of the microsporogenesis and pollen grains was made. The 2x and 3x plants presented 9m + 1sm and a satellited chromosome type 2 in each haploid genome. Physical mapping revealed a cluster of 18S-26S rDNA, proximally located on chromosome 6, and two 5S rDNA loci on chromosomes 3 and 5. Diploid plants presented 10II in meiosis while trivalents were observed in all triploids, with a maximum of 10III by cell. Diploid A. pintoi produced normal tetrads, but also triads, dyads and monads. Two types of pollen grains were detected: (1) normal-sized with a prolate shape and (2) large ones with a tetrahedral morphology. Karyotype and meiotic analysis demonstrate that the 3x clone of A. pintoi arose by autopolyploidy. The occurrence of unreduced gametes strongly supports unilateral sexual polyploidization as the most probable mechanism that could have led to the origin of the triploid cytotype. This mechanism of polyploidization would probably be one of the most important mechanisms involved in the origin of economically important species of Arachis, either by triploid bridge or bilateral sexual polyploidization.

Relevance of unilateral and bilateral sexual polyploidization in relation to intergenomic recombination and introgression in Lilium species hybrids

Sexual polyploids were induced in diploid (2n = 2x = 24) interspecific F1 hybrids of Longiflorum × Asiatic (LA) and Oriental × Asiatic (OA) Lilium hybrids by backcrossing to Asiatic (AA) parents as well as by sib-mating of the F1 LA hybrids. A majority of the BC1 progenies were triploid and the progenies from sib-mating were tetraploid or near tetraploids. Genomic in situ hybridization (GISH) technique was applied to assess the intergenomic recombination in the BC1 populations of LA and OA hybrids obtained after unilateral sexual polyploidization. A total of 63 LA (LA × AA and AA × LA) and 53 OA hybrids were analysed. LA hybrids were originated through the functioning of 2n gametes either as 2n eggs or 2n pollen while those of OA hybrids originated through functional 2n pollen of diploid OA genotype. In both type of crosses, a majority of the progenies had originated through First Division Restitution (FDR) mechanism of functional 2n gamete either with or without a cross over. However, there were nine LA- and four OA-genotypes where Indeterminate Meiotic Restitution (IMR) was the mechanism of 2n gamete formation. Based on GISH, total amount of introgression of Longiflorum and Oriental genome into Asiatic genome was determined. Most of the BC progenies exhibited recombination and the amount of recombination was higher in LA hybrids as compared to OA hybrids. Intergenomic recombination was also determined cytologically in the 16 plants of sib-mated LA hybrids where both parents had contributed 2n gametes. Based on these results the nature of interspecific lily hybrids obtained from uni- and bilateral sexual polyploidization leading to allotriploid and allotetraploid formation is discussed in the context of introgression and intergenomic recombination.

Field performance and resistance to tuber soft rot of potato clones derived from sexual polyploidization

Unilateral sexual polyploidization through 4x x 2x crosses is widely used to introgress genes from the diploid gene pool to the tetraploid cultivated potato Solanum tuberosum. Clones of eight 4x x 2x families, from crosses between four tetraploid cultivated potatoes and two diploid S. phureja - S. tuberosum hybrids, produc- ing 2n pollen, were evaluated for yield performances and resistance to tuber soft rot. Significant differences in terms of tuber yield and number were found between families and clones, with several clones outyielding their tetraploid parents. The source of variation due to females (4x females GCA) was significant for both tuber yield and number. A number of clones displayed inheritance of resistance to tuber soft rot. No relationship was found between agronomic traits and resistance to soft rot. Results suggest that the genotypes produced represent genet- ic material with good potentiality in the constitution of new genotypes with interesting trait combinations.

Unreduced gametes and neopolyploids in natural populations of Achillea borealis (Asteraceae)

Polyploidy is a major mechanism of speciation and adaptation, yet little is known about the origins of polyploids in natural species. I investigated gametic nonreduction and neopolyploid formation in natural tetraploid populations of Achillea borealis (Asteraceae), an autopolyploid complex consisting of tetraploid and hexaploid cytotypes. Cytological analyses of tetraploid populations revealed the occurrence of reduced (n=2x) as well as unreduced 'big' (2n=4x) and 'jumbo' (4n=8x) pollen grains, which were clearly distinguished by size. Production of unreduced pollen was monitored in two tetraploid populations in 1997 and 1998. Mean population-level frequencies of unreduced pollen ranged from 0.030 to 0.538%, with as few as one-third and as many as one-half of sampled plants producing unreduced grains. Eight individuals were found to produce >1% unreduced pollen, with highest observed frequencies of 7.0, 13.2 and 15.8%. Experimental crosses using high unreduced pollen producers as male parents generated viable seeds. However, the frequency of neohexaploids in the progeny of experimental crosses (0.388%) was similar to that observed in progeny of randomly selected, open-pollinated control parents (0.465%). These results suggest that unreduced eggs are the most likely source of new polyploids. In spite of the inefficiency of unreduced pollen in unilateral sexual polyploidization, the overall rate of neohexaploid formation (one in 233) was several orders of magnitude greater than estimates of genic mutation rates.

