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Article

Towards Introgression Between Watermelon (Citrullus lanatus) and Its Wild Relative, Bitter Apple (C. colocynthis)

by
Lana W. Al-Qadumii
1,
Monther T. Sadder
2,*,
Bayan Alkharabsheh
3,
Samih Y. Salem
3,
Mohammad S. Salem
3 and
Karam Bani-Yaseen
3
1
Faculty of Agricultural Technology, Department of Agricultural Biotechnology and Genetic Engineering, Al-Ahliyya Amman University, Amman 19328, Jordan
2
Department of Horticulture and Crop Science, School of Agriculture, University of Jordan, Amman 11942, Jordan
3
Global Agricultural Establishment, P.O. Box 245, Amman 11732, Jordan
*
Author to whom correspondence should be addressed.
Horticulturae 2025, 11(11), 1304; https://doi.org/10.3390/horticulturae11111304
Submission received: 12 September 2025 / Revised: 25 October 2025 / Accepted: 27 October 2025 / Published: 31 October 2025
(This article belongs to the Special Issue Germplasm Resources and Genetics Improvement of Watermelon and Melon)
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Abstract

The genetic diversity of cultivated crops is limited, largely as a result of domestication bottlenecks and the selective pressures imposed during modern breeding. An introgression cross was initiated by mating bitter apple (Citrullus colocynthis), as a wild founder parent, with ‘Charleston Grey’ watermelon (Citrullus lanatus) commercial cultivar, focused on identifying and utilizing trait-enhancing alleles from crop wild relative (CWR). Successful crosses resulted in diverse families, including F1 hybrids, F2 population, and backcross (BC) progenies. The study revealed substantial variation among the founder parents and their derived progeny in plant growth and major agronomic fruit traits, highlighting the value of this genetic diversity for breeding programs and demonstrating the potential of Citrullus introgression lines to enhance desired traits in cultivated watermelon. Morphological analysis demonstrated that F1 progeny resembled the maternal parent for the majority of investigated fruit traits. A considerable proportion of the introgression progeny in the F2 generation outperformed both parents in total soluble solids and lycopene content, suggesting that crop wild relatives hold strong breeding value through beneficial allelic recombination. BC1 siblings were closer to the wild watermelon, which is presumably maternally controlled through plastome and mitogenome in crosses between cultivated watermelon and wild bitter apple, which is expected to be retained in successive backcrosses. The study uncovers novel alleles of CWR that preserve extensive genetic variation that is essential for enhancing resilience traits in current breeding lines. These introgression-derived resources provide a critical platform for advancing genetic studies and enhancing crop resilience.

1. Introduction

Thousands of years ago, early humans began experimenting with agriculture, which resulted in the domestication of a wide range of wild plant species [1]. ‘Domestication syndrome’ is a common phenomenon in almost all cultivated crops [2,3,4]. Intensive selection during domestication has markedly reduced the genetic diversity of many crop species, often by more than half, when compared with their wild relatives. This loss is particularly evident in traits linked to adaptation and fitness, as documented in crops such as tomato [5], eggplant [6], and others [7].
Cultivated watermelon is one of the crops most severely impacted by domestication bottlenecks, which led to markedly reduced genetic diversity, making most varieties vulnerable to new diseases. Bitter apple (Citrullus colocynthis (L.) Schrad.), a crop wild relative (CWR) of watermelon, is resistant to many important diseases, making it a valuable resource for expanding genetic diversity and increasing disease resistance [8]. The major drawback is that the wild relative has white pulp, coarse flesh, and a bitter taste, which would negatively impact watermelon cultivation. Therefore, understanding the specific genomic regions contributing to variations in fruit quality traits, notably fruit morphology, sweetness, acidity (pH), and lycopene content, is essential when employing bitter apple in introgression breeding [9]. This approach can help preserve historical genetic bottlenecks, which involves systematic exploration and utilization of CWRs.
In fact, CWRs have endured diverse natural habitats and extreme climatic conditions [10]. They serve as valuable sources of allelic variants that confer environmental adaptation [11] and are increasingly being deployed for developing crop genotypes resilient to changing environmental conditions [12,13,14]. Recent studies have demonstrated the significant role of exotic germplasm in broadening the genetic diversity of modern crops [15,16], with major crops such as maize, wheat, sunflower, potato, and rice benefiting substantially from their wild relatives [17,18].
The deployment of introgression breeding between bitter apple C. colocynthis and C. lanatus ((Thunb.) Matsum. & Nakai) is still limited, despite its potential in providing novel alleles crucial for crop improvement [8,9]. C. colocynthis thrives under diverse natural conditions and represents a valuable source of genetic variation for environmental adaptation [19,20]. Among CWRs, C. amarus and C. mucosperrous are immediate progenitors with high cross-compatibility with C. lanatus [21]. Agronomically important traits, including resistance to various biotic and abiotic stresses, were identified within these CWRs and their hybrid derivatives [22,23,24]. Nonetheless, many wild genetic resources remain underrepresented in global gene banks and underutilized in many countries.
DNA marker analysis has shown that C. colocynthis possesses vast genetic diversity, making it a major genetic resource for improving watermelons [25]. This study builds on ongoing efforts involving introgression crosses between C. lanatus and C. colocynthis to utilize the genomic diversity preserved in CWRs for breeding and crop improvement programs. Systematic genetic introgression was performed via an artificial hybridization approach for the transfer of novel traits from the wild parents into adaptable commercial cultivars with the aim of establishing genetically wide/diverse pre-breeding germplasm, vital for future crop improvement.

