After reading this article you will learn about:- 1. Origin of Muskmelon 2. Production of Muskmelon 3. Botany 4. Pollination 5. Genetic Resources 6. Genes 7. Breeding Objectives 8. Breeding Methods 9. Biotechnology 10. Seed Production 11. Varieties.
Contents:
- Origin of Muskmelon
- Production of Muskmelon
- Botany of Muskmelon
- Pollination of Muskmelon
- Genetic Resources of Muskmelon
- Genes in Muskmelon
- Breeding Objectives of Muskmelon
- Breeding Methods of Muskmelon
- Biotechnology of Muskmelon
- Seed Production of Muskmelon
- Varieties of Muskmelon
1. Origin of Muskmelon:
Cucumis comprises a genus of nearly 40 species including several of considerable economic importance such as cucumber, muskmelon and West Indian gherkin (C. anguria L.).
All species are indigenous to East Africa which apparently were introduced to the West Indies from Africa and C. sativus and C. hardwickii, are natives of Asia. India, Persia, China and Southern Russia are considered secondary centres of diversity for muskmelon.
2. Production of Muskmelon:
Muskmelon (Cucumis melo L., 2n = 2x = 24) encompasses the netted, salmon-flesh cantaloupe, the smooth – skinned green fleshed ‘Honey Dew’, the wrinkled – skinned, white – fleshed, ‘Golden Beauty’ and several other dessert melons in USA. Other forms with very different plant and fruit characters are seen in Orient and India. In addition several wild forms occur in Africa and India and all of these are inter-fertile.
Muskmelon is one of the most economically important cucurbits, cultivated in many tropical, subtropical and temperate regions around the world. Winter production in parts of Africa (e.g. Sudan and Kenya) for export to northern Europe has increased its importance as a cash crop. It is a good cash crop in Asia and South American countries.
It is produced as protected crop under unheated polythene tunnels in the Mediterranean region. India grows melons substantially (about 1.66 lakh ha). As per FAO-statistics-2005, the world production of melon in 2005 has been 28 million tons in which main contributors were China (15.1 mt), followed by Turkey, Iran, Spain, and USA each with production figure of 1-1.7 million tons. Indian market is estimated to be of 10 tons of hybrid melon and 300 tons of OPs worth a total of Rs. 25 crores of seed business.
This crop is enriched with great variability as indicated below:
Plant size: 1-10 m
Fruit weight: 10 g – 10 kg
Fruit flesh TSS: 3 – 18%
Fruit flesh acidity: pH 3 – 7
Its polymorphism in leaf, flower, fruit shape, and colour, allowed the classification of horticulturally important melons into seven groups:
1. C. melo var. cantaloupensis Naud.: medium size fruits, round shape, smooth surface, marked ribs, orange flesh, aromatic flavour and sweet.
2. C. melo var. reticulatus Ser.: medium size fruits, netted surface, few marked ribs, flesh colour from green to red orange.
3. C. melo var. saccharinus Naud.: medium size fruits, round or oblong shape, smooth surface with grey tone sometimes with green spots, very sweet flesh.
4. C. melo var. inodorus Naud.: smooth or netted surface, flesh commonly white or green, lacking the typical musky flavour. These fruits are usually later in maturity and longer keeping than cantaloupensis.
5. C. melo var. flexuosus Naud.: long and slender fruit eaten immature as an alternative to cucumber.
6. C. melo var. conomon Mak.: small fruits, smooth surface, white flesh. These melons ripen rapidly, develop high sugar content but little aroma.
7. C. melo var. dudaim Naud.: small fruits, yellow rind with red streak, white to pink flesh.
3. Botany of Muskmelon:
Three important species of the genus Cucumis are distinguishable on the basis of following key:
Fruits spiny, muricate or echinate:
Leaves deeply lobed, fruits small, prickly lemon – yellow when mature – C. anguria.
Leaves shallowly lobed, fruits large, mostly oblong – C. sativus.
Fruits glabrous or pubescent, smooth or netted, mostly with sutures – C. melo.
