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Fruit set and S-alleles in pear

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Written by Ryan Sheick and Stefano Musacchi, WSU Tree Fruit Research and Extension Center, Wenatchee. June 2020.


European pear (Pyrus communis L.) is an important fruit crop to the Pacific Northwest, and grower profitability is correlated with fruit production and quality. Fruit set in pear is largely dependent on pollination (Webster 2002). During pollination, pollen is transferred to the floral stigma where it begins to germinate. Pollen tubes from germinated pollen penetrate the stigma and grow down the style and toward the ovary until they reach the ovules and complete fertilization. Fertilized ovules develop into seeds, which help promote fruit growth as an endogenous source of phytohormones (Zhang et al, 2007). High ovule fertilization rates are associated with the development of high-quality fruit (Claessen et al., 2019), which highlights the importance of pollination to the pear industry.

The role of insect pollinators on fruit set

The primary facilitator of pollination in pear is the European honeybee (Apis mellifera) (Quinet et al., 2016). Honeybees forage for pollen and nectar as a source of protein and carbohydrates, respectively, but floral attraction and visitation is influenced by nutritional chemistry, which varies among plant species (Farkas et al., 2002; Russo et al., 2019).

Pear blossoms have been noted to be less attractive to honeybees than other flowering perennials, such as apple (Quinet et al., 2016), and differences in nectar composition between Pyrus species and cultivars within a species is reflected in honeybee visitation (Farkas et al., 2002b; Seo et al., 2019). Cultivars that produce nectar with balanced sugar profiles that include glucose, fructose, and sucrose host more bee activity than cultivars that produce hexose-dominant nectars composed of primarily glucose and fructose (Farkas et al., 2002; Seo et al., 2019). Furthermore, cultivars that produce floral nectars with relatively high sugar concentrations have been observed to attract more bees than cultivars that produce nectar with lower sugar concentration (Farkas et al., 2002). Although nectar yield, chemistry, and temporal production is cultivar-dependent, these factors are also influenced by environmental conditions and year-to-year variability (Farkas et al., 2002b).

Parthenocarpy in pear

Pear production is primarily driven by pollination efficiency and cross compatibility of pollen sources, however, various degrees of parthenocarpy have been reported in some pear cultivars (Weinbaum et al., 2001; Moriya et al., 2005; Nishitani et al., 2012). Parthenocarpic cultivars are able to produce seedless fruit from unpollinated blossoms, and the extent of parthenocarpy can depend on both genetic and environmental factors (Weinbaum et al., 2001). Integrating parthenocarpic traits into pear breeding programs has been suggested as a strategy to improve production in the face of challenging pollination conditions (Nishitani et al, 2012). Furthermore, synthetic PGRs can be applied at bloom to promote fruit set in pear without pollination (Crane 1969; Zhang et al., 2008). Although parthenocarpic pear cultivars may help mitigate problems arising from inadequate pollination, some studies have observed these fruits to be smaller, misshapen, and lacking in key quality metrics such as soluble solid content in comparison to fully-seeded fruit (Weinbaum et al., 2001; Moriya et al., 2005; Zhang et al., 2008).

Self-incompatibility in pear

Pears, like other members of Rosaceae, are considered self-incompatible and generally require cross-pollination to promote seed and fruit set (Quinet et al., 2016; Claessen et al., 2019). This is due to a multi-allelic genetic mechanism that enables the rejection of “self” pollen tubes before they reach the ovary. This mechanism is known as gametophytic self-incompatibility (GSI) and is enabled by the genetic diversity at the S locus (the region of the genome that carries genes involved in GSI) (de Nettancourt, 1977). Because pears are usually diploid (meaning they have two sets of genes: one from each parent cultivar), self-incompatibility genotypes, or S-genotypes, consist of two different alleles (variations of the same gene), although three or more S-alleles may exist in polyploid cultivars (Takasaki et al, 2013). The S-alleles of pear cultivars dictate levels of cross-compatibility with other cultivars: full incompatibility occurs when both the S-alleles of the pollen donor cultivar are identical to the S-alleles carried by the maternal cultivar; full compatibility occurs when none of the pollen donor S-alleles match any of the S-alleles carried by the maternal cultivar, and semi-compatibility occurs when one (but not both) of the pollen donor S-alleles is identical to one of the maternal cultivar S-alleles. In the case of semi-compatibility, pollen may still be used for fertilization; however, the proportion of useful (i.e., compatible) pollen produced is reduced by 50%. S-alleles in European pear follow numerical designations, beginning with S101 (Goldway et al., 2009). See Table 1 for a summary of S-genotypes of some pear cultivars that have been reported in the literature.