2. Materials and Methods

2.1. Plant Material

The plant material used in this study comprises bitter apple (Citrullus colocynthis) accession LY25, sourced from a germplasm collection native to Libya, obtained through a donation from the Agricultural Research Institute of Libya. The bitter apple LY25 accession has a creamy white leathery flesh, no sugars, tan to black-colored seeds, and elliptical flat fruits weighing around 0.235 kg. The rind of LY25 showed a medium-green marbled appearance characterized by a yellowish-white blotchy inter-stripes pattern. Cultivated watermelon parent used in the introgression breeding was farmer-preferred cultivar elongated ‘Charleston Grey’; pure line 184 (Global Agricultural Est., Amman, Jordan). It has light green stripes, the fruit weighs about 3 kg, the seeds are brown, and it has sweet red flesh with a 9.72 Brix value [26,27]. Seeds were sown in 98-cell pro-trays in nurseries during September 2020 (for F1), 2021 (for F2), and 2022 (for BC1). Seedlings were grown in sterile peat moss as the growing medium. One week before transplanting, seedlings at the true leaf stage were acclimated in an open field for hardening. Transplanting was performed 14 days after sowing (DASs) at the two-true-leaf stage. They were grown in an experimental greenhouse in the Jordan Valley (Deir Alla, Jordan; GPS coordinates of the plot: 31°50′44.9″ N 35°52′37.2″ E). The plant-to-plant and row-to-row spacings were 1.2 m and 0.6 m, respectively. Irrigation was provided via a drip system, and each plant received 80 g of N:P:K (20:20:20) fertilizer supplemented with micronutrients, applied evenly throughout the cultivation period through the drip irrigation system.

2.2. Crossing and Breeding Scheme

Both watermelon (elongated ‘Charleston grey’) and bitter apple were grown at different time intervals to synchronize flowering to facilitate hybridization crosses and reciprocal crosses. Fifty crosses were carried out between watermelon as female and bitter apple as male (Figure 1A). Thereafter, two F1 lines were advanced and used for the generation of F2 and BC1 lines. For the reciprocal cross, another fifty crosses were carried out with bitter apple as female and watermelon as male (Figure 1B). Thereafter, five F1 lines were advanced and used for the generation of F2 and BC1 lines. Usually, the first 5–7 flowers were males, and the remaining were females. Closed buds were selected for pollination. After pollination, flowers were covered and labeled. All pollinations were made between 8 am and 12 noon. Mature fruits were used to extract seeds. Seeds were grown as above, and flowers were either selfed or backcrossed.

