Avocado is one of the major fruit crops in the world. It is regarded as the most nutritious of all fruit and has a significant volume in international trade, with an estimated world production of 3.2 million Mg in 2005 (U.S. Department of Agriculture, 2006b). In the United States, avocado affords a significant income to producers each year (U.S. Department of Agriculture, 2006a).
Although avocado is a major subtropical tree crop and is important for human nutrition, genetic improvement of avocado is rudimentary, resulting from obstacles to breeding that are related to the biology of the avocado tree itself. As a consequence, the process of avocado breeding continues to rely on the selection of promising open-pollinated cultivars, without knowledge of the pollen source. Promising open-pollinated progeny are field tested, usually in several locations, but progress is slow and field testing is expensive and time-consuming. Hybridization has played an important role in the origin of common avocado cultivars, many of which combine attributes from the three botanical races of avocado: the Guatemalan, Mexican, and West Indian races. The Guatemalan race likely originated in upland Guatemala and includes cultivars with fruit distinctive by their thick, rough, green skin. Trees have acceptable cold tolerance, although inferior to that of trees belonging to the Mexican botanical race. Mexican race avocados are thought to have originated in upland Mexico, and their fruit are covered by thin, smooth, black skin. The fruit flesh is rich in oils that confer a superior sensory quality. Leaves of this race are strongly aromatic (anise scented) when crushed. The West Indian race, despite its name, is thought to have originated in lowland coastal Guatemala, is characterized by large fruit with a watery consistency and has salinity tolerance. Marked sensitivity to cold temperatures renders West Indian cultivars unsuitable for cultivation in California, and hence are of little relevance to the study reported here.
Major obstacles to the genetic improvement of avocado include the fact that a typical avocado tree produces more than one million tiny flowers, most of which abscise before fruit set, and only about 0.1% or fewer of flowers are destined to yield mature fruit (Davenport, 1986; Davis et al., 1998), so controlled pollination is virtually impossible. It is reported that even when an avocado tree is caged with a beehive, the exclusive formation of selfed progeny cannot be guaranteed (Degani et al., 2003). Second, the large size of avocado trees makes large-scale experimental trials labor intensive and expensive in land resources, and the widespread use of rootstocks introduces additional maintenance costs because the rootstock may overgrow the scion and requires vigilant pruning. Lastly, avocado trees are slow to mature, and reliable evaluation of fruit yield and quality is rarely obtained until 4 to 5 years after planting, and this induces high labor and land usage costs. The tools of modern genetics can be used to identify pollen donors retrospectively, to accelerate the breeding cycle and to improve breeding efficiency. However, implementation of these tools requires both an abundant supply of genetic markers and an improved understanding of the genetic determination of economically important traits.
Usually commercially important traits of avocado, such as tree growth rate, fruit precocity, fruit quality, flavor, and so on, are controlled by multiple genes, and those of large effect are called quantitative trait loci (QTLs). These traits are also influenced by environmental factors, so the genetic determination of phenotypic variation must be assessed using the statistical framework of quantitative genetics. Microsatellites [or simple sequence repeats (SSR)] provide an abundant supply of markers. Ashworth et al. (2004) developed 127 microsatellite markers for avocado and used a subset to study genetic relationships among avocado cultivars (Ashworth and Clegg, 2003). Thus the stage is set to apply marker-assisted selection on commercial traits such as fruit yield and quality, based on the association of QTLs with SSR markers.
As a first step in QTL detection we report a study based on clonal replicates of 204 progeny with the avocado cultivar Gwen as maternal parent and multiple pollen parents (mainly ‘Bacon’, ‘Fuerte’, and ‘Zutano’, according to this study) grown in two different locations in California. ‘Gwen’ is a cultivar that was selected from the University of California avocado breeding program. Its grandparent is believed to have been ‘Hass’, which likely derived from a cross of a Mexican type × Guatemalan type backcrossed to a Guatemalan-type avocado (H. Chen, P.L. Morrell, M. de La Cruz, and M.T. Clegg, unpublished). Genetic analyses suggest that the parent of ‘Gwen’ was ‘Thille’, a Guatemalan-type cultivar (Davis et al., 1998). ‘Gwen’ has similar flavor to ‘Hass’, but with higher yields and its fruit stores on the tree better (Bergh and Whitsell, 1982). ‘Bacon’, ‘Fuerte’, and ‘Zutano’ are all popular cultivars grown in California since the early 20th century. Knowledge of their history is largely anecdotal, but recent surveys of DNA sequence diversity, from a wide sample of wild and cultivated avocados, suggest that ‘Fuerte’ is a Mexican-type avocado, whereas ‘Bacon’, like ‘Hass’, is predominantly Guatemalan with about 20% Mexican ancestry. ‘Zutano’ appears to have about equal Mexican and Guatemalan ancestries (H. Chen, P.L. Morrell, M. de La Cruz, and M.T. Clegg, unpublished). Figure 1 shows the photographs of fruit of these cultivars.
The phenotypic variance between trees of the same genotype provided an estimate of the environmental variance (VE). Broad-sense heritabilities (H) were estimated by subtraction of VE from the total phenotypic variance taken across all trees (VP). These data also permitted the calculation of phenotypic correlations among traits such as growth rate, flower abundance, and fruit set. Lastly, estimates of the average effect of different pollen donors on each of these traits are reported.
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