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Raymond J. Schnell and Robert J. Knight Jr.

Five isozyme systems were used to detect zygotic seedlings from five polyembryonic cultivars of mango (Manifera indica L.). Significant differences were found between cultivars (x2 = 35.53, P < 0.001) for the percentage of zygotic and nucellar seedlings detected. The range of variation in the percentage of off-types was from 0% in 13-1 to 64% in Golek. The percentage in Sabre was 4%, and 24% and 36% in Tupentine and Madoe, respectively. Three of eight rootstock mother trees of Turpentine were determined to be off-types.

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Raymond J. Schnell and Robert J. Knight

Genetic relationships between commercial mango cultivars are often speculative and only the maternal parent is generally known. RAPD™ primers were used with the polymerase chain reaction (PCR) to provide markers useful in determining individual identity, family relationships, and linkage mapping analysis. In mango, 53 RAPD primers were screened for markers and 27 proved useful. Genomic DNA was isolated from 70 clones of mango maintained in the USDA germplasm collection. DNA from these clones was amplified with each of the 27 primers. Data were scored as the presence or absence of bands. Groupings of the clones using UPGMA based on Nei's genetic distance gave distinct clusters. RAPD clusters vs. clusters based on isozyme analysis are compared.

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Michael K. Hennessey, Robert J. Knight Jr. and Raymond J. Schnell

Seventeen avocado selections from the U.S. Dept. of Agriculture Miami National Clonal Germplasm Repository were bioassayed for antibiosis to Caribbean fruit fly eggs and larvae. Two colony-reared strains of flies were used. Fourteen of the selections did not support any development of immature stages to the adult stage. The results support the contention that highly resistant cultivars would not pose a high risk of spreading Caribbean fruit fly to foreign markets even without postharvest disinfestation treatment.

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Thomas L. Davenport, Zhentu Ying and Raymond J. Schnell

The synchronously dichogamous flowering behavior of avocado has historically been assumed to promote cross-pollination. Preliminary studies in southern California have revealed that self-pollination is more typical. The primary objective of the California research is to determine the paternity of individual fruit sampled during early and late fruit development using SSR markers. Cultivars included Hass as the primary cultivar and Bacon, Ettinger, Fuerte, Harvest, Lamb Hass, Marvel, Nobel, SirPrize, and Zutano serving as cross-pollinizing cultivars. We were able to: 1) estimate proportions of self- and cross-pollinated `Hass' fruit with cultivars planted in rows of varying proximity to the `Hass' rows; determine if the proportion of outcrossed fruit increased during maturity due to preferential abscission of self-pollinated fruit; and 2) determine if there is preferential retention of fruit cross-pollinated by a specific cultivar during maturation. On average, cross-pollination by any individual cultivar in 2004 was 6% or less in marble-sized fruit. Over 70% of the fruit were self-pollinated. This is greater than the proportion of self-pollination (about 30%) observed in near-mature fruit harvested in the previous year, 2003. Proportions of marble-sized fruit pollinated by each cultivar within each row were compared to the proportions of self or cross-pollinations in fruit harvested from the same trees at near-maturity. We observed about a 10% increase in proportion of self-pollinated fruit and a concomitant decrease in retained fruit derived from cross-pollination. Self-pollination appears to be the dominant mode of pollination. These preliminary results indicate that trees benefit from it, perhaps in preference over cross-pollination.

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Alan W. Meerow, Richard Criley and Raymond J. Schnell

Plumeria is a small genus of succulent trees and shrubs in the Apocynaceae native to tropical America. It is favored as a landscape ornamental in tropical and tropical regions due to its tolerance of hot, dry conditions, ease of propagation, and long season of bloom. Flowers of certain varieties are important components of leis in Hawaii. Numerous cultivars have been developed, chiefly from either seedling selections of P. rubra, a Mexican species, and P. obtusa, broadly distributed in the Caribbean basin, or hybrids between these species and among older cultivars. Little is known of the breeding behavior of the species in nature or cultivations, but very few of the cultivars set an abundant number of fruits. We used 21 microsatellite DNA (SSR) loci developed in our lab from Plumeria rubra to investigate the genetic relationships of 83 cultivars of Plumeria from a germplasm collection at the University of Hawaii, now duplicated in Miami. All 21 loci were highly polymorphic, with three to 15 alleles observed across the cultivar population. Six of the 21 loci exhibit heterozygote excess across the cultivars; the majority of the remaining 15 have an excess of homozygotes, suggesting that the cultivars are largely inbred. Clustering with Bayesian analysis suggests that there are five main groups represented among the cultivars, with varying degrees of admixture among the five. The data also suggest that identical genotypes have received different cultivar names at times. We are also analyzing seedling populations from fruits spontaneously set on several cultivars to determine if they are predominantly the result of self-pollination or out-crossing.

