Crop landrace possesses various traits which enabled adaptation to local conditions and farming practices, and have been established through long period of natural and artificial selection. Some of these traits were inherited from their ancestral wild species, while others were acquired after domestication for adaptation to new environments encountered (Cortinovis et al. 2020). In case of wheat vernalization genes, for adaptation to warm conditions, cultivated wheat acquired spring alleles of Vrn-A1, Vrn-B1, and Vrn-D4 after domestication and spread of wheat cultivation (Yan et al. 2004; Kippes et al. 2015). By combining these alleles, new cultivars with various types of adaptability can be developed. Therefore, local landraces are important genetic resources for crop breeding. Genetic diversity in landrace population also provides important evidence to uncover crop evolution history as already reported in many crop species (Pourkheirandish et al. 2015).
Reflecting long history of cultivation, various types of usage and adaptation to various parts of the world, melon (Cucumis melo) is known as most diversified among Cucurbitaceae crops, and great diversity is reported in morphological traits such as fruit weight and seed length which ranged from 50 g to 15 kg and from 4.5 mm to 15.0 mm, respectively (Nuñez-Palenius et al. 2008; Akashi et al. 2002). C. melo is divided into 19 horticultural groups based primarily on geographical origin, morphology and horticultural traits: Agrestis, Kachri, Chito, Tibish, Acidulus, Momordica, Conomon, Makuwa, Chinensis, Flexuosus, Chate, Dudaim, Chandalak, Indicus, Ameri, Cassaba, Ibericus, Inodorus, and Cantalupensis (Pitrat 2016). Fujishita and Nakagawa (1973) focused on seed size variation and indicated that melon is classified into large-seed type (≥ 9.0 mm) and small-seed type (< 9.0 mm). Melon accessions of groups Cantalupensis and Inodorous are mostly classified as large-seed type and groups Agrestis, Conomon, and Makuwa as small-seed type according to Fujishita (1983) and Akashi et al. (2002). In Asia, groups Dudaim, Flexuosus, Chandalak, Ameri and Inodorus of large-seed type are mainly distributed in west and central Asia and India, whereas groups Chinensis, Conomon, Makuwa and Acidulus of small-seed type in southeast and far-east Asia and India (Stepansky et al. 1999; Akashi et al. 2002; Yashiro et al. 2005; Tanaka et al. 2007; Pitrat et al. 2008; Tanaka et al. 2013).
Among Asian melon, small- and large-seed types proved to be diversified in independent maternal lineages, Ia and Ib, respectively, by the analysis of chloroplast genome (Tanaka et al. 2013). India is considered as the secondary center of diversity (McCreight, 2004; Dhillon et al. 2007; Fergany et al. 2011), and both of small- and large-seed types and both of Ia and Ib types are frequently distributed. However, landraces of Conomon and Makuwa are not reported in India. Conomon and Makuwa are mostly distributed in areas from China to Japan, and considered to be diversified in small-seed melon with Ia type cytoplasm in areas somewhere lying between India and China (Akashi et al. 2002; Tanaka et al. 2007; Serres-Giardi and Dogimont, 2012). In contrast to India and far-east Asia, little is known about melon landraces in southeast Asia. Myanmar melon landraces were first investigated by Yi et al. (2009), and proved to possess large genetic variation as like Indian melon. Myanmar melon consists both of small- and large-seed types, and they classified small-seed type accessions as Conomon, Momordica, or Agrestis. However, most of Myanmar Conomon accessions were clustered separately from Conomon and Makuwa of far-east Asia. Although one accession was regarded as Makuwa, they could not rule out the possibility of its recent introduction from other countries such as China and Japan. More recently, Nhi et al. (2010) and Duong et al. (2021) analyzed Vietnamese melon landraces, and showed that all except one accession were small-seed type. Vietnamese melon consisted of seven cultivar groups among which “Dua le” and “Dua vang” were regarded as Makuwa and “Dua bo” and “Dua gang-andromonoecious” as Conomon. In contrast, “Dua thom” and “Montok” showed genetic similarity with Indian and Maynmar landraces. Based on these results, the presence of groups Conomon and Makuwa was first confirmed in Vietnam. Therefore, to uncover the origin of groups Conomon and Makuwa, melon landraces of Cambodia, Laos, and Thailand should be investigated.
Recently, we have conducted germplasm collection expedition in Cambodia from 2014, and successfully introduced germplasms of melon landraces (Matsunaga et al. 2015; Tanaka et al. 2016; Tanaka et al. 2017; Tanaka et al. 2020). Genetic resources of Cambodian melon are not available in major Genebanks other than NARO Genebank, Japan, and are expected to contribute to breeding for disease resistance and to the analysis of diversification of melon in Asia. More specifically, genetic analysis of Cambodian landraces is indispensable to understand the distribution of groups Conomon and Makuwa and genetic relationship with Vietnamese landraces. Therefore, in this study, we aimed to uncover genetic diversity and genetic structure of Cambodian melon using RAPD and SSR markers, and discussed genetic diversification in Asian melon.