It is a polymorphic species where most cultivars are andromonoecious (staminate and perfect flowers) but other sex forms are also available. Stem is soft-hairy to glabrous, striate or angled, leaves orbicular to ovate to reniform, usually five angled, sometimes shallowly three to seven – lobed, hairy or somewhat scrabrous, 3-5 inches across (Fig. 27.1).
The staminate flowers are clustered. The pistillate flowers are solitary on short stout pedicels. There are various horticultural forms within C. melo based on fruit characteristics, namely, cantaloupes, nutmeg muskmelons, winter melons, white- skinned melons, snake melons, oriental pickling melons, mango melons, pomegranate melons which hybridize readily with each other and there is apparently very little sterility even among progenies from crosses involving variant types. Under open natural system, it is partly cross and partly self-pollinated by bees but there are reports of obtaining vigorous lines by inbreeding up to 25 generations.
4. Pollination of Muskmelon:
Melons may be andromonoecious (hermaphrodite and staminate flowers), gynoecious (only pistillate flowers), or monoecious (pistillate and staminate flowers). Monoecious and andromonoecious are most common. Hand pollination of andromonoecious types is a two-step process. On the day prior to anthesis, the hermaphrodite flower is emasculated.
Both pistillate and staminate flowers are covered to prevent insect contamination. Emasculation is not required on gynoecious and monoecious types.
Hand pollination is done at anthesis by gently rubbing pollen from anthers of staminate parent flower on the stigma of the pistillate parent flower. Pistillate flower after pollination is covered to prevent contamination by insects like honey bees and Apis spp. etc. Emasculation and pollination can be done as one step procedure also in afternoons.
5. Genetic Resources of Muskmelon:
Gene banks in different countries are active. The largest collections are in Russia (2900 acc), USA (2300 acc), France (1800 acc) and China (1200 acc), Melon is not included in the International treaty for multilateral access to plant genetic resources for food and agriculture and this could reduce the exchange of accessions collected after 1993.
Collaborations between gene banks is increasing for instance with the European Cooperative Programme for Crop Genetic Resources Networks (ECP/GR). Descriptors for melon have been prepared by the International Plant Genetic Resources Institute (IPGRI). It seems that the wild melons are underrepresented in the gene bank.
Up-to now no exclusive character has been observed only in wild melons; for instance wild melon can be resistant to powdery mildew, but the same trait is also found in cultivated landraces.
Genetic resources can be structured by geographical origin and by phenotypic traits (botanical groups). Other characters have also been utilized: Isozymes have been used as markers of the polymorphism since the 1980. Polymorphism has also been studied with different types of molecular markers (RAPD, RFLP, AFLP, SSR).
Relationship between the botanical groups and biochemical and molecular markers has been investigated. In general, accessions belonging to a botanical group fall in the same cluster determined by the molecular markers.
But in many cases, accessions of different geographical origin belonging to one cultigroup are in different clusters. Molecular markers have not yet been used for the management of collections, for instance in the definition of core-collections.
In India, there is enormous variability and several landraces adapted to local situations are available. The pockets of rich diversity in India are Eastern UP, Lucknow, Shahjahanpur, Bareilly, Mathura, Agra, Meerut and Rajasthan. The germplasm lines are conserved at IIVR, Varanasi and SAUs like PAU, GBPUAT and Rajasthan Agriculture Univ, Research Station, Jaipur.
Sex Expression:
From the crosses of hermaphrodite x monoecious types, it has been usually concluded that F2 segregation conforms to a typical di-genic ratio as follows:
G – A – = ++ = 9 monoecious
G – aa = + a =3 andromonoecious
ggA – = g + = 3 gynomonoecious
ggaa = ga = 1 hermaphrodite
However, it should be kept in mind that environmental factors and interaction with other genes may give rise to various other sex forms in muskmelon. Usually, there is an association between fruit shape and sex form. Fruit shape of monoecious and gynoecious lines is oblong and perfect flowers produce round fruits, however, exceptions have also been reported.
Pitrat (2008) has illustrated genetic control of sex expression in melon and effect of ethylene and silver nitrate on melon sex modification as follows:
6. Genes in Muskmelon:
The genes reported in melons can be roughly categorized as:
36 genes as listed by Robinson (1976) are given in Table 27.1. An updated list is now available.
