Avocado is a diploid (2n = 24), outcrossing tree with a genome size of 883 Mb (Arumuganathan and Earle, 1991). Widely grown throughout tropical and subtropical regions of the world, it is valued for its nutritious fruit (Bergh, 1992, Ding et al., 2007). Avocado is native to Mexico and Central America where it was domesticated an estimated 2900 to 8400 years ago (Smith, 1966), although more recent sources (Galindo-Tovar et al., 2008) narrow the range to ≈4000 to 6500 years ago. Domestication took place at least three times, yielding the three distinct races of cultivated avocado that are used today (see Ashworth et al., 2011). This history of separate domestications results in a wide base of genetic variation (Chen et al., 2008, 2009) and accounts for the broad range of phenotypic variation observed among avocado cultivars.
The avocado fruit includes an array of antioxidants (e.g., Ding et al., 2007; Kim et al., 1998, 2000) with manifold beneficial properties, including cholesterol reduction (Lopez-Ledesma et al., 1996) and anticarcinogenicity (D’Ambrosio, 2007; Ding et al., 2009). Dissecting the genetic and environmental factors that control the level of individual nutrients in avocado will provide important information to avocado breeders, growers, and consumers for the selection of nutritionally enriched cultivars. Conventional breeding approaches in avocado involve considerable time and resources, mainly as a result of the avocado’s long generation time and extended land requirements. Our goal is to estimate the genetic determination of nutritive traits in avocado as a prelude to the implementation of a program of marker-assisted selection (MAS). In this article we present data on the heritability of three compounds that are known to confer a health benefit (carotenoids, β-sitosterol, and vitamin E).
Avocados are a significant source of dietary phytosterols, in particular β-sitosterol, that has been shown to decrease cholesterol absorption in the intestine, thus lowering blood cholesterol levels (Moghadasian and Frohlich, 1999). On average, avocados contain ≈760 μg β-sitosterol per gram of fruit or ≈114 mg per average avocado (Duester, 2001). Clearly, selection for increased β-sitosterol content is appealing, but little is known about genetic determination of β-sitosterol content among avocado genotypes. In a study of rapeseed (Brassica napus), heritability for β-sitosterol was high: 0.90, 0.82, and 0.85 in three populations grown in three different environments (Amar et al., 2007), making β-sitosterol a good candidate for selection in rapeseed.
Total carotenoids in avocado are mainly composed of β-carotene, α-carotene, β-cryptoxanthin, lutein, and zeaxanthin with lutein being the largest component (Lu et al., 2009). The carotenoids in general are antioxidants and are also considered important in combating cancer, eye disease, and other health conditions (Johnson, 2002). Heritability of carotenoids in lettuce (Lactuca sativa) has been estimated to be 0.50 (Gupta et al., 2008), and a study in maize (Zea mays) reported heritability values ranging from 0.49 to 0.87 (Wong et al., 2004). Studies on carrot (Daucus carota) and sorghum (Sorghum bicolor) carotenoids showed higher heritability at ≈0.90 (Fernandez et al., 2008; Santos and Simon, 2002, 2006). Carotenoid-fortified crops have been developed for other species (Römer and Fraser, 2005; Römer et al., 2000).
Vitamin E is an important antioxidant that functions by binding free radical intermediates. It acts as an inhibitor of platelet aggregation and protects lipids by preventing the oxidation of polyunsaturated fatty acids (Traber and Stevens, 2011). In this study we are measuring α-tocopherol, the most biologically active form of vitamin E. The U.S. Department of Agriculture (USDA), Agricultural Research Service reports vitamin E content for a Florida-grown ‘Hass’ avocado to be ≈19.7 μg·g−1. A study of tocopherol content in sunflower (Helianthus annuus) seeds reported a heritability ranging from 0.67 to 0.78 (Del Moral et al., 2012), making vitamin E a good candidate for selection in that species.
Molecular tools have the potential of improving current avocado breeding efforts (Tester and Langridge, 2010). Selection on molecular markers such as simple sequence repeats (SSRs) or single nucleotide polymorphisms (SNPs) associated with traits of economic value promise to increase breeding efficiency for beneficial traits, especially in long-lived tree crops like avocado. SSR- and SNP-based MAS offers the potential to combine target traits in the same genotype more precisely and with shorter selection cycles than conventional mass selection procedures. We have begun to identify SSR and SNP markers in genes from the carotenoid, β-sitosterol, and vitamin E pathways to search for markers that are predictive of high concentrations of these compounds in avocado fruits.
In this article, we examine phenotypic variation for carotenoids, β-sitosterol, and vitamin E in a breeding population of avocado, and we determine the broad-sense heritability of these traits in an experimental design based on estimating trait variances within and between clonal genotypes to determine if nutrient concentration will respond efficiently to artificial selection. This design does not permit the estimation of narrow sense heritability, which is important for mass selection, but MAS schemes allow the exploitation of non-additive sources of variation. To measure individual levels of these traits, we developed simple assays for nutrient content that can be applied to hundreds of fruit easily and cheaply. This information will be useful in understanding the relative role of environmental vs. genetic factors in the determination of nutritive trait variation and will assist in providing a basis for selection for nutritive content in avocado fruit.
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