Quantitative trait locus analysis in avocado: The challenge of a slow-maturing horticultural tree crop

Authors: ASHWORTH, VANESSA - University Of California; CHEN, HAOFENG - University Of California; CALDERON-VAZQUEZ, CARLOS - University Of California; ARPAIA, MARY - University Of California; Kuhn, David; DURBIN, MARY - University Of California; TOMMASINI, LIVIA - University Of California; DEYETT, ELIZABETH - University Of California; JIA, ZHENYU - University Of California; CLEGG, MICHAEL - University Of California; ROLSHAUSEN, PHILIPPE - University Of California

Submitted to: Journal of the American Society for Horticultural Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/4/2019
Publication Date: 9/3/2019
Citation: Ashworth, V., Chen, H., Calderon-Vazquez, C.L., Arpaia, M.L., Kuhn, D.N., Durbin, M.L., Tommasini, L., Deyett, E., Jia, Z., Clegg, M.T., Rolshausen, P.E. 2019. Quantitative trait locus analysis in avocado: The challenge of a slow-maturing horticultural tree crop. Journal of the American Society for Horticultural Science. 144(5):352–362. https://doi.org/10.21273/JASHS04729-19.
DOI: https://doi.org/10.21273/JASHS04729-19

Interpretive Summary: Avocado ( Persea americana ) is an important tropical and subtropical fruit crop. Recent interest in healthy nutrition has singled out the avocado as an excellent source of several phytonutrients. We wanted to associate favorable traits with particular regions of the genome (quantitative trait loci, QTL), especially in regions affecting phytonutrient production. However, as avocado is a sizeable, slow-maturing tree crop, it has been largely neglected by genetic studies, owing to a long breeding cycle and costly field trials. We used a small, replicated experimental population of 50 progeny, grown at two locations in two successive years, to explore the feasibility of developing a dense genetic linkage map and to implement quantitative trait locus (QTL) analysis for seven phenotypic traits. Additionally, we tested the utility of candidate-gene single-nucleotide polymorphisms developed to genes from biosynthetic pathways of phytonutrients beneficial to human health. The resulting linkage map consisted of 1346 markers distributed across 12 linkage groups. Numerous markers were associated with flowering type and vitamin E (a-tocopherol) content of the fruit. Despite the limited statistical power associated with small sample size, this study provided useful mapping information on these two important traits, demonstrating that the pursuit of linkage mapping and QTL analysis is worthwhile, even when population size is small. This and future QTL studies can guide the breeding of high-value and nutritionally enhanced avocado cultivars. Results of QTL analysis are important to scientists and breeders worldwide and will lead to the identification of new, improved cultivars of avocado for distribution to growers.

Technical Abstract: The glossy, green-fleshed fruit of the avocado (Persea americana) has been the object of human selection for thousands of years. Recent interest in healthy nutrition has singled out the avocado as an excellent source of several phytonutrients. Yet as a sizeable, slow-maturing tree crop, it has been largely neglected by genetic studies, owing to a long breeding cycle and costly field trials. We use a small, replicated experimental population of 50 progeny, grown at two locations in two successive years, to explore the feasibility of developing a dense genetic linkage map and to implement quantitative trait locus (QTL) analysis for seven phenotypic traits. Additionally, we tested the utility of candidate-gene single-nucleotide polymorphisms developed to genes from biosynthetic pathways of phytonutrients beneficial to human health. The resulting linkage map consisted of 1346 markers (1044.7 cM) distributed across 12 linkage groups. Numerous markers on Linkage Group 10 were associated with a QTL for flowering type. One marker on Linkage Group 1 tracked a QTL for b-sitosterol content of the fruit. A region on Linkage Group 3 tracked vitamin E (a-tocopherol) content of the fruit, and several markers were stable across both locations and study years. We argue that the pursuit of linkage mapping and QTL analysis is worthwhile, even when population size is small.