A few genes involved in controlling fruit flesh colour, sweetness, bitterness, skin colour and flavour as reviewed by Pitrat (2008) are as follows:
A two genes model is available for the genetic control of flesh colour. The orange-fleshed cultivars have a much higher level of β-carotene. White flesh (symbol wf) and green flesh (symbol gf) are epistatic with the following genotypes and phenotypes: wf+/ – – is orange, wf wf / gf gf is green and wf wf / gf+ – is white. However other genes are involved in the genetic control of fruit colour and the intensity of the colour.
Fruit bitterness usually disappears when fruits approach maturity but in the case of fruits harvested immature (var. flexuosus for instance), one must be careful to eliminate bitterness due to the presence of the triterpene cucurbitacins. Non-bitter plants at the seedling stage produce non-bitter fruits but other genes can be involved.
In a cross between a non-sweet melon (var. flexuosus) and a sweet melon, sucrose accumulation was shown to be controlled by one recessive gene. QTLs have also been detected in crosses between medium-sweet and sweet melons. Usually sweet melons have a high pH (low organic acid content). Combination of high sucrose and high organic acid, mainly citric acid, controlled by the gene Sour (symbol So) leads to new flavours.
Genes involved in the biosynthesis of esters, the main volatile components of the Alcohol Dehydrogenase (ADH) and Alcohol Acyl-Transferase (AAT) families have been isolated but their phenotypic effect has no’ yet been determined.
Genetic control of fruit skin colour has been studied and some genes with strong effect have been described. These include Mottled rind pattern (symbol Mt and Mt-2), ridged fruit surface (symbol ri), striped epicarp (symbol st and st-2), sutures (symbol s and s-2), white colour of mature fruit (symbol w), White colour of immature fruit (symbol Wi), or Yellow colour of the fruit (symbol Y),
Breeders have tried to improve the shelf-life or the keeping quality after harvest. Melon is one of the few species where climacteric and non-climacteric fruits have been selected. Peach, apricot, banana, tomato, melons belonging to var. cantalupensis, dudaim, makuwa or momordica are climacteric fruits with a burst of respiration and autocatalytic ethylene production at fruit maturity.
Strawberry, citrus, grape, melons belonging to variety inodorus with the cultigroups piel de sapo, honeydew, tendral, yuva and casaba are non-climacteric. In melon, some traits are ethylene independent like sugar and carotenoid accumulation; while other traits like production of volatile compounds, peduncle abscission, change of skin colour, flesh softening are ethylene dependent. Decreasing the intensity of the climacteric crisis enables to increase the shelf-life.
7. Breeding Objectives of Muskmelon:
1. Attractive round shape/spherical fruit shape
2. Thick flesh with attractive orange/green colour
3. Small seed cavity
4. Sweet, juicy, musky flavorsome fruits
5. TSS not less than 10%
6. Tough netted skin of fruit
7. High early and total marketable yield
8. Resistance to common diseases (powdery mildew, downy mildew, virus, fusarium wilt, gummy stem blight)
9. Resistance to important insect-pests (aphid and leaf miner)
8. Breeding Methods/Selection Criteria of Muskmelon:
Melon is a semi-allogamous species. Presence of nectar in male and female flowers attracts bees and other insects. Cross pollination rate is higher in monoecious plants than in andromonoecious. Usually there is no inbreeding depression due to the homozygous state and pure line can be cultivated. Heterosis or hybrid vigour is clearly observed in F1 hybrids between two different parental line.
Heterosis is more important when parents are more divergent; for instance the F1 Galia is a hybrid between an Ogen line from central Europe and a netted Asiatic parent. Most of the modern cultivars are F1 hybrids which allow also a protection of breeder’s investments.
Flowers are quite large and hand pollination is easy to handle. Male flowers (in inflorescences) appear before female or bisexual flowers at the nodes of the main stem and after the second or third node of the lateral branches, while the solitary pistillate flowers are present at the first and second nodes of the lateral branches.
