Advances in melon breeding ( Cucumis melo L.): An update

Melon ( Cucumis melo L.) a member of family Cucurbitaceae is extensively cultivated for its fleshy fruits. Based upon the agroclimatic zones of cultivation as well as concerning the regional preferences, melon displays significant variability phenotypic and biochemical attributes. Below, an effort is put forth to considerably evaluate the scope of achievements while in the growth as well as the enactment of melon breeding programs by employing the newest solutions. Melon breeding has achieved critical milestones throughout the previous century, and we hope this trend will go on to persist down the road. However, experiments have to understand new genetic information for genes associated with the challenges imposed by climate change. The identification of valuable hereditary and also metabolic variability in the form of landraces and melon wild relatives will be useful for harvest diversification and also for the broadening of the cultivated melon genetic base. Whereas, considerable information on genomics, and melon metabolomics, is beneficial for dissecting the basis of the inheritance of important traits. Overall, we hope the manuscript is going to serve as a crucial resource for the melon breeders.

. There is an extensive perturbation in the morphology of the fresh fruits of melon, in shape, colour, texture and flavour (Kirkbride, 1993). Melon fruit can reach a size of up to 20 kg, while the shape can vary from spherical to long type. Whereas, in flavour, fruits can be from bland to sweet, sour, or maybe bitter (Kirkbride, 1993;Stepansky et al., 1999;Liu et al., 2004;Monforte et al., 2004).
The recent breeding effort has developed several hybrids aiming to enhance storage and shelf life, disease resistance and resistance to abiotic stresses. Generally, cultivars with a longer shelf life (LSL types) are thought of by customers as having low fruit quality and therefore, present minimal acceptability (Wolff and Dunlap, 1995). Although, most of the breeders concur that powdery mildew resistance and fusarium are essential and fundamental demands of farmers for the modern melon varieties (Capel et al., 2010; Branham et al., 2018). Also, insect pest management is a significant component which benefits commercial yield by providing a much better yield safety measures, reduced waste along with much better fruit quality (Ramamurthy and Waters, 2015). Melon leaf curl New Delhi virus, which is transmitted by whiteflies is causing a considerable yield loss throughout Asia and Europe. More recently, a QTLs are identified for the disease resistance in melon (Saez et al., 2017).
Similarly, the quantitative genes of yield and also yield-related traits in melon are also thoroughly studied. Additionally, many researchers have reported on the merging ability and heterosis benefits in fruit yield along with other morphological characteristics of melon under even under biotic and abiotic stress conditions (Kamer et al., 2015;Varinder and Vashisht, 2018). Recently, due to its very affordable cost and high sensitivity, the next-generation sequencing is possible for melon. Especially GBS (genotyping by sequencing) and RAD sequencing of multiplexed samples are reproducible, fast, and accessible (Ganal et al., 2014;Shang et al., 2020;Zhang et al., 2016). Below, an attempt is put forth to significantly assess the scope of accomplishments while in the development and enactment of melon breeding programs by employing the latest technologies. We hope this manuscript will serve as an important resource for the melon breeders.

DIVERSITY
Cucumis melo belongs to family Cucurbitaceae is a horticultural crop of great economic importance and showed a wide range of diversity for agro-morphological and fruit traits (Zitter et al., 1996;Silberstein et al., 2003). Leaves are simple, three or five-lobed, borne singly at nodes, have significant variation for colour, shape and size (Kirkbride, 1993). Tendrils are simple and borne on leaf axils. Melons fruits are classified as Pepo with three ovary sections (Font-Quer, 1979). Variations in melon fruits were observed for shape, size, internal and external colour.
Flesh color varied from orange, light orange, pink, white, green; rind color varied from green, white, orange, yellow and red grey; rind texture as smooth, striped, warty, rough and netted; shape from round, elongated, flattened; size 4cm in C. melo var. agrestis to 200 cm in C. melo var. flexuosus (Kirkbride, 1993  Flowers are yellow, epigynous and actinomorphic. Flowering in muskmelon starts 40-45 days after sowing (Choudhary and Pandey, 2016; Revanasidda and Belavadi, 2019). Anthesis takes place in the early morning between 5.30 to 6.30 am, and anther dehiscence occurs 5.00 to 6.00 am. Pollens remains viable between 5.00 am to 2 pm (Choudhary and Pandey, 2016) and stigma become receptive 2 hr before and 2 -3 hr after anthesis (Munshi and Alvarez, 2005). Two significant genes A and G determine the sex expression in musk melon. Allele G is responsible for the production of unisexual male flower, either andromonoecious or monoecious (Kubicki, 1969;Kenigsbuch and Cohen 1990). Allele A with G produces monoecious flowers and with gg has gynoecious flowers (Kubicki 1962;Kenigsbuch and Cohen 1990). According to Choudhary and Pandey, the different genotypes produced by the combination of two genes can be represented as aagg-hermaphrodite, aaG_-andromonoecious, A_gg-gynomonoecious and A_G_monoecious. Now a day's use male sterility for heterosis exploitation is a major area of interest for plant breeders to develop high yielding commercial hybrids. Use of male sterility reduces the total cost of hybrid seed production by eliminating the need for labors for the emasculation of female parents (Dhillon and Kumar 2008). All of these five genes are recessive, non-allelic and have unique phenotype (Pitrat, 1991). The first gene for genetic male sterility was reported in 1949 by Bohn and Whitaker. The second gene for male sterility was identified by Bohn and Principe (1964)

