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.
David N. Kuhn, Giri Narasimhan, Kyoko Nakamura, J. Steven Brown, Raymond J. Schnell and Alan W. Meerow
Raymond Schnell, J. Steven Brown, Cecile Olano, Alan Meerow, Richard Campbell and David Kuhn
Mangifera indica L. germplasm can be classified by origin with the primary groups being cultivars selected from the centers of diversity for the species, India and Southeast Asia, and those selected in Florida and other tropical and subtropical locations. Accessions have also been classified by horticultural type: cultivars that produce monoembryonic seed vs. cultivars that produce polyembryonic seed. In this study, we used 25 microsatellite loci to estimate genetic diversity among 203 accessions. The 25 microsatellite loci had an average of 6.96 alleles per locus and an average PIC value of 0.552. The total propagation error in the collection, i.e., plants that had been incorrectly labeled or grafted, was estimated to be 6.13%. When compared by origin, the Florida cultivars were more closely related to Indian than to Southeast Asian cultivars. Unbiased gene diversity (Hnb) of 0.600 and 0.582 was found for Indian and Southeast Asian cultivars, respectively, and both were higher than Hnb among Florida cultivars (0.538). When compared by horticultural type, Hnb was higher among the polyembryonic types (0.596) than in the monoembryonic types (0.571). Parentage analysis of the Florida cultivars was accomplished using a multistage process based on introduction dates of cultivars into Florida and selection dates of Florida cultivars. Microsatellite marker evidence suggests that as few as four Indian cultivars, and the land race known as `Turpentine', were involved in the early cultivar selections. Florida may not represent a secondary center of diversity; however, the Florida group is a unique set of cultivars selected under similar conditions offering production stability in a wide range of environments.
J. Steven Brown, R.J. Schnell, J.C. Motamayor, Uilson Lopes, David N. Kuhn and James W. Borrone
A genetic linkage map was created from 146 cacao trees (Theobroma cacao), using an F2 population produced by selfing an F1 progeny of the cross Sca6 and ICS1. Simple sequence repeat (SSR) markers (170) were used principally for this map, with 12 candidate genes [eight resistance gene homologues (RGH) and four stress related WRKY genes], for a total of 182 markers. Joinmap software was used to create the map, and 10 linkage groups were clearly obtained, corresponding to the 10 known chromosomes of cacao. Our map encompassed 671.9 cM, approximately 100 cM less than most previously reported cacao maps, and 213.5 cM less than the one reported high-density map. Approximately 27% of the markers showed significant segregation distortion, mapping together in six genomic areas, four of which also showed distortion in other cacao maps. Two quantitative trait loci (QTL) for resistance to witches' broom disease were found, one producing a major effect and one a minor effect, both showing important dominance effects. One QTL for trunk diameter was found at a point 10.2 cM away from the stronger resistance gene. One RGH flanked the minor QTL for witches' broom resistance, implying possible association. QTLs mapped in F2 populations produce estimates of additive and dominance effects, not obtainable in F1 crosses. As dominance was clearly shown in the QTL found in this study, this population merits further study for evaluation of dominance effects for other traits. This F2 cacao population constitutes a useful link for genomic studies between cacao and cotton, its only widely grown agronomic relative.
James W. Borrone, Cecile T. Olano, David N. Kuhn, J. Steven Brown, Raymond J. Schnell and Helen A. Violi
Avocado (Persea americana Mill.) possesses a unique flowering mechanism, thought to promote out-crossing, in which the male and female parts of the perfect flower function at different time periods. Cultivars are classified as Flowering Type A, where flowers are functionally female the morning of one day and functionally male the afternoon of the next day, or Flowering Type B, where flowers are functionally female in the afternoon and functionally male the next morning. Avocado growers typically interplant cultivars of opposite flowering types to maximize yield. Recently, it has been hypothesized that 90% to 95% of avocado flowers are self-pollinated in southern Florida. However, this hypothesis does not address whether mature, marketable avocado fruit in Florida are the result of outcrossing. To determine whether avocado fruit in southern Florida result from self-pollination or outcrossing, fruit were harvested from a commercial orchard in Miami-Dade County, Florida, from a block consisting of two cultivars, Simmonds (Flowering Type A) and Tonnage (Flowering Type B), interplanted in approximately equal numbers. Seeds were germinated and the resulting progeny were genotyped using eight fully informative, microsatellite markers. Seventy-four percent of the ‘Simmonds’ progeny and 96% of the ‘Tonnage’ progeny were judged to be the result of cross-pollination, with an estimated overall outcrossing rate of 63% to 85% within this particular block of the orchard. Seedlings judged to be the result of cross-pollinations between ‘Simmonds’ and ‘Tonnage’ are being maintained at the U.S. Department of Agriculture-Agricultural Research Service, Subtropical Horticulture Research Station and are being evaluated for segregation of important agronomic and horticultural traits.
R. Karina Gallardo, Kara Grant, David J. Brown, James R. McFerson, Karen M. Lewis, Todd Einhorn and Mario Miranda Sazo
Advances in precision agriculture technologies provide opportunities to improve the efficiency of agricultural production systems, especially for high-value specialty crops such as fresh apples (Malus domestica). We distributed an online survey to apple growers in Washington, New York, and Michigan to elicit stakeholder perceptions of precision agriculture technologies. Findings from this study demonstrated that growers are willing to adopt precision agriculture technologies when they receive results from applied research projects and are engaged with active extension programs. The availability of customized services and purchasing and rental options may minimize the effects of the economies of size that create barriers to adopting increasing access to technologies. Finally, respondents deemed collaborative efforts between industry and academic institutions crucial for adapting the innovation to better address the needs of growers.