Watermelon (Citrullus vulgaris) (Cucurbitaceae) includes several other economically important species such as cucumber (Cucumis sativus), melon (Cucumis melo L), squash (Cucurbita pepo), calabash (L. siceraria), and pumpkin (Citrullus maxima). C. lanatus is an annual species, which has wild, cultivated, and feral forms.

In the finding that the watermelon and its sister species are West African plants suggests that the natural range of watermelon may have extended into Libya or into Egypt during more humid periods of the Pleistocene and Holocene(Schulz, 1987, 1991). Alternatively, watermelon seeds may have been traded from West Africa to Northern Africa. The illustrations found in Egyptian tombs of watermelon served on a tray suggest that these fruits were eaten raw, perhaps as a dessert. Seeds found in ancient Egyptian tombs, including that of Thutankhamun (Hepper, 1990) should ideally be studied using ancient DNA approaches.

Genetically, the cultivated watermelon is closest to plants from West Africa that represents the gene pool from which watermelon was domesticated (Chomicki and Renner, 2014). The sister species of watermelon is C. mucosospermus, the egusi melon from Nigeria to Senegal and described by Fursa (1983), Che et al. (2003), and Dane and Liu (2007). In consideration of the findings by Fursa (1983)and Chomicki and Renner (2014), the egusi-type watermelon is recognized as C. mucosospermus throughout this paper.

The "egusi" watermelon of West Africa is cultivated for its oleaginous and nutritious seeds that are important in the social and cultural live of several peoples (Burkill, 1985; Oyulu, 1977; van der Vossen et al., 2004; Achigan-Dako et al., 2008). For example, dried slightly toasted and ground seeds are used as soup thickener. The "egusi" seeds are reported to be rich in nutrients ~60% lipids and ~30% proteins (Loukou et al., 2007). Edible oil can also be extracted from the seeds. Commonly found in many traditional cropping  systems,  the  plant  is  well  adapted to extremely divergent agro-ecosystems and various cropping systems characterized by minimal inputs. C. lanatus thus represents an excellent plant model for which improved cropping systems implementation can insure the economic prosperity of rural women that are the main producers in tropical Africa. The west African watermelon is generally considered as allogamous, monoecious, entomophilous, and protandrous(Gusmini and Wehner, 2003). This species is named as C. lanatus var. lanatus or as C. lanatus subsp. Mucosospermus (Fursa, 1972) or C. colocynthis(Hutchinson and Dalziel, 1954).

Particularly, in Côte d’Ivoire, indigenous and minor crops for local consumption have almost disappeared in favor of exotic species. Some farmers still continue to produce these crops not far from their home or on small plots cleared after harvest of the main annual crops (yams, corn, peanut, etc.) (Zoro Bi et al., 2003). Thus, some of them such as oleaginous cucurbits are still found in the market. According to Zoro Bi et al. (2003) and Djè et al. (2006), C. mucosospermus (but identified as C. lanatus by these authors in Côte d’Ivoire) is the species most widely distributed and most commercialized in Côte d’Ivoire. This plant is mainly grown in savanna areas in the North, in the pre-forest areas in Central, East, and South(Djè et al., 2006). Two distinct cultigroups have been described on the basis of the morphology of seeds. The first one (wlèwlè) is characterized by glossy seeds with a tapered proximal extremity. This morphotype is subdivided into three cultivars according to the size of the seeds(Djè et al., 2006). Fruit color of this cultigroup, is almost uniform, greenish or whitish. The second (bebu), with one cultivar has green fruit also, but with dark longitudinal bands. The seeds are heavier and have a flat ovoid shape with rugged and thick ends (Zoro Bi et al., 2003).Although the oleaginous type of Citrullus is an economical important crop in west Africa. Few studies were conducted on genetic resources of this plant (Ndabalishye, 1995; Djè et al., 2006) previously reported to be morphological differentiated especially in terms of fruit and seed characters.

The objectives of the present study were to survey and analyze variations in plant, fruit and seed morphology in C. lanatus oleaginous type collection from Côte d’Ivoire. This represents the first study on collection and characterization of C. lanatus oleaginous type germplasm in Nangui Abrogoua University.

Study site

The trials were regularly monitored throughout the growing season in 2011 and 2012 (May to November). Farm experiment was conducted in the village of Manfla, located in the centre (6°49′34.38″N, 5°43′47.68″W) 400 km North Abidjan (Côte d’Ivoire). Annual rainfall varies from 800 to 1400 mm with a long-term mean of 1200 mm, and the annual mean temperature is 27°C. A complete description of study site was done by Kouassi and Zoro Bi (2010).