Table 1. S-genotypes of Pyrus communis cultivars under the most recent S-allele designation system defined by Goldway et al. (2009).
Cultivar S-genotype Reference
Abbé Fétel S104/S105 Goldway et al. (2009), Takasaki et al. (2006), Zuccherelli et al. (2002), Sanzol (2009)
Alexandrine Douillard S103/S104 Goldway et al. (2009), Takasaki et al. (2006), Mota et al. (2007), Sanzol (2009)
Bartlett (Williams, Bon-Chrétien) S101/S102 Goldway et al. (2009), Takasaki et al. (2006), Mota et al. (2007), Sanzol and Herrero (2002)
Beurré Bosc (Kaiser) S107/S114 Goldway et al. (2009), Tassinari (2005), Zuccherelli et al. (2002), Zisovich et al. (2009)
Beurré d’Anjou S101/S114 Goldway et al. (2009), Moriya et al. (2007)
Beurré de l’Assomption S102/S106 Goldway et al. (2009), Moriya et al. (2007), Sanzol (2009)
Beurré Hardy S108/S114 Goldway et al. (2009), Zuccherelli et al. (2002), Moriya et al. (2007), Sanzol (2009)
Carmen S105/S120 Sanzol (2009)
Cascade S101/S104 Goldway et al. (2009), Takasaki et al. (2006), Zuccherelli et al. (2002), Zisovich et al. (2009)
Concorde S104/S108 Goldway et al. (2009), Tassinari (2005), Sanzol (2009)
Devoe S108/S118 Goldway et al. (2009), Takasaki et al. (2006), Sanzol (2009)*
Doyenné du Comice S104/S105 Goldway et al. (2009), Takasaki et al. (2006), Zuccherelli et al. (2002)
General Leclerc S102/S118 Goldway et al. (2009). Takasaki et al. (2006), Mota et al. (2007), Sanzol (2009)*
Harrow Sweet S102/S105 Goldway et al. (2009), Takasaki et al. (2006), Sanzol (2009)
Harvest Queen S101/S102 Goldway et al. (2009), Takasaki et al. (2006), Sanzol (2009)
Max Red Bartlett S101/S102 Goldway et al. (2009), Takasaki et al. (2006), Sanzol (2009)
Rocha S101/S105 Goldway et al. (2009), Mota et al. (2007), Sanzol (2009)
Rogue Red S105/S114 Goldway et al. (2009), Moriya et al. (2007), Sanzol (2009)
Starkrimson (Clapp’s Rouge, Kalle, Red Clapp’s) S101/S108 Goldway et al. (2009), Takasaki et al. (2006), Mota et al. (2007), Zisovich et al. (2004), Zisovich et al. (2009)
Winter Nelis S103/S107 Goldway et al. (2009), Takasaki et al. (2006), Sanzol (2009)
* Sanzol (2009) reports these cultivars carry S12 (former designation); S12 (EF418043) reported by Sanzol (2009) is synonymous with Sq (AB236424), which was reassigned to S118 by Goldway et al. (2009).



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Stefano Musacchi
Endowed Chair for Tree Fruit Production Systems and Horticulture
Washington State University
Tree Fruit Research and Extension Center
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