Transmission of Verticillium wilt resistance to tetraploid potato via unilateral sexual polyploidization

Verticillium wilt is a serious disease of potato and is caused by the soil-borne fungi Verticillium dahliae and V. albo-atrum. No major cultivar is resistant to this disease. Two diploid interspecific potato clones, C287 and C545, were previously identified with consistently high levels of Verticillium wilt resistance and are thought to have the same genotype for the loci that confer resistance to V. dahliae stem colonization. The purpose of this study was to determine whether resistance to V. dahliae stem colonization could be transferred to the tetraploid level in potato via unilateral sexual polyploidization (USP). Progenies in eight families obtained by crossing C287 and C545 to two tetraploid breeding clones, S438 and S440, and the cultivar ‘Atlantic’ were planted in a V. dahliae-infested field and a field without a significant V. dahliae infestation. Resistance was evaluated relative to C545 and C287. There were differences among families for stem colonization and yield. No differences between the mean stem colonization of C545 and C287 progeny were detected. Family differences due to the tetraploid parents indicate that they contributed resistance to the progeny. Stem colonization data from this experiment were consistent with the proposed complementary two-gene model for Verticillium wilt resistance in the diploid parents. Unilateral sexual polyploidization is an effective method for transferring V. dahliae stem colonization resistance to the tetraploid level.

Advantages of Wild Diploid Solanum Species Over Cultivated Diploid Relatives in Potato Breeding Programs

For breeding programs of the tetraploid potato (Solanum tuberosum), both wild and cultivated diploid relatives are valuable sources of genetic diversity. While both types of germplasm are used in breeding programs, there are several advantages to using wild relatives. Diploid relatives are typically crossed with haploids (2n = 2x = 24) from tetraploid S. tuberosum to improve daylength adaptation. Most haploids are male sterile, so they are typically used as female parents. Cultivated diploids, such as members of the Phureja Group, produce male sterile hybrids when crossed as females to haploids; wild relatives, such as S. tarijense, often produce male fertile hybrids. Tuber yield following crosses of haploids to cultivated or wild relatives is often high. However, cultivated relatives generally produce hybrids with a high set of small tubers; hybrids from wild relatives are variable, but many are similar to cultivars in tuber size and set. While tubers of hybrids from cultivated relatives are typically rough, with deep eyes and raised internodes, those from wild relatives are often smooth. Tuber dormancy in hybrids with cultivated relatives is generally short, while that in hybrids with wild species is longer, allowing for storage over winter. Finally, resistance to several major diseases and stresses has been found in wild species and their hybrids with S. tuberosum haploids. The desirable traits in hybrids are transmitted to tetraploids via unilateral sexual polyploidization (4x × 2x or 2x × 4x crosses in which the diploid parent produces 2n gametes). Wild Solanum species are recommended for use in potato breeding programs as sources of genetic diversity that can be adapted easily following hybridization with S. tuberosum haploids.

Occurrence, inheritance and use of reproductive mutants in alfalfa improvement

The widespread occurrence of 2ngametes (i.e. gametes with the somatic chromosome number) in the Medicago sativa-coerulea-falcata complex supports the concept that gene flow from diploid to tetraploid species occurs continuously in nature and plays a key role in alfalfa evolution. Breeders realized early that gene transfer between ploidy levels via 2n pollen and 2n eggs would have had potential use in cultivated alfalfa improvement. Cytological investigations provided insights into the types of meiotic abnormalities responsible for the production of 2n gametes. Alterations were defined as genetically equivalent to first (FDR) or second division restitution(SDR) mechanisms. For breeding purposes,data have proven that 2n gametes of the FDR type are more advantageous than those obtained by SDR for transferring parental heterozygosity and retaining epistatic interactions. The use of diploid meiotic mutants that produce 2ngametes is now recognized as one of the most effective methods available for exploiting heterosis and introgressing wild germplasm traits into cultivated tetraploid alfalfa via unilateral (USP) and bilateral sexual polyploidization (BSP) schemes. Both2n egg and 2n pollen producers could be used for direct gene transfer from wild diploid relatives into cultivated alfalfa by means of 2x-4x and4x-2x crosses. Although data have shown that forage yield improvement can be achieved when plants are sexually tetraploidized, problems related to reduced plant fertility and seed production remain largely unexplained. Apomixis has the potential of cloning plants through seed and thus provides a unique opportunity for developing superior tetraploid cultivars with permanently fixed heterosis and epistatic effects. A main goal in alfalfa breeding could be the introduction of functional apomixis (i.e. Apomeiosis and parthenogenesis) in cultivated alfalfa stocks. In the future, the efficiency of alfalfa breeding programs based on the use of reproductive mutants could be improved by direct selection at the genotype level using RFLPs and PCR-based markers. Suitable DNA markers and detailed linkage maps of alfalfa mutants should help to discover apomictic mutants and address basic genetic issues such as the extent of genomicre combination in polyploid hybrids and the effect of sexual polyploidization on heterosis. Molecular markers have recently been used in alfalfa for studying the inheritance of 2n gamete formation and identifying polymorphisms associated to genes involved in meiotic abnormalities. Molecular tagging of 2n egg and 2n pollen formation not only should explain the genetic control and regulation of these traits, but may also be an essential step towards marker-assisted selection of 2n gamete producers and implementation of USP and BSP breeding schemes. Future perspectives include strategies for the map-based cloning of genomic DNA markers,and screening of EST mini-libraries related to flowers at different developmental stages from meiotic mutants and wild-type scan lead to the identification of mRNAs and thus of candidate genes that control 2n gamete formation in alfalfa.