In the case of gynecious or hermaphrodite plants, female or perfect flowers respectively appear at all the nodes of primary or secondary branches. Controlled pollinations can be achieved by working in insect-proof greenhouses or by closing the flowers before and after pollination with bags or other means.
The typical number of stamens is five but partial fusion leads generally to two large bilocular and one small unilocular stamens. The stamens open and release the pollen the day the corolla opens. To perform crosses, perfect flowers must be emasculated before stamens open. The stigma is receptive one day before and one day after the corolla opens.
Pollination can be achieved at the same time as emasculation. It is usually more efficient to make pollinations in the morning, as in the afternoon nectar can wet the pollen. One successful pollination will produce 300 to 500 seeds.
Classical breeding methods using crosses and selfing such as pedigree or backcrosses can be used. Usually backcrosses are used to introduce a new character in a cultigroup, for instance disease resistance from an exotic accession in a Charentais or Piel de Sapo type. Within a cultigroup, pedigree and recurrent selection are used to accumulate favourable traits.
For characters with a simple inheritance selection is efficient in the first generations (F2 or F3). For instance, tests can be performed on individual F2 plants for many disease resistance; moreover techniques have been developed allowing multiple tests on a single plant, for instance by using leaf disks or detached cotyledons.
For characters with a more complex inheritance, such as fruit quality, earliness or yield, selection is more efficient if applied in more advanced generations (F4 or F5).
Controlled Inbreeding:
Usually inbreeding depression has not been reported in melons, therefore, melons can be handled as self-pollinated crop for breeding purposes through judicious application of selfing and selection for desirable traits.
Pedigree Method:
This method is applicable to develop genotypes by crossing parental lines having complementary traits followed by selection of desired types in the selfed/inbred generations up to 5-6 generations till homozygosity is attained.
Heterosis Breeding:
Despite the lack of inbreeding depression in general in melons, heterosis for earliness, fruit size, fruit weight, flesh thickness and soluble solids has been observed. This and other earlier zeal for hybrid cultivars of vegetables have triggered great interest in F1 hybrids which provide seedsmen plant variety protection and also offer opportunity to deploy dominant gene (s) conditioning disease and insect resistance from either parent.
Melons have a wide range of sex forms (andromonoecious, gynoecious, monoecious). Gynoecious lines are available but need further improvement for fruit quality and stability of this trait under sub-tropical conditions as pointed out by several workers, namely, T.A. More, V.S. Seshadari, C.E. Peterson, K.S. Nandpuri, Tarsem Lal etc.
Male sterility has immense potential for production of hybrid cultivars of muskmelon. Five recessive nuclear male sterile genes (ms-1, ms-2, ms-3, ms-4, ms- 5) are known.
The genetic male sterile line MS-1, (genotype ms-1) of muskmelon was introduced in India more than 20 years ago and is still the only line available in India. The development of the superior F1 cultivar of Punjab Hybrid (MS-1 X Hara Madhu) is the result of research and breeding work using MS-1.
The male sterile line (ms1 ms1) is maintained by crossing to the isogenic maintainer heterozygous for recessive sterility allele as given below:
Now onwards the seed set on ms1 ms1 will always segregate into a ratio of 1 fertile : 1 sterile. This seed shall be used to maintain mS1 gene by planting this seed in isolation and collecting the seeds set only on male sterile plants. Further, this stock seed can be used as seed parent in hybridization block to produce F1 seed with a standard male parent as Hara Madhu for Punjab Hybrid.
However, in the seed production block, the 50% of the fertile segregates (Ms1 ms1) will have to be removed from the female row. For this purpose, in the early hours of morning, anthers of three anthesised flowers from each plant of the female parent are pierced with needle. Male fertile flowers give light yellow powdery mass, whereas from the male sterile anthers sticky green immature pollen are produced.
Identified male sterile plants are tagged every day. This practice is completed before sunrise when the activity of bees is absent. All the male sterile plants are subjected to confirmation on the last day of roguing period. The pollinator (Hara Madhu) is homozygous fertile (Ms1 Ms1) to ensure that all F1 hybrid plants are fertile.
