Molecular Plant Breeding 2025, Vol.16, No.1, 1-12 http://genbreedpublisher.com/index.php/mpb 2 This study aims to review the current state of QTL (Quantitative Trait Loci) mapping research on key agronomic traits in cucumber, focusing on exploring the genetic basis of these traits. It summarizes the findings from various studies, highlights the significance of identified QTLs, and discusses their implications for cucumber breeding programs. The goal is to provide insights for future research and breeding strategies to enhance the improvement potential of this economically important crop. 2 Overview of Agronomic Traits in Cucumber 2.1 Description of key traits: yield, fruit quality, disease resistance, and growth habits Cucumber (Cucumis sativus L.) is a globally significant vegetable crop, valued for its fresh and processed forms. The key agronomic traits in cucumber include yield, fruit quality, disease resistance, and growth habits. Yield in cucumber is influenced by several factors, including fruit size, fruit number, and flowering time. Quantitative trait loci (QTL) mapping has identified numerous QTLs associated with these traits. For instance, QTLs for fruit length and weight have been detected, with some explaining a significant portion of phenotypic variance (Wei et al., 2014; Zhu et al., 2016; Pan et al., 2022). Additionally, traits like fruit carpel number and sex expression also impact yield (Pan et al., 2020). Fruit quality encompasses attributes such as fruit size, shape, flesh thickness, and skin characteristics. QTLs for fruit size and shape have been extensively studied, with major-effect QTLs identified on various chromosomes (Pan et al., 2017; Gao et al., 2020; Pan et al., 2022). For example, the QTL FS5.2 has been linked to fruit shape, influencing both longitudinal and radial growth (Pan et al., 2017). Other quality traits like fruit neck length are also controlled by specific QTLs, such as Fnl7.1, which encodes a late embryogenesis abundant protein (Xu et al., 2020). Disease resistance is a critical trait for maintaining cucumber yield and quality. Numerous QTLs have been identified for resistance to various pathogens. For instance, the legendary cucumber inbred line WI2757 possesses resistance against nine pathogens, with QTLs mapped for powdery mildew resistance and other diseases (Lee et al., 2020; Pan et al., 2020). The identification of these QTLs facilitates marker-assisted selection in breeding programs (Wang et al., 2020b). Growth habits, including flowering time and plant architecture, are essential for optimizing cucumber cultivation. QTLs for flowering time have been mapped, with some playing significant roles in photoperiod-dependent flowering (Pan et al., 2017). Additionally, traits like fruit stalk length and plant height are influenced by specific QTLs, contributing to the overall growth habit of the plant (Yuan et al., 2008). 2.2 Agronomic importance and challenges associated with each trait High yield is a primary goal in cucumber breeding. However, achieving consistent high yields can be challenging due to environmental variability and the complex genetic basis of yield-related traits. The identification of stable QTLs across different environments is crucial for developing high-yielding varieties (Wei et al., 2014; Zhu et al., 2016). Fruit quality directly affects marketability and consumer preference. Breeding for desirable fruit quality traits, such as size, shape, and texture, is challenging due to the polygenic nature of these traits. Fine mapping and cloning of major QTLs, like FS5.2 and Fnl7.1, are essential for improving fruit quality through marker-assisted selection (Pan et al., 2017; Xu et al., 2020; Pan et al., 2022). Disease resistance is vital for sustainable cucumber production. The narrow genetic base of cucumber poses a challenge for breeding disease-resistant varieties. The identification and utilization of QTLs for disease resistance, such as those found in WI2757, are critical for developing resilient cultivars (Lee et al., 2020; Pan et al., 2020). Optimizing growth habits is important for efficient cultivation and harvesting. Traits like flowering time and plant architecture need to be finely tuned to suit different growing conditions. The genetic complexity of these traits requires comprehensive QTL mapping and the development of molecular markers for effective breeding (Yuan et al., 2008; Pan et al., 2017).
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