**3.3 Heterosis breeding**

Cucumber is a monoecious and cross pollinated crop, there is a great scope for exploitation of heterosis. High level of hybrid vigor could be obtained with the

**207**

*Classical Genetics and Traditional Breeding in Cucumber (*Cucumis sativus *L.)*

involvement of diverse parents. Several studies have been conducted to identify the best heterotic combination for earliness, yield and quality traits in cucumber. Appreciable heterosis was observed over better parent and top parent for many economic traits like node number of first female flowers, number of fruits per plant, days to fruit set, days to first fruit harvest, yield per plant [82–89]. Significant heterosis has also been reported for earliness characters using gynoecious lines [82, 83, 90] and quality characters [88]. The hybrid combinations of gynoecious × gynoecious and gynoecious × monoecious showed maximum heterosis followed by monoecious × monoecious hybrids for earliness and yield per plant [83, 90]. Therefore yield of the cucumber can be enhanced by using gynoecious line as one of the parent in future breeding programme. Hence breeder should concentrate mainly number of fruits rather than the fruit size to increase yield in cucumber.

Traditional breeding has the major limitations of its dependency only on selection of traits based on morphological markers (leaf characters, flowering traits including sex ratio, fruit color, fruit size, shape etc.) from a segregating population. Traditional breeding has been effectively utilized for the improvement of qualitative characters. Traits like beta carotene containing cucumbers (linked to orange flesh) and parthenocarpy (linked to seedless) are the classical examples of selection using morphological markers. Traditional breeding has also been exploited for major changes in important quantitative characters including early maturity, fruit size, free from bitterness and fruit yield. Further genetic improvement in quantitative traits using traditional strategies will be more time consuming. For examples, selection for parthenocarpy will be difficult using visual appearance as was done in the past. It is fairly straight forward to select cucumber genotypes containing high beta carotene from asegregating population, but it is difficult to distinguish levels of

The most effective method for selecting a multiple allele trait is to utilize multiple markers to identify a majority of the alleles. This is especially true to the cucumber crop, which requires a large amount of space to evaluate. However, phenotypic markers will have an intrinsic disadvantage if the trait is influenced by the environmental factors. Development of disease resistance lines are often challenging for cucumber crop. Many disease resistance traits are quantitative; their expression is affected by environmental factors and requires complex inoculation procedure. Cucumber mosaic virus, downy mildew and powdery mildew are the good examples of diseases where development of new cultivar with high level of resistance has thus far proven difficult. Host plant resistance strategy should be utilized for control of these diseases. For these diseases several resistant sources have been identified [91] across the world but these genotypes do not withstand

Molecular markers have the potential to overcoming the limitations of traditional selection methods, since they are non-destructive, eliminate environmental variation associated with disease resistance and can be evaluated for multiple traits simultaneously. However, the molecular breeding requires the development of segregating populations for the traits of interest and the trait must be properly identified during marker identification. Recently, due to the advance technologies of genome sequencing, genome wide association study (GWAS) or Linkage disequilibrium (LD) mapping have gained popularity which is a powerful and alternative

with high disease pressure under multiple locations.

**3.5 Rationale for molecular breeding**

*DOI: http://dx.doi.org/10.5772/intechopen.97593*

**3.4 Limitations of traditional breeding**

carotene based on color.

*Classical Genetics and Traditional Breeding in Cucumber (*Cucumis sativus *L.) DOI: http://dx.doi.org/10.5772/intechopen.97593*

involvement of diverse parents. Several studies have been conducted to identify the best heterotic combination for earliness, yield and quality traits in cucumber. Appreciable heterosis was observed over better parent and top parent for many economic traits like node number of first female flowers, number of fruits per plant, days to fruit set, days to first fruit harvest, yield per plant [82–89]. Significant heterosis has also been reported for earliness characters using gynoecious lines [82, 83, 90] and quality characters [88]. The hybrid combinations of gynoecious × gynoecious and gynoecious × monoecious showed maximum heterosis followed by monoecious × monoecious hybrids for earliness and yield per plant [83, 90]. Therefore yield of the cucumber can be enhanced by using gynoecious line as one of the parent in future breeding programme. Hence breeder should concentrate mainly number of fruits rather than the fruit size to increase yield in cucumber.

