Cacao (Theobroma cacao L.) is an important cash crop in tropical growing regions of the world and particularly for small cacao farmers. Over the past two decades, ‘CCN 51’ has become one of the most planted cultivars in Ecuador, mainly as a result of its high productivity and disease resistance. Intermixing of Nacional fine flavor Ecuadorian beans with beans of ‘CCN 51’ has become common practice, reducing overall bean quality and decreasing value. The primary goals of this study were to determine the genetic identity, structure, and allelic richness of ‘CCN 51’, its maternal origin and to compare ‘CCN 51’s’ agronomic characteristics against a composite group of Nacional cultivars. To investigate the complex genetic background of this cultivar, 70 simple sequence repeat loci were used. The high heterozygosity observed (56 of 70 loci) for ‘CCN 51’ is not characteristic of traditional Nacional cultivars. Comparison of agronomic characteristics between ‘CCN 51’ and several Nacional cultivars indicates significant differences in cacao dry bean weight, yield potential, production efficiency, percent healthy pods, and witches' broom [Moniliophthora perniciosa (Stahel) Aime & Phillips-Mora] disease incidence. Additionally, physical, chemical, and organoleptic characteristics suggest that ‘CCN 51’ is different from those of Nacional lineage. Based on population structure analysis, the predominant ancestries for ‘CCN 51’ are Iquitos (45.4%), Criollo (22.2%), and Amelonado (21.5%) genetic groups. A lesser proportion of its genome was accounted for by genetic groups Contamana (3.9%), Purús (2.5%), Marañon (2.1%), and Nacional (1.1%) admixtures. Results of phylogenetic analyses using the unweighted pair group method with arithmetic mean yielding high bootstrap values strongly support the relatedness of ‘CCN 51’ with Iquitos, Criollo, and Amelonado genetic groups. Moreover, seven mitochondrial simple sequence repeat loci revealed that ‘CCN 51’ maternally inherited the ‘IMC 67’ cytotype. ‘CCN 51’ constitutes a valuable cacao genetic resource that is currently used not only in its country of origin, but also in many other national breeding and selection programs worldwide.

No abstract

No abstract

Recent enhancement of the pool of known molecular markers for avocado has allowed the construction of the first moderately dense genetic map for this species. Over 300 SSR markers have been characterized and 163 of these were used to construct a map from the reciprocal cross of two Florida cultivars 'Simmonds' and 'Tonnage'. One hundred thirty-five primer pairs amplified 163 usable loci with 20 primer pairs amplifying more than one locus. 'Tonnage' was heterozygous for 152 (93%) loci, whereas 'Simmonds' was heterozygous for 64 (39%). Null alleles were identified at several loci. Linkage maps were produced for both reciprocal crosses and combined to generate a composite linkage map for the F 1 population of 715 individuals. The composite map contains 12 linkage groups. Linkage groups ranged in size from 157.3 cM (LG2) to 2.4 cM (LG12) and the number of loci mapped per group ranged from 29 (LG1) to two (LG12). The total map length was 1,087.4 cM. Only seven markers were observed to have segregation distortion (α≤0.05) across both sub-composite (reciprocal) maps. Phenotypic data from traits of horticultural interest are currently being collected on this population with the ultimate goal of identifying useful quantitative trait loci and the development of a marker-assisted selection program.

Avocado sunblotch viroid (ASBVd) causes an important disease of avocado, Persea americana. Symptoms of avocado sunblotch were first observed in the avocado germplasm collection at the National Germplasm Repository in Miami in the early 1980s; however, the extent of infection was unknown. An ASBVd-specific reverse transcription polymerase chain reaction (RT-PCR) protocol was developed in 1996 and used to screen every tree in the collection. Surveys in 1996 and 2000 found that although 23 newly infected trees were detected, the proportion of ASBVd-positive accessions remained unchanged at 19%. However, in a 2009 survey, 50 newly infected trees were detected for an overall infection rate of 21%. Results of spatial analyses indicate that for the older plantings, the effective range of spread increased more than threefold during the 13 year span, while in the newer plantings, the pattern of infection indicates a reintroduction of the viroid rather than natural spread. Despite strict sanitization procedures in field and greenhouse operations, ASBVd infections have increased in the USDA collection. Although genetic diversity in the collection would be reduced, eliminating all ASBVd-positive plants may be necessary to ensure that other accessions in the collection do not become infected.

The genetic improvement of tropical fruit trees is limited when compared to progress achieved in temperate fruit trees and annual crops. Tropical fruit tree breeding programs require signi fi cant resources to develop new cultivars that are adapted to modern shipping and storage requirements. The use of molecular markers in tropical fruit tree breeding is greatly assisting in solving a number of dif fi cult challenges for breeders such as the development of complex family structures for recombination mapping and for recurrent selection. A review of the literature on molecular markers development and new techniques for increasing single-nucleotide polymorphic markers is discussed. The development of marker-assisted breeding for these tropical tree crops is also discussed.

No abstract

Avocado (Persea americana Mill.) has an unusual flowering mechanism, diurnally synchronous protogynous dichogamy, that promotes crosspollination among avocado genotypes. In commercial groves, which usually contain pollinizer rows adjacent to the more desirable commercial cultivars, the rate of outcrossing has been measured with variable results. Using microsatellite markers, we estimated outcrossing in a commercial California ‘Hass’ avocado orchard with adjacent ‘Bacon’ pollinizers. Seedlings grown from mature harvested fruit of both cultivars were genotyped with five fully informative microsatellite markers and their parentage determined. Among the 919 seedlings of ‘Hass’, 688 (75%) were hybrids with ‘Bacon’; the remaining 231 (25%) seedlings were selfs of ‘Hass’. Among the 850 seedlings of ‘Bacon’, 382 (45%) were hybrids with ‘Hass’ and the remaining 468 (55%) seedlings were selfs of ‘Bacon’. The high outcrossing rate observed in the ‘Hass’ seedlings was expected, because adjacent rows of opposite flowering types (A versus B) are expected to outcross. However, the high selfing rate in ‘Bacon’ was unexpected. A previous study in Florida using the cultivars ‘Simmonds’ and ‘Tonnage’ demonstrated differences in outcrossing rates between complementary flowering type cultivars. In both Florida and California, the A type parents (‘Hass’ and ‘Simmonds’) had similar outcrossing rates (≈75%); however, the B type parents (‘Bacon’ and ‘Tonnage”) had highly skewed outcrossing rates of 45% and 96%, respectively. Two new avocado lethal mutants were discovered among the selfed seedlings of ‘Hass’ and ‘Bacon’. These were labeled “spindly” and “gnarly” and are similar in phenotype to mutants described in Arabidopsis and other crop species.