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Ram K. Birhman, Sylvain R. Rivard, and Mario Cappadocia

Using restriction fragment length polymorphism (RFLP) analysis, the genetic architecture of some anther-culture-derived S. chacoense Bitt. plants was studied, and their origins were elucidated. Our RFLP analyses showed that 1) several plants, even of different ploidy but otherwise genetically identical (clones), can be regenerated from callus originating from a single microspore and, conversely, that 2) some plants regenerated from single callus can have different genetic constitutions and, therefore, must have originated from two different microspore. These findings imply that previous anther culture efficiency estimates might have to be reconsidered.

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Zhigiang Zhu and Paul G. Thompson

The polymorphisms of phosphoglucose isomerase (PGI) in sweetpotato and I. trifida were examined. Horizontal starch gel electrophoresis was used to analyze leaf and pollen tissue of parents and progenies of 10 crosses. Analyses revealed that PGI was a dimeric enzyme system controlled by 5 loci. The segregation ratios did not suggest that PGI was a duplicate system and therefore did not indicate hexaploidy. Only 2 loci appeared to be present in I. trifida. No observed band was related to different ploidy levels in I. batatas and I. trifida. No linkage was identified among the loci.

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M.M. Hossain, H. Inden, and T. Asahira

Pollen morphology was examined in amphidiploid and amphidaploid interspecific hybrids between Brassica oleracea L. and B. campestris L. Pollen of the amphidiploid interspecific hybrids between B. oleracea var. capitata and B. campestris var. pekinensis, and between B. campestris var. chinensis and B. oleracea var. capitata, were significantly longer and wider than those of their diploid parents, presumably due to the phenotypic expression of the hybrid genomes and ploidy effects. The exine ridges and pores of the amphidiploids were well-developed and significantly larger than those of their diploid parents, but they were poorly developed in the amphihaploids.

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Perry E. Nugent and Dennis T. Ray

Since 1968, three spontaneous 4× melons (Cucumis melo L.) plants were discovered in our field or greenhouse plantings. Two were found in the cultivar Planters Jumbo and one in the virescent marker C879-52. Each of these 4× plants had rounded cotyledons, shorter internodes, thicker stems and leaves, more hairs, and smaller fruits, with larger stem and blossom scars, than their 2× counterparts. Also, their flowers, pollen grains, stomates, and seeds were larger. The discovery of a 4× virescent plant in 1987 allows easier germplasm transfer between ploidy levels. Morphological characteristics of 2× and 4× melons will allow identification without need for chromosome counts.

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Bill Rhodes, Xingping Zhang, Tom Garrett, Tom Platt, and Chao Fang

A selection of Congo produced fruit that were not infected by blotch (pathogen Acidovorax avenae subsp. citrulli) in a replicated trial interplanted with infected seedlings. Ninety percent of Congo fruit not infected with the bacterial pathogen had a darker green background than those infected. PI 295843 and PI 299318 selections were also not infected. Infection rates in susceptible checks ranged from 22.5% to 47.6% and from 0 to 13.9% among triploids. Both ploidy level and genotype significantly affected infection rates. Infestation rates in triploid seeds were reduced but not eliminated by dry heat up to 75C. Heat treatment necessary to kill the pathogen was detrimental to germination.

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Asim Esen and Robert K. Soost

Abstract

The analysis of ploidy levels in progenies from 2x X 4x crosses during embryogenesis and after germination of the seeds indicates that they are mixtures of triploids and tetraploids. The frequency of tetraploids varies from 6 to 94% depending on the pistillate parents used. Chromosome number determinations in the embryo and endosperm of sectioned young seeds provide conclusive evidence that the megagametophytes which produce tetraploids when fertilized contain diploid eggs and polar nuclei. The occurrence of diploid megagametophytes in diploids provides an additional approach for producing tetraploid stocks following 2x X 4x crosses.

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Janice R. Bohac, Alfred Jones, and Daniel F. Austin

Previous work in this laboratory identified high levels of unreduced (2n) pollen in the tetraploid (4×) Ipomoea spp. Acc. 81.2. This work provided indirect evidence that 2n pollen was involved in the evolution of the 6x ploidy level of the cultivated sweetpotato (I. batatas). To further study the role of 2n pollen in sweetpotato evolution, we examined plants of Acc. 81.2. plants of five sweetpotato cultivars, and 100 randomly selected heterozygous sweetpotato seedlings. The 4× Acc. 81.2 was determined to be I. batatas. High levels of large 2n pollen were confirmed in Acc. 81.2, and low levels of 2n pollen were observed in `Sulfur' and in 16% of the sweetpotato seedlings. Presence of monad, dyad, and triad sporads confirmed that the large 2n pollen grains were the result of nonreduction in the sporad stage. These new findings are direct evidence that 2n pollen was involved in the evolution of the 6× ploidy level of sweetpotato. This is the first report of a 4× accession classified as I. batatas; it is also the first report of 2n pollen in 6× I. batatas The widespread presence of 2n pollen in sweetpotato suggests that the trait can be used to advantage in breeding programs to introgress genes from wild 4× Ipomoea spp. into cultivated 6× sweetpotato without adverse effects on genetic stability or fertility.

