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Title: Genetic differentiation, races and interracial admixture in avocado (Persea americana Mill.), and Persea spp. evaluated using SSR markers

item BOZA, EDWARD - US Department Of Agriculture (USDA)
item Tondo, Cecile
item LEDESMA, NORIS - Fairchild Tropical Botanic Garden
item CAMPBELL, RICHARD - Fairchild Tropical Botanic Garden
item BOST, JAY - University Of Hawaii
item SCHNELL, RAYMOND - Mars, Inc
item Gutierrez, Osman

Submitted to: Genetic Resources and Crop Evolution
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/15/2018
Publication Date: 2/21/2018
Citation: Boza, E., Tondo, C.L., Ledesma, N., Campbell, R., Bost, J., Schnell, R., Gutierrez, O.A. 2018. Genetic differentiation, races and interracial admixture in Avocado (Persea americana Mill.) analyzed using SSR markers. Genetic Resources and Crop Evolution. 65, no. 4(2018):1195-1215.

Interpretive Summary: Avocado is a sub-tropical evergreen tree native of Mesoamerica and an economically important crop, member of the Lauraceae family. Based on their ecological adaptation, botanical, and physiological traits, avocados have been classified in three horticultural races (Guatemalan, Mexican, and West Indian). Genetically diverse avocado resources are essential for avocado improvement programs. This diversity constitutes the necessary genetic pool to be used in the development of modern varieties of avocado, with the aim of increasing yield, pest and disease resistance, improved oil content and enhanced fruit quality. Accurate methodologies, such as molecular marker analysis, allow geneticists and curators to accomplish the characterization and evaluation of avocado genetic diversity, as well as detect mislabeling errors. Molecular markers are currently used in other crops to address these matters and accelerate cultivar development. Using microsatellite markers, scientists at the USDA-ARS in Miami, FL, studied the genetic diversity of 354 trees from avocado collections at the USDA-ARS Miami, FL, Fairchild Tropical Botanic Garden, Homestead, FL, and from wild collections in Mexico. Microsatellite marker analysis results indicated high variability in the allele frequency for the study group. The main source of variation among the evaluated trees was found to be within trees across all races, followed by variation among races, and finally variation among trees within races. Results indicated trees were divided into groups associated with Guatemalan, Mexican, and West Indian races; interracial admixture; hybrids and wild avocados. A mislabeling error of 0.28 % was estimated. This research improves our understanding of the complex nature of genetic interrelationships between avocado races and results can be applied in our breeding program.

Technical Abstract: Avocado (Persea americana Mill.) is a subtropical domesticated fruit tree indigenous to Mesoamerica. It is a member of the Lauraceae family and is separated into three horticultural races (Guatemalan, Mexican, and West Indian) mainly corresponding to their ecological adaptation, botanical, and physiological traits. Main objectives of this study were to characterize the population structure, genetic diversity, and horticultural race of a total of 354 Persea spp. trees whose origin is as follow: 221 trees [P. americana, (218), P. nubigena (2) and P. krugii (1)] from the USDA-ARS-Subtropical Horticultural Research Station, Miami; 105 trees from the Fairchild Farm [P. americana (104) and P. schiedeana (1)], and 28 trees collected in Mexico [P. schiedeana (23) and P. americana (5)]. The complexity of their interracial admixture; as well as mislabeling frequency was also evaluated. Molecular marker analysis utilizing a set of 55 simple sequence repeat (SSR) markers amplified a total of 869 alleles with a mean number of alleles per locus of 15.8 and average polymorphism information content value of 0.71, indicating a high variability in the allele frequency for the collection. Significant deviations from Hardy–Weinberg equilibrium were identified after Bonferroni correction for a large number of loci (48; 87%) due to the presence of null alleles. The main source of variation for this population was found to be within individuals (66.84%), with 19.30% variation among populations, and 13.86% variation among individuals within populations. Moreover, population specific inbreeding indices (FIS) were calculated for West Indian, Guatemalan, and Mexican [(0.1918; p value 0.0000), (0.1879; pvalue 0.0000), (0.0925; p-value 0.0022)], respectively. Bayesian analysis divided the individual genotypes into groups associated with the Guatemalan, Mexican, West Indian races; interracial admixture; complex hybrids and P. schiedeana species. Also, results of the multivariate clustering method (PCA) and genetic distance analyses calculated among all possible individual combinations within the SSR diversity data agreed with Bayesian or Structure analyses results. The 55 SSRs provided complete resolution of all individuals and the estimated mislabeling error was approximately 0.28%.