#### **3.4 Limitations of traditional breeding**

*Cucumber Economic Values and Its Cultivation and Breeding*

total yield in pickling cucumber. Breeding technique for achieving these goals differ among breeders as does his/her definition of quality fruits. Gynoecious sex has also been combined with parthenocary trait to set fruit without pollen particularly for green house cultivation, since it eliminates the need for pollination and produces superior quality fruits. Gynoecious sex expression has been commercially exploited for the hybrid seed production since the female plants do not produce male flowers. Recently, the cucumber breeders have been interested in the development of carotene rich (yellow cucumber) lines. Several cucumber varieties have been developed for early characters, high yield, and resistance to diseases using heterosis breeding. The most commonly used breeding methods for the improvement of cucumber is inbreeding, single plant selection from segregating populations, heterosis breeding and presently marker assisted selection (MAS). The most common breeding procedure followed in cucumber is the selection from a local cultivar. Several F1 hybrids have been developed in cucumber both from public and private sector through hybridization across the globe. Cucumber is easily grown, indeterminate plant types, offer plenty of large flowers to work with over a long period of time where a large variation is observed for quantitative and qualitative characters. The use of gynoecious lines as one of the parent in breeding programme has made possible to enhance the area under hybrid cultivation. Recent QTL mapping and cloning studies in cucumbers for many quantitative traits will present a complete picture on the genetic architecture of these traits. The identification of molecular markers (SNPs and SSRs) for earliness, yield and quality traits would be directly useful in genome selection to expedite cucumber breeding in different market classes. The cloning of QTLs and genome wide association studies (GWAS) for important quantitative traits will be more innovative and future-focused for trait specific breeding of cucumber.

Several breeding methods have been employed for the genetic improvement of cucumber depending on the specific breeding objectives. Single plant selection, single seed descent method, mass selection, simple backcross breeding, pedigree selection, hybridization, use of sex inheritance and chemicals in breeding, and population improvement and extraction of inbred lines are the most common methods used. In recent time, marker assisted selection derived lines have been developed for the improvement of quantitative and qualitative traits in different cucumber market classes. Simple backcross breeding is quite useful for transferring characters governed by single genes e.g. disease resistance or quality traits from donor lines to more stable recurrent parents. Often, six generations of selection and backcrossing to the recurrent parent are required to recover the desired genotypes (recurrent parent + additional trait). Single seed descent method is useful for the development of inbred lines by self pollination. In chemical breeding the gynoecious lines are treated with Silver nitrate/silver thiosulphate to induce hermaphrodite flowers for pollination. The population improvement method is based on recurrent selection and aims at long term gains for the characters having low to moderate heritability. The exploitation of hybrid vigor in cucumber is desirable due to high heterosis for earliness, yield and disease resistance. Heterosis breeding can exploit the genetic diversity present in cucumber for various growth and yield characters. In Western

countries almost 90% of the area of cucumber is under F1 hybrids.

Cucumber is a monoecious and cross pollinated crop, there is a great scope for exploitation of heterosis. High level of hybrid vigor could be obtained with the

**206**

**3.3 Heterosis breeding**

**3.2 Breeding methods**

Traditional breeding has the major limitations of its dependency only on selection of traits based on morphological markers (leaf characters, flowering traits including sex ratio, fruit color, fruit size, shape etc.) from a segregating population. Traditional breeding has been effectively utilized for the improvement of qualitative characters. Traits like beta carotene containing cucumbers (linked to orange flesh) and parthenocarpy (linked to seedless) are the classical examples of selection using morphological markers. Traditional breeding has also been exploited for major changes in important quantitative characters including early maturity, fruit size, free from bitterness and fruit yield. Further genetic improvement in quantitative traits using traditional strategies will be more time consuming. For examples, selection for parthenocarpy will be difficult using visual appearance as was done in the past. It is fairly straight forward to select cucumber genotypes containing high beta carotene from asegregating population, but it is difficult to distinguish levels of carotene based on color.

The most effective method for selecting a multiple allele trait is to utilize multiple markers to identify a majority of the alleles. This is especially true to the cucumber crop, which requires a large amount of space to evaluate. However, phenotypic markers will have an intrinsic disadvantage if the trait is influenced by the environmental factors. Development of disease resistance lines are often challenging for cucumber crop. Many disease resistance traits are quantitative; their expression is affected by environmental factors and requires complex inoculation procedure. Cucumber mosaic virus, downy mildew and powdery mildew are the good examples of diseases where development of new cultivar with high level of resistance has thus far proven difficult. Host plant resistance strategy should be utilized for control of these diseases. For these diseases several resistant sources have been identified [91] across the world but these genotypes do not withstand with high disease pressure under multiple locations.

#### **3.5 Rationale for molecular breeding**

Molecular markers have the potential to overcoming the limitations of traditional selection methods, since they are non-destructive, eliminate environmental variation associated with disease resistance and can be evaluated for multiple traits simultaneously. However, the molecular breeding requires the development of segregating populations for the traits of interest and the trait must be properly identified during marker identification. Recently, due to the advance technologies of genome sequencing, genome wide association study (GWAS) or Linkage disequilibrium (LD) mapping have gained popularity which is a powerful and alternative

genetic mapping approach for the identification and dissecting important QTL regions which harbor candidate genes of interest in plants [92]. When, GWAS performed on large set of diversity panel, it provides the higher resolution mapping of traits associated variants because it exploits historical recombination events. GWAS study identifies genomic regions harboring loci controlling different traits. The sequencing data will help in the development of desirable genotypes in the form of cultivar which will help the farmers to receive higher returns on their investments. The potential of molecular breeding to save money is in their long term utilization in combination with multiple markers for a wide variety of traits; this will allow cucumber breeder to select for multiple traits from the large populations.