Experimental design

Planting was done according to a completely randomized block design, with seven replications. Each replicate consisted of a 24×27 m containing 70 holes at a depth of 3 cm, resulting in 7 holes per accession. The planting distance was 3 m between and within rows with 1.5 m of edges. A manual weeding was carried out during plant development to prevent weed invasion. Disease and pest control was carried out using a carbamate-based insecticide applied when necessary.

Traits measurement

Accessions were examined using 11 quantitative traits subdivided into four vegetative and seven yield characters selected from standard descriptors for cucurbits(Maggs-Kölling et al., 2000; Marr et al., 2007; Morimoto et al., 2005; Koffi et al., 2008). Data were measured in five plants and five fruits per accession.

Vegetative traits measured were limb length (LL), limb width (LWI), plant length (PL), and number of branches from the central taproot (BN) for each plant. Yield characters measured were weight (FWE), length (FL), width (FWI), seeds cavity diameter (SCD), seeds number (SN), and seeds weight (SW). Seed traits analyzed included 100-seeds weight (100-SWE). Measurements on 100- seeds weight were scored using five individuals randomly selected on each plant (Table 3).

Data analysis

Statistical  analyses  were  carried  out  with  STATISTICA  software package version 7.1(StatSoft, 2005). Principal Component Analysis (PCA) was applied to identify most discriminant parameters followed by hierarchical cluster analysis using unweighted pair-group method using arithmetic average (UPGMA) method. Multivariate analysis of variance (MANOVA) and a factorial discriminant analysis (FDA) were performed to check difference between the variable means for each group obtained with the clustering analysis. A confusion matrix was constructed, to check the reliability of groups as defined by the hierarchical clustering.

According to confusion matrix, the composition of groups as defined by the hierarchical clustering is different. Group 1 contains 41 accessions instead of 46. Accessions CL-146 and CL-172 have been reclassified in group 2. Similarly, accessions CL-142 and CL-085 have been reclassified in group 3 and accession CL-131 has been reclassified in group 4. Group 2 consists of 44 instead of 48 accessions. An enrollment of 50 accessions belongs to group 3 instead of 53. Group 4 includes 23 accessions instead of 24 (Table 7).

In this study, 171 accessions of Citrullus of Nangui University collection were characterized based on morphological traits. In the analysis made to estimate the relative contributions of the different traits studied towards the overall phenotypic variation among the 171 accessions, a total of five principal components (PCs), explained 81.2% of variability. Fruit-related parameters were the most discriminant. In fact, fruit length, fruit diameter, and seed cavity diameter were parameters which contributed more to the first principal component (24%), implying that 24% of the variability was explained by these parameters. These results suggest that fruit size is a useful criterion for separating Citrullus accessions. In the cucurbit family, the significant contribution of the fruit traits to morphological variability has been reported for watermelon (Gusmini and Wehner, 2003; Maggs-Kölling et al., 2000). According to Achigan-Dako (2015) diagnostic morphological markers among species include fruit such as fruit size.

The least discriminant parameters were numbers of seeds and seeds weight. This finding is due to the fact that medium-sized seeds represented 60% of sample analyzed. Contrary to these results, Achigan-Dako et al. (2015) demonstrated that other major differentiation traits is related to seed.

Although our accessions originated from seven different countries, they did not cluster according to geographical distribution. This may be suggestive of the single ancestry of the analyzed plants (Dane and Liu, 2007). This may be because genetically, the cultivated watermelon is closest to plants from West Africa that represents the gene pool from which watermelon was domesticated (Chomicki and Renner, 2014). The sister species of watermelon is C. mucosospermus, the egusi melon, ranging from Nigeria to Senegal (Renner et al., 2014). Plants from Benin closely resemble the collection type of the name C. mucosospermus (which is from Ghana). In order to explain the low genetic diversity of C. mucosospermus (but identified as C. lanatus), Dane and Liu (2007) demonstrated that the watermelon varieties have diverged into small populations. Moreover, the domestication   and   the   resulting selection  may  have reduced the genetic diversity from the original watermelon crops. Levi et al. (2001) also revealed that cultivated watermelon exhibited narrow genetic base as a results of many years cultivation and selection for specific qualities.