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Monica J. Norby and Michael J. Havey

Phytophthorainfestans is the casual agent of late blight and is a major threat to potato production worldwide. There are no curative control agents available and resistance genes offer promise in controlling late blight. To date, the primary source of late-blight resistance has been from hexaploid (6x) [4 Endosperm Balance Number (EBN)] Solanum demissum. Mexican diploid (2x) (1EBN) Solanum species possess a wealth of late-blight resistances, but have been neglected due to crossing barriers. Manipulation of EBN and ploidies should allow integration of 2x (1EBN) germplasm into cultivated potato. Synteny between late-blight resistance loci from Solanum species of disparate ploidies and EBNs may facilitate the identification of unique resistance alleles and loci. Isolate MSU96 (US8/A2) of P. infestans revealed a late-blight resistance locus (Rpi1) from 2x(1EBN) S. pinnatisectum (PI 253214) that mapped to chromosome seven (MGG 265:977-985). MSU96 was also avirulent on the late-blight differential R9-Hodgson 2573 (LB3), revealing the presence of the avirulence gene for R9 originating from S. demissum. To test the relationship between Rpi1 and R9, we evaluated a family segregating for R9 and revealed that it does not map to chromosome seven. The independent inheritance of R9 and Rpi1 indicates that Rpi1 is a unique resistance locus. We are conducting a variety of crossing schemes to introgress Rpi1 into cultivated potato.

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Robert H. Bors and J. Alan Sullivan

Fragaria species from the center of diversity have not been integrated into octoploid commercial strawberry cultivars because of ploidy level differences. Even though traits such as disease resistance, enhanced flavor, cold hardiness, and vigor are known to exist in the diploid, tetraploid, and hexaploid species, they cannot be easily used for breeding. The synthetic octoploid method circumvented introgression difficulties by combining lower ploidy species and doubling to the octoploid level. Although easily crossed to cultivars, the use of synthetic octoploids has been minimal as it has been extremely difficult to create them. By working to improve bottlenecks of the original system, improved methodology has been developed and 170 synthetic octoploids have been produced. This represents more than a 100-fold increase in efficiency. The following factors played a major role in improving the system: wide germplasm base; use of F. vesca as a common genome; embryo rescue; 5% colchicine applied in vitro by dropper method for 24 hours followed by a quick rinse and continuous light in a 18C growth chamber. F. vesca, F. nilgerrensis, F. nubicola, F. viridis, F. orientalis, and F. moschata have been incorporated into synthetic octoploids in this study.

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Bruce L. Dunn and Jon T. Lindstrom

Ploidy level and fertility status are often the two biggest barriers a breeder must overcome when trying to incorporate novel characteristics among related taxa. This study was aimed at developing an efficient chromosome doubling method for Buddleja L., commonly known as butterfly bush, with the goal of equalizing the ploidy level and restoring the fertility of a diploid (2n=38) F1 interspecific hybrid that has a unique orange color but happens to be sterile. This method would ease the crossing of the hybrid to the tetraploid (2n=76) B. davidii Franch. cultivars commonly found in the industry. An antimitotic treatment of oryzalin was tested on 02-25-142 (B. madagascarensis Lam. × B. crispa Benth.) in vitro using nodal sections. A factorial of varying concentrations [3, 5, and 7 μM (micromolar)] by different exposure times (1, 2, and 3 day) plus controls was set up. Oryzalin appeared to be an efficient agent for chromosome doubling in Buddleja. Significant differences in the number of polyploids were not seen between chemical concentrations and exposure times. However, higher chemical concentrations and exposure times did have a significant effect on the number of nodes that survived tissue culture. Increased leaf size and color, stem thickness, shortened internode length, and upright growth habit were all good early phenotypic indicators of polyploidy induction as later confirmed by flow cytometry. Significant increases in pollen viability accompanied chromosome doubling as crosses between 02-25-142 × B. davidii cultivars produced viable seedlings.