Culinary melons (Cucumis melo subsp. agrestis var. acidulus) of South India, locally referred as Mangalore melon or Sambhar Southe, is a non-dessert melon belonging to the family Cucurbitaceae with somatic chromosomal number 2n = 2x = 24 (Pitrat et al. 2000). It is one of the popular vegetables grown in the coastal districts of Karnataka, used for making sambar (a kind of lentil soup), hence the name Sambar Southe. Mangalore melons are widely grown in the coastal parts of southern states of Karnataka, Kerala, Tamil Nadu, Andhra Pradesh, Telangana and Goa, referred by vernacular names Mangaluru cucumber, Mangaluru southekayi, sambar cucumber, sambar southe, thouthe, mage-kaayi, moge-kaayi, mogem, dosakaya, budamekaya, bollari, Malabar cucumber, Madras cucumber, vellari, kanivellari, vellarikka and so on (Manchali et al. 2019; Ramesh Babu and Hariprasad Rao 2018; Silpa et al. 2019; Swamy 2017). Asian melon (C. melo subsp. melo f. agrestis) has been reported as the ancestor of culinary melons of South India and have been considered to be originated and domesticated in India (Endl et al. 2018; Sebastian et al. 2010).
Mangalore melons are characterized by dark green foliage, bearing monoecious flowers that produce non-climacteric fruits with varied size, shape and color (Fergany et al. 2011; Kamagoud and Shet 2018; Manohar and Murthy 2012). Flesh of Mangalore melons is white, very firm and crispy, non-sweet and slightly aromatic to non-aromatic. Seeds of Mangalore melons are 4–10 mm and are yellow colored (Fergany et al. 2011; Swamy 2017). Often Mangalore melons are referred as ‘conomon’ melons (C. melo ssp. agrestis var. conomon) (Gondi et al. 2016; Kamagoud and Shet 2018; Lakshmi et al. 2017a; Lakshmi et al. 2017b; Ramesh Babu and Hariprasad Rao 2018; Silpa et al. 2019; Swamy 2017; Vinutha et al. 2017). But, Mangalore melons differs from their counterpart ‘conomon’ melons of South East Asia with respect to larger fruits with varied shapes and size, long shelf life of more than six months and small seed size (Dhillon et al. 2012; Fergany et al. 2011; Koli and Murthy 2013; Manohar and Murthy 2012; Roy et al. 2012).
Both mature and immature fruits of Mangalore melons are cooked as vegetable. Off-white, turgid and crunchy flesh of Mangalore melon that holds crunchiness even after cooking is a delicious, yummy and low-calorie vegetable. It encompasses about 3.6% protein, 4% fat, and 2.5% carbohydrates and are also good sources of phosphorous, potassium, iron, zinc, ascorbic acid and carotenoids (Fergany et al. 2011). Consumption of Mangalore melons prevent macular degeneration in the eyes, the antioxidants in the seeds keep blood cholesterol under check and decrease the risk of cancer (Pawar 2018). Slightly bitter seeds of Mangalore melon that are usually discarded are hugely in demand by both the beauty and nutrition industries are also good sources of oils and fatty acids (Pawar 2018). Mangalore melons seeds contain 34.68% oil on dry weight basis, primarily comprises of nine fatty acids with high concentration of polyunsaturated fatty acids (80.54%). Linoleic acid (66.55%), oleic acid (13.70%), palmitic acid (11.65%) and stearic acid (7.49%) were the major fatty acids, which enhance the immunity and reduce atherosclerosis (Manohar and Murthy 2014).
In spite of its high nutritional and medicinal values, cultivation of culinary cucumber is limited to humid tropics of Southern India and are available for about 5 to 6 months during the spring and kharif season in the South Indian markets (Dhillon et al. 2012). Exploration of genetic diversity is crucial for broadening the genetic base for sustainable crop improvement. Molecular markers are invaluable tools for assessing the genetic variation within and among the plant genetic resources. Characterization of South Indian melon landraces with SSR markers (Fergany et al. 2011), RAPD markers (Lakshmi et al. 2017a; Manohar and Murthy 2011) and AFLP markers (Lakshmi et al. 2017b) have revealed substantial genetic variation among the culinary melons collected from different parts of South India. However, genetic relatedness among the landraces from different south Indian states need to be explored with large collections.
Long shelf of Mangalore melons up to one year is a breeding treasure for enhancing the shelf life of melons. Contrarily, dessert type cantaloupe melons (Cucumis melo L.) that are widely cultivated and consumed all over the world have short shelf life of less than 15 days (Burger et al. 2010). Enhancing the shelf life of cantaloupes for long distance transport is one of the major objectives of cantaloupe breeding (Nunez-Palenius et al. 2008). Opportunistically Mangalore melons have same chromosome numbers that of cantaloupe and are readily crossable with each other with crossability index ranging from 50–70% (Subramanian 2008)(Dr. Ratnakar Shet, personal communication). Thus, Mangalore melons with long shelf life with firm fruits grant an opportunity to exploit Mangalore melons in enhancing the shelf life of cantaloupe. But non-dessert type Mangalore melons, characterized with low sweetness, unacceptable flesh color and flavour and other undesirable agronomic traits, pose risk of linkage drag. Identification of genes linked to shelf life is crucial for employing Mangalore melons in breeding of cantaloupe for enhanced shelf life. Candidate genes of enhanced shelf life in most of the fruits belong to the fruit ripening biochemical pathways such as cuticle function, cell-wall integrity, ethylene biosynthesis and upstream components that regulate ripening (Friedman 2019).
In this study, 79 accessions of Mangalore melon were collected from the five states of Southern India and the diversity in the fruit shape, skin color and shelf life were evaluated, and their genetic diversity was assessed using six inter simple sequence repeat (ISSR) markers. Putative candidate genes of shelf life were identified by assessing the expression levels of key shelf life related genes of ethylene biosynthesis and cell wall metabolism using quantitative reverse transcriptase PCR (qRT-PCR) in comparison with the cantaloupe.