**2.2 Sex expression**

*Cucumber Economic Values and Its Cultivation and Breeding*

anti-cancer benefits. Cucumbers are also used for digestive benefits and mood stability when modulating stress. Cucumbers fortify cells so they may retain hydrated and work at the highest levels, and may slow age-related cellular deteriorations [5]. The available genetic diversity within the cultivated cucumber is very low [2, 6–9] which is the major impediment in the genetic improvement of various cucumber market classes [10]. Thus, increasing the genetic diversity of cultivated cucumber is an important task for public sector research [11]. The cultivated cucumber has narrow genetic base with 3–8% polymorphism within the cultivated genotypes, and 10–25% between botanical varieties [12]. Earliness, high yield, uniform fruit shape, size, color and better quality are prerequisites for the release of the cucumber varieties and F1 hybrids for open field condition. In addition to these characters, gynoecious and parthenocarpic traits are desirable for green house cucumber production. Identification of genotypes tolerance to drought is also one of important breeding objective in cucumber [13]. Cucumber is a monoecious vegetable crops species. However, several gynoecious varieties and F1 hybrids have been developed by introgression of *F* locus (gynoecious gene) in the background of different market classes of the cucumber for commercial production. The utilization of gynoecious lines are economical and easier for hybrid seed production by reducing the cost of male flower pinching and hand pollination [14]. The present day cucumber F1 hybrids for open field production derived from the cross between gynoecious × monoecious and monoecious × monoecious where as green house grown F1 hybrids are the result of the cross of gynoecious × gynoecious lines with parthenocarpic traits. The evaluation and selection of the genotypes only based on phenotype characters for high yield and stability of gynoecious sex form require many years in multiple environments which is very expensive and time consuming process. The whole genome sequencing in cucumber [15] have opened the way to utilize the DNA markers viz. simple sequence repeats (SSRs) for gene mapping, marker assisted selection and marker trait association for several economic traits in cucumber [16–18]. With the development of high density linkage map several traits have been mapped in cucumber including, flowering time [19] fruit quality [20], diseases [17, 21], yield [22], fruit spines [23], fruit color [8], chromosomal mapping and QTL analysis of resistance to downy mildew [24–26], yellow fruit flesh [27], pleiotropic andromonoecy and carpel number [28, 29]. QTLs have been identified in Sikkim cucumber (*Cucumis sativus* var. *sikkimensis*) for important horticultural traits including flowering time, fruit size, flesh thickness, fruit spines, fruit

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color [30].

**2. Classical genetics in cucumber**

**2.1 Genetics of quantitative and qualitative characters**

The deep understanding about cucumber crops biology was only possible due to mendelian's classical genetics which have made possible for the cucumber breeders to develop improved varieties and F1 hybrids. The knowledge about different genes which affects economic traits facilitated breeders to develop proper genetic resources for the development of trait specific genetic stock for further use for genetic improvement of cucumber. For example, the size of the population will be much smaller if a breeder is selecting for a trait controlled by a single gene, than if the trait is controlled by multiple genes with a large environmental influence. The application of Mendelian genetics using classical techniques in cucumber has facilitated the discovery of a number of genes for yield, quality, plant architecture, and disease and pest resistance in both slicing and pickling cucumber. The yield in

In commercial cucumber cultivars, the type of sex forms (gynoecious or monoecious) and the amount of their expression is important because these have a direct effect on harvesting date, production and productivity of this crop. Flowering time in cucumber played a critical role in fetching early market price and increase in fruit yield for the growers. In cucumber, the type of sex from (gynoecious or monoecious) and the amount of their expression have direct effect on harvesting time, production and productivity. The sex expression also played a vital role in seed production as well as development of new plant types. The flowering traits like node number at which first female flower appear, days to first pistillate flower opening, and male: female (♂:♀) flower ratio (sex ratio) are the important traits for determination of earliness and fruit yield. The sex expression in cucumber is controlled by three genes, *F*, *M*, and *A* [31]. The degree of female flower expression is controlled by *F/f* gene [31, 32]. The *F* locus determines the amount of femaleness (*FF* > *Ff* > *ff*). Gynoecious sex expression in F1 hybrid of cross of gynoecious × monoecious is governed by partial dominance ([33]; Perl-Treves and Rajagopalan [34] where as in gynoecious × subandroecious, it was governed by multiple genes [35]. single gene with dominant or incomplete dominance [36], single dominant gene [8, 37–39] and oligogene with some modified genes [40], three major QTLs conferring subgynoecy in cucumbers [41]. Seven gynoecious QTLs were detected on chromosomes 5 and 6 in backcross population [42]. These studies suggested that gynoecious is an important economic trait for determinant of earliness and yield in cucumber.