Dhillon (1994) studied F1 hybrid seed production in muskmelon using genetic male sterility in relation to two factors, viz. four population densities (3, 4, 5, 6 seedlings per hill) of seed parent (Msms + msms), and roguing of male fertile plants (Msms) spread over 10, 15, and 20 days. Planting of 6 seedlings per hill and roguing of male fertile plants for 10 days gave the cost effective hybrid seed yield (109.0 kg/ha).
Considering the problems associated with use of male sterile gene(s) for production of muskmelon hybrid (maintenance of single recessive gene in heterozygous condition and problem of identifying and roguing of 50% male fertile plants from the female row at the time of flowering) and also to overcome the problems of andromonoecious sex forms, the scientists at IARI took keen interest in developing true breeding monoecious lines, viz. M1, M2, M3 and M4.
Of various combinations, M3 x Durgapura Madhu i.e. Pusa Rasraj was found to be of outstanding performance. It was identified for release by the Project Directorate of Vegetable Research in 1990 for commercial cultivation in Delhi, U.P., Punjab, Haryana, Bihar and Rajasthan followed by release by IARI committee in 1991 and by central sub-committee on crop standards, notification and release of varieties in 1993.
Backcrossing:
Backcrossing is usually followed to introduce disease resistance etc. from horticulturally undesirable type (s) to an acceptable variety. Backcrossing is simple when the trait to be transferred is caused by a single dominant gene, but becomes complex in case of one or more recessive genes. Powdery mildew resistance breeding in muskmelon is of historic importance using a backcrossing programme, hence that is being highlighted here (Whitaker, 1979).
A notable milestone in plant breeding history occurred in 1937 when l.C. Jagger and G.W. Scott reported research that led to the release of ‘Powdery Mildew Resistant Cantaloupe 45’. Powdery mildew is a devastating disease of muskmelons in the arid valleys of the Southwest, USA and causes severe economic losses in India.
The fungus responsible is Sphaerotheca fulginea (Schecht. ex Fr.) Poll. It produces round, cottony white masses of asexual spores on both surfaces of the leaves of the host plant. The pathogen destroys the epidermal cells of the leaves and stems. The end result of a serious infection is a small, dwarfed, stunted melon, often sun-burned with light coloured, insipid flesh that lacks both flavour and sugar.
In 1925, Professor J.T. Rosa of the University of California, Davis, received a shipment of muskmelon seed from India through one of his former students. Dr. Rosa in collaboration with l.C. Jagger of the USDA found a few plants in this collection that showed field resistance to powdery mildew in the Imperial Valley. These Indian muskmelons had all sorts of objectionable characters.
The melons were large, smooth with wide vein tracts and they split open at maturity. The fruit flesh was white, mushy, tasteless and barely edible. Using this Indian material as a source of resistance, several backcrosses to the commercial type were made.
The backcross method was combined with selection for resistance to powdery mildew in the field. In 1937, a superior cultivar, Powdery Mildew Resistant Cantaloupe 45 (PMR-45) was released. Even today PMR-45 is one of the most known cultivars of contaloupe for shipping to distant markets. The resistance is governed by a single dominant gene.
Sources of Disease and Insect Resistance:
According to McCreight (1993) and Dhiman (1995) the sources of resistance to diseases and insect-pests in muskmelon are given in Table 27.2.
Fruit Quality Traits:
These include observations as follows on marketable fruits:
(i) Rind colour
(ii) Stem scar size (for those that slip at maturity)
(iii) Blossom scar size
(iv) Fruit size
(v) Fruit shape
(vi) Overall appearance
(vii) Percentage net cover
(viii) Net type
(ix) Surface cracks
(x) Ground spot (area contacting soil)
(xi) Rind roughness
(xii) Flesh colour
(xiii) Rind thickness
(xiv) Flesh thickness
(xv) Seed cavity
(xvi) Dryness
(xvii) Flesh firmness
(xviii) Flesh texture
(xix) Flavour
(xx) Total soluble solids
9. Biotechnology of Muskmelon:
Guis (1998) have reviewed this aspect extensively. The brief presentation here is based on this.