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Catherine M. Ronning, Raymond J. Schnell and Shmuel Gazit

The native American genus Annona contains many species that are cultivated in the tropics and subtropics for their edible fruit, including the custard apple (A. reticulata), soursop (A. muricata), cherimoya (A. cherimola), sugar apple (A. squamosa), and the interspecific hybrid, Atemoya. Random Amplified Polymorphic DNA (KAPD) analysis of the A. cherimola cultivars `Jete' and `Campa, 1A. squamosa `Lessard', and the Atemoya cultivars `Ubranitzki', `Mallali', and `Kaspi' resulted in very distinctive patterns, indicating that RAPD markers are an easy, efficient method of fingerprinting Annona species. Thirteen of 15 primers gave repeatable, polymorphic patterns. An F1 population of `Jete' × `Lessard' as well as selfed populations of `Jete' and of `Lessard' were analyzed to determine the inheritance of the KAPD banding patterns. The results indicate that PAPD analysis can be used in genetic and phylogenetic studies of Annona species.

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Jayasankar Subramanian, Richard E Litz and Raymond J. Schnell

Anthracnose, caused by Colletotrichum gloeosporioides, is the most serious production and postharvest problem of mango. Most mango cultivars are highly susceptible to this disease. Embryogenic nucellar cultures of two cultivars, `Hindi' and `Carabao', were recurrently selected with either the purified phytotoxin or the crude culture filtrate produced by the fungus. Mycelium growth was suppressed in dual cultures involving the recurrently selected lines. Enhanced extracellular production of proteins was observed in the embryogenic cultures and in regenerants, including a newly expressed acidic chitinase isozyme (`Hindi') and greater expression of two other chitinase isozymes (`Hindi' and `Carabao'). Activity of α-1,3-glucanase was also substantially increased in the recurrently selected lines. Resistant lines were characterized using RAPDs on the basis of polymorphisms generated by eight different primers. In most cases, the differences involved the absence of RAPD markers in resistant lines.

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David N. Kuhn, Giri Narasimhan, Kyoko Nakamura, J. Steven Brown, Raymond J. Schnell and Alan W. Meerow

Identifying genetic markers linked to disease resistance in plants is an important goal in marker-assisted selection. Using a candidate-gene approach, we have previously developed genetic markers in cacao (Theobroma cacao L.) for two families of genes involved in disease resistance: non-TIR-NBS-LRR (Toll/Interleukin-1 Receptor-nucleotide binding site-leucine rich repeat) resistance gene homologues and WRKY transcription factor genes; however, we failed to isolate TIR-NBS-LRR genes. Using a novel algorithm to design degenerate primers, we have now isolated TIR-NBS-LRR loci as determined by DNA sequence comparison. These loci have been developed as genetic markers using capillary array electrophoresis (CAE) and single-strand conformational polymorphism (SSCP) analysis. We have mapped three distinct TIR-NBS-LRR loci in an F2 population of cacao and demonstrated that one is located on linkage group 3 and the other two on linkage group 5.

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Yaseen Mohamed-Yaseen, Raymond J. Schnell, Robert J. Knight and T.L. Davenport

A procedure was developed to regenerate plants via tissue culture from embryonic axes of mature avocado seeds. Explants were cultured in Murashige and Skoog (MS) medium supplemented with benzyladenine (BA) and naphthalene-acetic acid (NAA) or thidiazuron (TDZ) and NAA. Culture were kept in the dark for 7-10 days to reduce browning resulting from phenolic oxidation. Multiple shoots (5-8) were formed after transfer to light. Further multiplication were achieved using different combination of BA and NAA or TDZ and NAA. Shoots were cultured in MS supplemented with 2mg/l indolebutyric acid (IBA) for 2 weeks then transferred to MS supplemented with lg/l activated charcoal for root induction. Complete plants were obtained in vitro.

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Yaseen Mohamed-Yaseen, Raymond J. Schnell, Robert J. Knight and T. L. Davenport

Guava (Psidium guajava L.) is an exceptional source of vitamin. C. It is also considered to be the most important cultivated species of the Myrtel family. Shoot tip and stem node were taken from seedling germinated in Murashige and Skoog medium (MS) and cultured in the same medium supplemented with 1-3mg/l benzylaminopurine (BA) and 0.1mg/l naphthaleneacetic acid (NAA) or 0.2-2mg/l thidiazuron (TDZ) and 0.1mg/l NAA. Multiple shoots (4-6) were obtained in 4-5 weeks from culture in 1-2mg/l BA and 0.1mg/l NAA, while TDZ caused abnormal shoot growth. Shoots were rooted successfully with 100% frequency in MS medium containing 2mg/l indolebutyric acid and further elongation of shoots was achieved in MS medium, supplemented with lg/l activated charcoal. Regenerated plantlets were successfully established in soil.