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.

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.

Reproductive behaviour of tetraploid alfalfa plants obtained by unilateral and bilateral sexual polyploidization

The objective of the present study was to evaluate the fertility of tetraploid plants obtained from diploid mutants that produced 2n gametes via bilateral (BSP) and unilateral sexual polyploidization (USP) schemes. Controlled matings between selected plants from BSP and USP were carried out according to a complete diallel. The level of male and female fertility of each plant was estimated within full-sib, half-sib and non-inbred crosses. Crosses with unrelated self-fertile and male-sterile testers were also performed. Pollen size and stainability were determined by acetocarmine staining. The number of ovules per ovary was counted and the callose deposition within ovules examined to evaluate embryo sac fecundity. Cross-fertility was generally much higher for USP than BSP plants (on average, 0.3 vs. 0.03 seeds per flower pollinated). Both male and female fertility were inversely related to the inbreeding level of cross combinations. The in vitro pollen germinability was positively correlated to male fertility of BSP plants. The level of in vivo embryo sac sterility was not correlated to female fertility in either BSP or USP plants. Female fertility was restored in both BSP and USP groups when plants were crossed with unrelated tetraploid testers. Male fertility also increased in USP plants but remained rather low in the BSP plants. Scaling up the ploidy level by means of 2n gamete union resulted in tetraploid plants with low fertility. In particular, the BSP process yielded virtually self-sterile and highly cross-sterile plants. The well documented positive effect of sexual tetraploidization on forage yield was accompanied by worsening of fertility traits.

Evaluation of FDR diploid and tetraploid parents in potato under two different day‐length environments

AbstractFive tetraploid clones of potato were crossed to each of eight diploid first division restitution (FDR) 2n pollen producers and four tetraploid males using a line × tester mating design. A total of 59 families were obtained and evaluated under long days at Rhinelander (USA). A subset of 49 families were grown under short days at four Peruvian locations. Tuber yield (kg per plant) of 4x‐2x families was similar to, or greater than, that of 4x‐4x families in short day environments. The diploid clones had breeding values greater than or equal to those of the tetraploid clones. Only families derived from FDR 2n pollinators combined high tuber yield with stability and high dry matter content. Reciprocal recurrent selection would be the best breeding scheme, in which the diploids are testers of the tetraploids and vice‐versa. Intra‐population improvement could be achieved through phenotypic recurrent selection. The best materials from both breeding pools could then be crossed to produce tetraploid hybrids through unilateral sexual polyploidization (4x‐2x crosses).

Occurrence and Inheritance of 2nPollen inMusa

Sexual polyploidization explains the formation of most polyploid crop species and provides means for their genetic improvement. This paper reports the occurrence and inheritance of 2 n pollen in diploid and polyploid Musa . Pollen samples of male fertile accessions available in the field gene bank of the International Institute of Tropical Agriculture were microscopically examined throughout a calendar year. Based on pollen diameter 2 n pollen producers were identified. The presence of 2 n pollen in diploid species suggests that unilateral sexual polyploidization (2 n×n ) could have been involved in the origin of triploid Musa species. Segregation data suggests that at least one dominant gene controls 2 n pollen production in Musa . Further introgression of desirable alleles from diploid species to polyploids could be achieved either through unilateral or bilateral (2 n×2n ) sexual polyploidization in Musa .

Potato germplasm development for warm climates: genetic enhancement of tolerance to heat stress

Complex potato hybrids were derived through unilateral sexual polyploidization between Solanum tuberosum cv. Atlantic and 11 diploid hybrids that produced 2n pollen through co-orientation of second division spindles. The hybrids represented the following genomic compositions: TAPB, TAPC, and TAPM where T = S. tuberosum, A = S. andigena, P = S. phureja, B = S. berthaultii, C = S. chacoense, and M = S. microdontum. The B, C, and M components of the hybrids had been selected from heat tolerant accessions. The heat tolerance and agronomic performance of the hybrids were assessed under both controlled environments and field tests. The hybrids exhibited good tuberization potential (tuber number, tuber weight and percent plants producing tubers) under heat stress conditions in controlled environments. Under severe heat stress conditions in the field in Israel, many of the hybrids tuberized when Atlantic failed to produce any tubers at all. Under more moderate heat stress in Virginia, the complex hybrids exhibited total tuber yield not significantly different from Atlantic, although the tuber set per plant was greater. However, the complex hybrids were more tolerant to heat necrosis and hollow heart than Atlantic. Total glycoalkaloids in field-grown tubers did not exceed the acceptable limit for 13 of 26 selections examined.