Melon Regeneration:
The development of efficient regeneration processes from in vitro cultures is absolutely necessary for application of biotechnological techniques. Over the years regeneration in melons by organogenesis has been described. Several factors influence the success of plant regeneration.
These include:
1. genotype/variety
2. explant source
3. culture condition
4. physical factors
The information is summarised in Tables 27.3, 27.4 and 27.5.
Genetic Transformation:
The successful genetic transformations of melon are summarized in Table 27.6. The primary focus has been on the introduction of genes for disease resistance. Other attempts have been made to improve resistance of environmental stress or to enhance fruit quality.
Genetic transformation of melon is commonly achieved through Agrobacterium tumefaciens using cotyledon explants. Among several strains available, LA 4404 is the most frequently used. Particle gun transformation has also been reported to be successful.
In the transformation protocols reported npt II is the most frequently used selectable marker gene, however, escapes are particularly high in melon. Non-transformed plants produce roots that are short and have no branches.
The typical root morphology for transgenic melon plants is branched, strong and elongated and this morphological criterion could be a useful marker for easy identification of transgenic plants in the Cucurbitaceae family.
More than 25 viruses are able to induce disease in the melon under natural conditions. Among them, five have an important economic impact in the world. Cucumber mosaic virus (CMV), zucchini yellow mosaic virus (ZYMV) and watermelon mosaic virus-2 (WMV2) are the most widespread. These viruses affect the development of the plants, which subsequently produce abnormal fruits or no fruit at all.
Traditional breeding techniques allow the selection of resistant melon plants, but few resistant commercial varieties are currently available. Cultural practices or control of insect vectors using insecticides or mineral oil sprays have a limited effectiveness.
Transformed melon plants over-expressing CMV coat protein or zucchini yellow mosaic virus coat protein have been produced by genetic engineering. CMV coat protein has been over-expressed in several cultivars of cantaloupe type melons.
Under greenhouse conditions, transgenic melon plants inoculated with CMV did not develop symptoms during a 46-day observation period whereas non-transgenic plants showed visible symptoms three days after inoculation. However, at a high inoculum concentration, the appearance of the symptoms is only delayed in transgenic plants.
Similar observations were made by Gonsalves (1994), but they also found among their 45 transgenic plants, five resistant ones that exhibited no symptom, even six months after inoculation. Variation among the expression of resistance observed between the different lines may be due to different sites of insertion of the transgene.
Extensive field trials have been conducted to test the level of resistance of five cantaloupe lines genetically modified with WMV2 and ZYMV-CP under natural infection conditions. At the end of the trial, transformed lines exhibited only a low infection rate whereas more than 60% of the control plants developed symptoms.
All the transformed cultivars exhibited significant disease reduction under extreme disease pressure and produced normal fruits. The growth rate, physical appearance and fertility were the same as in the non-transformed type. Transgenic plants including a polyribosome construction directed towards CMV coat protein, exhibited a good level of resistance.
The Charentais type melon is the most important melon produced in France. It is characterized by yellow-orange flesh, strong aromatic flavour and abundant sweetness. However, its storage capabilities are low, due mainly to rapid ripening.
Melon fruits with improved shelf-life have been generated by traditional breeding. Genetic manipulation has also allowed the regeneration of transgenic fruits expressing low levels of ethylene, the key hormone responsible for fruit ripening.
Ripening is greatly delayed in transgenic fruits expressing an antisense copy of the 1-aminocyclopropane-l-carboxylic-acid oxidase gene. Several parameters of ripening, including colour, firmness and sugar level have been determined in the transgenic fruit. The first difference observed between transgenic and wild-type fruit is the colour of the rind.
Wild-type melon develops a yellow colour throughout ripening, whereas transgenic fruit remains green, even at the latest stages of ripening. Only exogenous treatment with ethylene allows the recovery of the yellow colour.
Improvement in firmness is also observed in transgenic fruit. Flesh of transgenic fruit exhibits an almost complete inhibition of softening, whereas, wild-type fruit softens rapidly. Evolution of sugar content and flesh colour is the same in both transgenic and wild-type melon fruits.