Morphological and genetic characterization of Hungarian and Turkish accessions of watermelon (including C. lanatus and C. colocynthis) revealed that germplasm resources present a wide range of diversity (Sari et al., 2005; Solmaz and Sari, 2009; Solmaz et al., 2010). However, accessions of the two countries show many similarities and therefore cannot be separated clearly (Szamosi et al., 2009). In the present study, comparison of the four groups using a MANOVA combined with factorial discriminant analysis, allowed the classification of accessions according to agro-morphological characters. Accessions of group 2 had the biggest leaves. The smallest leaves were observed in accession of group 1. The largest number of ramification and longest plants were observed in group 4.

Groups 1 and 2 have the largest and heaviest fruits. The best seed yield is obtained with accessions of group 2, but 100 seed-weight was higher in groups 1 and 3 than in the two other groups. Morphological variation analysis in landraces of C. mucosospermus (but identified as C. lanatus by authors in Côte d’Ivoire) also allowed to classify accessions based on fruit and seed traits (Adjoumani et al., 2012).

Although, the Hierarchical Cluster Analysis gave a clear separation of accessions in distinct groups, the confusion matrix displayed a reclassification of some accessions. Accessions CL-146 and CL-172 has been reclassified in group 2. Accessions CL-142 and CL-085 has been reclassified in group 3 and accession CL-131 has been reclassified in group 4.

Traditionally, watermelon breeders focus on fruit quality traits. These traits include sugar content, flesh color, fruit size, and rind patterns (Hashizume et al., 2003). Investigations of the inheritance of fruit morphology and quality traits date back as far at the 1930’s (Porter, 1933, 1937). Since then, many efforts have been made to better understand traits associated with watermelon fruit quality and morphology. Many genes have been described that control the internal fruit quality and morphology in watermelon (Guner and Wehner, 2004). An internal fruit characteristic that has received attention is the Brix value.

A linkage map was developed and a QTL that accounts for 19% of the variation in Brix was mapped on linkage group (LG) 8 (Prothro, 2010). Fruit shape and weight are important external characteristics that breeders must consider when developing watermelon cultivars. 

Watermelon shape can be classified as being either round, oval, blocky or elongate (Wehner et al., 2001). The only gene described that controls fruit shape is the elongate fruit gene (O). Fruit weight has recently become an important  consideration for breeders due to increased consumer preference for smaller sized watermelons (Gusmini and Wehner, 2007). Watermelons have traditionally been classified into five categories based on fruit weight. These are icebox (less than 5.5 kg), small (5.5 to 8.0 kg), medium (8.1 to 11.0 kg), large (11.1 to 14.5 kg), giant (greater than 14.5 kg), and recently mini fruit size (1.5 to 4.0 kg) was added (Gusmini and Wehner, 2007; Maynard, 2001). No genes have been identified that are responsible for fruit weight inwatermelon (Gusmini and Wehner, 2007; Maynard, 2001). Fruit quality and morphological traits, such as the ones described earlier, gain the most attention from watermelon breeders. There are many other important traits in watermelon that receive little to no attention. The egusi seed trait is one of these traits. The egusi seed trait has been shown to be inherited as a single gene (Gusmini et al., 2004).

A single QTL was identified for rind thickness (Prothro, 2010). The objective when breeding for this trait should be to have a rind thickness that is a small percentage of the fruit diameter. Three QTL were identified for fruit length and two QTL were identified for fruit width, in the same region on LG5. Another QTL for fruit length was identified on LG3. But it is not known if they are associated with the elongate fruit gene. Kumar et al. (2009) produced a population segregating for fruit shape and found that his data did not fit the single gene theory for fruit shape inheritance. Watermelons varieties that produce fruit that fit into the weight category most preferred by consumers must be available for growers(Gusmini and Wehner, 2007). The QTL identified for fruit length and fruit width should be useful for fruit weight selection in a breeding program. Two QTL were identified for this trait on LG2. Another QTL was identified for seed oil percentage. This QTL falls between the locations of the two QTL identified for the egusi seed trait on LG2 (Prothro et al., 2012).

A great deal of diversity morphological traits has been observed in C. mucosospermus oleaginous type accessions from the Nangui Abrogoua University collection. From the present investigation, it was concluded that the indigenous oleaginous cucurbit C. mucosospermus displayed a wide range of diversity for most of the morphological traits studied. Traits related to fruit size and weight were the most accession distinctive. Consequently, fruit morphological traits could be used for accession identification during collecting missions.  Accessions had been divided into four groups in which group 2 had the highest seed yield. Therefore, accessions of this group must be used to enhance yield. These findings are important to plan future germplasm sampling and evaluation studies.

The authors have not declared any conflict of interests.