Pyrus (Rosaceae) contains ≈22 species of trees originating from Asia, northern Africa, and Europe that have been cultivated extensively as both fruit and landscape plants (Challice, 1973). Pyrus calleryana was first introduced into cultivation by E.H. Wilson in 1908 (Santamour and McArdle, 1983; Vincent, 2005). However, it was not until the cultivar ‘Bradford’ was released in the 1960s by the U.S. Department of Agriculture (USDA) that the species gained widespread popularity as a landscape tree (Bell et al., 2004). Many other cultivars of P. calleryana have been introduced since and are valued for their abundance of white flowers, showy fall color, broad pest resistance, attractive forms, and ability to thrive in the USDA plant hardiness zones 5 to 8 (potentially 9). Pyrus calleryana can be susceptible to fire blight as well as splitting and breakage of trunks and branches in older trees. More recently concerns have been raised regarding weediness (Dirr, 1998). Pyrus calleryana is listed by the U.S. Fish and Wildlife Service as a plant invader of mid-Atlantic natural areas (Swearingen et al., 2010). Birds eat the fruits and disperse seeds into nearby areas where it commonly naturalizes in old fields and along highways.
Development of highly infertile cultivars of P. calleryana and related hybrids would be desirable as an alternative to the fertile cultivars currently available. One approach for producing seedless plants is through the development of triploids. Triploids typically have low fertility due to a reproductive barrier whereby three sets of chromosomes cannot be divided evenly during meiosis yielding unbalanced segregation of chromosomes. Seedless bananas (Musa sp.), watermelons (Citrullus lanatus), and some citrus (Citrus sp.) are notable examples of triploid plants that have been purposefully developed to minimize seeds (Rounsaville, 2011). This approach has also been used to develop highly infertile triploid cultivars of various species that are valuable nursery crops, but potentially weedy in some environments, including trumpet vine (Campsis ×tagliabuana) (Oates et al., 2014), tutsan (Hypericum androsaemum) (Trueblood et al., 2010), maiden grass (Miscanthus sinensis) (Rounsaville et al., 2011), and ruellia (Ruellia simplex) (Freyre and Moseley, 2012).
Triploids are typically highly infertile; however, limited fertility and seed production can result from the formation of apomictic embryos or through the union of aneuploid or unreduced gametes (Ramsey and Schemske, 1998; Rounsaville et al., 2011). Flow cytometric screening of seeds and/or seedlings can often elucidate these reproductive pathways (Eeckhaut et al., 2005; Matzk et al., 2000). Diploid plants, with standard sexual fertilization, form a 2Cx (diploid) embryo and 3Cx (triploid) endosperm, where 1Cx represents the monoploid genome size of one complete set of chromosomes. In gametophytic apomixis, the unreduced embryo sac and gametophyte develop autonomously, without fertilization, also forming a 2Cx embryo. However, the unreduced endosperm will have either a 4Cx cytotype (autonomous) or greater if fertilized (pseudogamous) (Barcaccia and Albertini, 2013; Koltunow et al., 2013). For triploids that undergo gametophytic apomixis, the embryo will be 3Cx while the endosperm will be 6Cx or greater. Although gametophytic apomixis involves nonreduction of the embryo sac, male gametes may also be unreduced in triploids producing 2n pollen with a 3Cx cytotype (Ramsey and Schemske, 1998). Alternatively, aneuploids are a product of unbalanced chromosome segregation in meiosis and have either missing or extra individual chromosomes (Brownfield and Kohler, 2011). For triploids, these would result in gametes with ≈1.5Cx cytotypes, but potentially varying substantially.
Pyrus sp., including P. calleryana, have a base chromosome number of 17 and are primarily diploid (2n = 2x = 34) with occasional triploid and tetraploid variants (Zielinski and Thompson, 1967). Researchers at the North Carolina State University’s Mountain Crop Improvement Laboratory, Mills River, NC, developed a population of triploid Pyrus hybrids by crossing artificially induced tetraploids of P. calleryana with various diploid Pyrus taxa. The objective of this study was to evaluate fertility and reproductive pathways in newly developed triploid Pyrus hybrids for potential use as highly infertile alternatives to diploid cultivars.
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