Transgenic melon fruits were able to be stored for at least two weeks at 25°C without over- ripening or fungal attack. Meanwhile, wild-type fruits completely got rotten during this period. The shelf-life of the transgenic melon fruits is greatly extended relative to that of control fruits. In addition, growers will be able to harvest the fruit later, when more sugars have accumulated.
Losses of marketable fruit should decrease and the geographical distribution of harvested fruits can be expanded. Exposure to exogenous ethylene allows the recovery of the original quality attributes (i.e. colour of the rind, pattern of aroma volatiles) typical of wild-type fruit harvested at a suitable ripening stage.
Biochemical and Molecular Markers:
Melon, as most of the Cucumis species, is a diploid plant with 12 chromosomes (2n = 2x = 24). Karyotype is composed of eight median chromosomes., two sub-median and two sub-terminal with two satellite pairs. Their size is around 1.06 and 1.88 µm, and due to this small size, they are rather difficult to observe. The size of melon genome is 0.94-1.04 pg/1C which is around 3.5 times the size of the Arabidopsis thaliana genome.
Conventional breeding programmes have improved agronomic traits by combining characters of parental lines. The selection of simply inherited characters can be achieved by repeated backcrosses.
Combination of complex characters encoded by multiple genes (quantitative trait loci) or recessive genes is more difficult to achieve with classical methods that are mainly based on phenotypical characters and require large population size.
The use of molecular markers has allowed the connection of phenotypic characters with enzyme activity or genomic loci responsible for them, and are indispensable for plant improvement.
Genetic variability in melon has been studied using biochemical isozyme markers or molecular markers, such as restriction fragment length polymorphisms (RFLP) or random amplified polymorphic DNA (RAPD). Several authors examined melon accessions for isozyme variability and found few allelic variations.
However, the use of molecular markers like RFLP has allowed the detection of sufficient polymorphism (33% polymorphism within 44 C. melo accessions) to discriminate varieties belonging to different groups within the germplasm of cultivated melons.
Unfortunately, discrimination of varieties belonging to the same group was not possible. Microsatellite markers detect a high level of polymorphism. Microsatellite markers are a very promising tool for the detection of polymorphism in the Cucurbitaceae family.
More than 90 melon genes have been described, including genes controlling morphological characteristics and those involved in resistance. Identification of genetic linkages between various genes has allowed the correlation between some disease resistance genes and vegetative or flower biology characteristics.
Eight independent linkage groups, containing 23 genes have been described. These results were the first step towards a genetic map of C. melo. The first molecular map of the melon genome was generated from the analysis of an F2 population resulting from an intraspecific cross between two divergent inbred lines, a Charentais type and a Korean type. About 100 markers, mainly RFLP and RAPD markers, were used to define 14 linkage groups.
Integration of New Biotechnologies in Breeding Programmes:
Melon has a small genome size, estimated at 450-500 Mbp (1C). A chloroplast genome has been estimated at 150 kbp. Mitochondrial genome is of 2400 kbp. The maternal transmission of the chloroplast and the paternal transmission of the mitochondrial genome have been demonstrated as reviewed by Pitrat (2008).
An International Cucurbit Genomics Initiative started in 2005 with the aim of furthering better knowledge of cucurbits genomics (genetic and physical maps, EST-expressed sequence tags, B AC-bacterial artificial chromosome libraries and genome sequencing). Melon is the most advanced species for genomics among the cucurbits and will be used as a model for the family.
A gene list is regularly published by the Cucurbit Genetics Cooperative and can be accessed at http://cuke(dot)hort(dot)ncsu(dot);edu/cgc/index(dot)html. Different types of molecular markers have been used to study the genetic diversity and to develop genetic maps by crossing distant parents.
Several types of population have been used: F2, BC, doubled haploid (DH) and recombinant inbred lines (RILs). Genes and QTLs controlling phenotypic traits have been localized.
BAC and EST libraries are available and provide new sequences which can be used as markers. The next step will be the merging of these different maps in a consensus one using anchor points such as microsatellites (SSR).