GYNOECIOUS LINE
Gynoecious breeding lines provides several advantages for hybrid seed production as it reduces the cost of emasculation, manual pollination, rouging and use of chemical treatment and ensures 100% purity of hybrid seed production. Gynoecious lines also found advantageous over genetic male sterile lines as it avoids the need for identification and rouging of 50% male fertile plants. The use of gynoecious lines for hybrid seed production was also suggested by Frankel and Galun (1977) and Loy et al. (1979). Kenigsbuch and Cohen (1990) proposed possible genotype and sex expression. Kenigsbuch and Cohen (1990) suggested that for stable gynoecious condition, genotype AAgg and recessive mm combination is needed. At very first, Peterson et al. Gylan are round to oval with orange flesh and weight from 0.9 to 1.1 kg. This line produced good F1 with an andromonoecious Vedrantais and Tam-Uvalde (Cohen et al., 1993). A cross between W321 (gynoecious line) and N233 showed earliness in picking and 50% higher yield than Punjab hybrid (Dhaliwal and Lal, 1996). To induce perfect flowers for the maintenance of gynoecious lines, silver thiosulphate, Ethral and Alar had been suggested by Rudich et al. 1970 and More and Seshadri, 1987. Ma et al., (2010) identified two SSR markers MU5549-1and MU14723-2 linked with gene 'a' and 'm', respectively. However, instability of gynoecious lines under high-temperature conditions limits their utilization for hybrid seed production.

HETEROSIS
The primary breeding objectives in muskmelon include early maturity, medium to long vine length, high female flower to male flower ratio, high total fruit yield, fruit size, fruit shape, i.e., round to flattish to oval, outer skin colour, hard netted skin, small seed cavity, adequate flesh thickness, flesh texture, attractive flesh colour, total soluble solids, good resistance against downy mildew, powdery mildew, viral disease, fruit fly, red pumpkin beetle etc. Due to vigorous growth, early maturity, high yield and good quality of hybrids, heterosis breeding is the commonly used method in muskmelon (Banga and Banga, 2000). A good level of heterosis for different traits has been reported by various researchers working on muskmelon. The first study for heterosis in muskmelon was done by Munger (1942)
Melon fruit is rich in vitamins and folic acid and these nutrients act as potent antioxidants and essential compounds in human metabolic reactions. Ripening process in musk melon involves a series of changes in texture, colour, aroma, firmness, flavour (Lelievre et al., 1997;Jiang and Fu, 2000) and ethylene is known as ripening factor in climacteric fruits (Giovannoni, 2001). Two critical enzymes involve in ethylene biosynthesis pathway, enzyme ACC synthase  Table 2.

GENOMICS
Genomics is widely used in melon research because of its affordable price and very high sensitivity, and the next-generation sequencing is doable for melon and its diverse wild relatives

CONCLUSIONS AND FUTURE DIRECTIONS
Melon breeding has achieved several milestones during the last century, and we hope this trend will continue to persist in the future. Although, studies are required to recognize new genetic resources for genes related to the unique challenges imposed by climate change.
Researchers need to dissect and elucidate the molecular basis for resistance, yield traits and fruit quality by gaining insight into the regulatory factors, that synchronize at different development stages for contributing traits. Nevertheless, a sturdy, durable resistance strategy is required against plant pathogens as pathogens can overcome resistance by building new races. The recent sequencing of the melon genomes at a large scale is going to provide the ability to get, reliable information for disease resistance genes to different diseases and also genes for important biochemical attributes. Lates techniques like speed breeding have to be tailored for the cucurbits keep in view the different sex forms temperature requirement for flowering induction.
As the genome sequencing costs decreasing the use of RAD-sequencing, and DNA microarrays, will accelerate genome mapping and tagging of new QTLs. These QTLs might be used to send the resistance into high yielding melon genotypes and combine different QTL with substantial resistance genes to the other races. Moreover, in melon, GWAS (genome-wide association studies) are used for mapping traits to the one candidate gene level. This procedure continues to be recently called mGWAS (metabolite genome-wide association study). The identification of useful genetic and also metabolic variability forms the foundation for directed harvest diversification as well as genetic enhancement by breeding.