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Hamidou F. Sakhanokho and Nurul Islam-Faridi

Christia obcordata is an intriguing small-sized house plant with unusual and attractive features such as its striped leaves. Because very little is known about the plant, we conducted an investigation of its genome and chromosomes. The number of chromosomes was determined using a protoplast technique to prepare root tip chromosome spread and was found to be 2n = 2x = 20. Flow cytometry was used to determine nuclear DNA content (1C = 0.65 pg = 634.4 Mb) for C. obcordata and AT/GC composition was shown to be AT% = 62.8% ± 0.0% and GC% = 37.2% ± 0.0%. Finally, fluorescent in situ hybridization was used to locate ribosomal RNA gene families in C. obcordata. Ribosomal RNA gene families, viz. 18S-28S and 5S rDNA, are unique cytomolecular landmarks that provide valuable information about the evolutionary organization of a genome. We have identified one locus each of 18S-28S and 5S rDNA. The 18S-28S rDNA is located in the subterminal position on the secondary constriction region [also known as the nucleolus organizer region (NOR)] and the 5S rDNA is located interstitially close to a centromeric position. The basic information gathered in this study on C. obcordata will be helpful in understanding the genetics of this species.

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Cecil T. Pounders and Hamidou F. Sakhanokho

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Hamidou F. Sakhanokho and M. Nurul Islam-Faridi

We report for the first time the incidence of spontaneous autotetraploidy in Solanum aethiopicum (PI 636107). Stomatal dimensions and frequency, number of chloroplasts per guard cell, flow cytometry, and chromosome counts were used to differentiate the diploid plants from tetraploids. The impact of increased ploidy on pollen viability as assessed by in vitro germination and on selected morphological traits was evaluated. In vitro pollen germination was reduced in tetraploid plants, but no significant differences were found in fruit production per plant between diploid and tetraploid plants. Compared with the diploids, the tetraploid plants were significantly shorter and had wider leaves and smaller fruits; therefore, tetraploid S. aethiopicum plants can be valuable for future breeding programs, particularly those aiming to develop shorter, more compact plants. Moreover, some S. aethiopicum selections are grown for their edible leaves, so tetraploid plants producing large leaves would be desirable. Additionally, the availability of tetraploid S. aethiopicum could remove hybridization barriers caused by ploidy differences with other tetraploid Solanum species.

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Cecil Pounders, Tim Rinehart and Hamidou Sakhanokho

Production of viable interspecific seedlings from a cross between Lagerstroemia indica L. ‘Tonto’ × L. speciosa (L.) Pers. was confirmed by comparison of morphological traits and genetic markers. Traits such as plant height and width showed marked variation within the seedling population whereas variation in other traits such as flower size and color was very limited. Seedlings were found to be functionally sterile as either male or female parents. Observed sterility prevents the maximum introgression of important complex traits such as cold hardiness by sib mating or backcrossing into clones derived from this parental combination. ‘Princess’ was confirmed to be a sterile hybrid of L. indica and L. speciosa whereas ‘Monia’ was indicated to have L. indica in its ancestry but not L. speciosa.

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Cecil T. Pounders, Hamidou F. Sakhanokho and Leopold M. Nyochembeng

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Hamidou F. Sakhanokho, Kanniah Rajasekaran and Rowena Y. Kelley

An efficient primary somatic embryo (SE) and secondary somatic embryo (SSE) production system was developed for the ornamental ginger Hedychium bousigonianum Pierre ex Gagnepain. Addition of two ethylene inhibitors, salicylic acid (SA) and silver nitrate (AgNO3), to the culture media improved the system. Callus was initiated and proliferated on a medium containing Murashige and Skoog (MS) basal salts supplemented with 9.05 μM 2,4-dichlorophenoxyacetic acid and 4.6 μM kinetin. Friable callus was transferred to a liquid medium containing MS basal salts, B5 vitamins, 0.6 μM thidiazuron, and 8.9 μM 6-benzylaminopurine to induce somatic embryogenesis. The effects of various concentrations of SA (0, 25, 50, 75, 100, 125, 150 μM) and AgNO3 (0, 10, 20, 30, 40, 50, 60 μM) on callus growth, SE, and SSE development was further evaluated. The rate of callus growth decreased as the concentrations of SA or AgNO3 increased. AgNO3 and SA at all concentrations stimulated SE and SSE development better than the control although a decrease in embryo production was observed at higher concentrations of both SA and AgNO3. The best concentrations for SA were 75 and 100 μM, whereas for AgNO3, they were 30 to 50 μM for both SE and SSE production.

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Ebrahiem M. Babiker, Stephen J. Stringer, Barbara J. Smith and Hamidou F. Sakhanokho

Blueberry leaf rust caused by Thekopsora minima is a serious threat to blueberry production. To investigate the host range and characterize new sources of resistance, 15 southern highbush accessions (Vaccinium corymbosum), two interspecific hybrids (V. elliottii × V. pallidum and V. corymbosum × V. pallidum), and accessions from five diploid Vaccinium species were inoculated with an isolate of T. minima. Of 15, only two southern highbush accessions displayed resistance, whereas both accessions of V. arboreum displayed immunity against T. minima. Accessions of V. darrowii exhibited necrosis but with limited sporulation, indicating a high level of resistance. Sporulating lesions and brown spots were observed in accessions of V. elliottii and V. tenellum. Brown lesions, large pustules, and abundant sporulation were observed on V. pallidum accessions and their interspecific hybrids. As the lesions expanded, defoliation was observed in V. pallidum accessions. When tested against rabbiteye (V. virgatum) and southern highbush blueberries, urediniospores of T. minima from overwintering leaves of V. pallidum were found to be virulent, suggesting that T. minima overwinters on V. pallidum. Based on symptoms and scanning electron microscopy (SEM) of urediniospores, we hypothesize that V. elliottii, V. tenellum, V. pallidum, and V. corymbosum exhibit no host specificity to T. minima.