The development of genetic maps allows the use of marker assisted selection (MAS). Breeders can use markers closely linked with genes or QTLs of horticultural interest such as disease resistance, fruit quality or flower biology. One of the main problems in MAS is the recombination event between the gene and the marker. Markers should be developed in different elite material genetic background.
When a gene has been cloned, polymorphic sequences between the alleles allow to define markers within the allele. For instance a single nucleotide difference has been found between the alleles nsv and nsv+ controlling resistance and susceptibility to MNSV.
Agrobacterium tumefaciens has generally been used as the vector. An attenuated non-aphid transmissible strain of ZYMV has also been used as vector for herbicide resistance or for producing antiviral and antitumor proteins.
10. Seed Production of Muskmelon:
Isolation Distance:
1. Breeder/foundation seed – 800 m
2. Certified seed – 400 m
3. Different groups of melons within C. melo are all cross-compatible.
4. No viable seed production if cross pollinated with cucumber (C. sativus)
Harvesting:
Fruits of cantaloupe and muskmelon types tend to separate from the stem at the base of the fruit as the fruit becomes fully mature. This stage of separation by formation of an abscission layer is known as ‘Full-Slip’. Fruits are harvested/collected at this stage.
When there is no abscission layer formation, the fruit maturity is indicated by external rind colour change from green to yellow or yellow to white. The seeds are washed without fermentation.
Seed Yield:
1. 300-500 kg/ha
2. 1000 seed weight – Approximately 25 g
11. Varieties of Muskmelon:
Hara Madhu:
This is a variety developed at PAU, Ludhiana from the local material of Kutana type (a local collection of UP). It was identified in 1975 for zones IV and VII. Vines are 3-4 m long and vigorous. Fruits are large, round, slightly tapering towards the stalk end. There are 10 prominent green sutures. Average fruit weight is 1.0 kg. Flesh is green, with small seed cavity. TSS is 12-13%. The yield potential is 125 q/ha.
Pusa Sharbati:
This has been developed at IARI from a cross of Kutana x PMR 6 of USA. It was identified in 1975 by the vegetable workshop for northern-Gangetic plains. It is suitable for riverbed conditions also. Fruits are round with netted skin. Flesh is thick and orange. TSS content is 11-12%. Yield potential is 150 q/ha.
Arka Rajhans:
It is a selection from local collection (IIHR-107) of Rajasthan at IIHR, Bangalore. It was identified in 1975 for better fruit quality and yield attributes. Fruits are round-slightly oval, medium large, with white, firm flesh having 11-14% TSS. Average fruit weight is 1.25-2.0 kg. It is moderately resistant to powdery mildew.
Arka Jeet:
It is an improvement over the commonly available local type of Lucknow area. It was developed at IIHR, Bangalore (IIHR 103). It was identified in 1975 for zones VI and VII. Fruits are small, flat-round, with attractive orange flesh, weighing 300-500 g. Flesh is white and sweet with medium soft texture.
Punjab Hybrid:
This is an F1 hybrid developed at PAU, Ludhiana having the parentage as male sterile MS-1 X Hara Madhu. Identified in 1985 for zones IV and VIII, Punjab Hybrid has 2-2.5 m long vines, vigorous luxuriant growth, globular fruits with distinct sutures, weighing about 800 g. Flesh is creamy yellow. Rind is netted. TSS is about 12%. It is early in maturity, has good post-harvest life and transportability. It is moderately resistant to powdery mildew.
Pusa Rasraj:
A monoecious line M-3 developed at IARI and crossed with Durgapura Madhu gave rise to the F1 hybrid, Pusa Rasraj. It has been recommended for commercial cultivation in Delhi, UP, Punjab, Haryana, and Bihar by Project Directorate, All India Coordinated Vegetable Improvement Project in June, 1990.
Finally, this hybrid has been released as Pusa Rasraj. Private Sector Veg. Seed Companies are largely selling hybrid seeds of muskmelon imported from Taiwan, China, Japan, etc. These hybrids have better shelf-life. Deepti is most common hybrid.