2.3. Data Collection

Our study trials consisted of P1, P2, F1 lines, F2 lines, and BC1 lines. Fruits were collected at 85–90 DAS, corresponding to their horticultural maturity. Major quality attributes, including total soluble solids (TSS), acidity (pH), and lycopene content, were evaluated. Fully matured fruits from each genotype were harvested, and the edible portion was separated for analysis.
Approximately 20 g of fresh fruit pulp was homogenized using a laboratory blender to obtain a uniform juice sample. The homogenate was filtered through a fine mesh to remove particulate matter, and the filtrate was used for subsequent measurements. The TSS content was measured using a handheld digital refractometer calibrated with distilled water at 20 ± 2 °C. A few drops of the clarified juice were placed on the prism surface of the refractometer, and readings were expressed as degrees Brix (°Brix).
The pH of the fruit pulp juice was determined using a calibrated digital pH meter prior to measurement, and the instrument was standardized with buffer solutions of pH 4.0, 7.0, and 9.2 at room temperature (25 ± 2 °C). Approximately 10 mL of the filtered juice was placed in a beaker, and the electrode was immersed until a stable reading was obtained. All measurements were conducted in triplicate, and the mean of three replicates was considered as the final representative value for each genotype.
Lycopene, which imparts varying coloration among the parental lines and their progeny, was quantified using the low-volume hexane extraction method as described by reference [28]. Lycopene levels across the generations were subsequently analyzed.
L y c o p e n e = A 503 × M W × D F × 1000 Ƹ × L
where MW represents the molecular weight of lycopene (536.9 g/mol), DF is the dilution factor, L denotes the optical path length in centimeters, and Ƹ corresponds to the molar extinction coefficient of lycopene (172,000 L mol/cm) [29]. The TSS of fruits was measured by using a hand-held refractometer. In addition, morphological traits were measured, including fruit dimensions: length, width (diameter), and fruit shape index (FSI), which is used to quantify the shape of a fruit and is calculated by dividing the fruit’s length by its diameter, rind thickness [30], flesh color [31], and fruit color [32].

3. Results

3.1. Growing Plants and Crossings

Seeds were successfully germinated in trays with peat moss media. Repotted seedlings were successfully established in the greenhouse for crossing (Figure 2A). All planned crossings were carried out, and successful fertilization resulted in small fruits resampling bitter apple parents (Figure 2B,C). The fruits were harvested at maturity for further analysis.
Reciprocal hybridization between C. colocynthis (25LY) and C. lanatus (184) was conducted. When C. colocynthis (25LY) served as the female parent, 34% F1 hybrids were successfully obtained, whereas the reciprocal cross with C. lanatus (184) as the female parent yielded a 20% success rate.
During the subsequent season, each F1 genotype was used for self-pollination and backcrossing with C. lanatus (184). The progeny derived from C. colocynthis as the female parent exhibited markedly higher fruiting performance, producing 69 selfed fruits and 5 backcross fruits per F1, corresponding to a fruit-set rate of 90%. In contrast, the reciprocal family produced only 43 total fruits, with a fruit-set rate of 71.7%. These results indicate that the maternal genotype of C. colocynthis enhanced reproductive success and compatibility in hybrid formation compared with the reciprocal direction.

3.2. Fruit Traits

Harvested mature fruits were brought to the lab for further analysis. External and internal agronomical traits were recorded, and quality measurements were taken. All these features were recorded for two reciprocal cross families. In the first cross (watermelon accession 184 as female × bitter apple accession Y25 as male), the following lines were achieved: two F1 lines (F1:66 and F1:67), seven BC1 lines (including BC1:16 and BC1:23), and 38 F2 lines (including F2:16 and F2:23) (Figure 3).
The generated F1 plants had fruits with an intermediate size between the two parents (smaller than watermelon and larger than bitter apple). In addition, F1 fruits showed red flesh and intermediate rind color between the two parents (Figure 3). On the other hand, the segregating F2 plants showed huge phenotypic variation, with flesh colors including red, yellow, white, beige, and pink, while fruits showed different shapes, including round, ovate, oblong, and lemon-shaped, with all possible rind colors (green shades) and stripe density and format (Figure 3). Nonetheless, the backcrossed plants showed bitter apple retro-traits, e.g., white flesh and small fruits.
In the reciprocal cross (bitter apple accession Y25 as female × watermelon accession 184 as male), we grew 5 F1 plants, 16 BC1 plants, and 107 F2 plants (Figure 4, Figure 5, Figure 6 and Figure 7). The two families (F1:154 with F2:20 and BC1:20) and (F1:156 with F2:18 and BC1:18) are shown in Figure 4. In the reciprocal cross, the F1 resembled the bitter apple female parent, while the F2 segregating plants showed an array of flesh colors (red, yellow, white, beige, and pink). Recorded fruit shapes include round, ovate, oblong, and pear-shaped, with variations in rind colors (Figure 4). Similar to the first cross (Figure 3), the BC1 in the reciprocal cross showed bitter apple retro-traits. Similar trends were achieved for the families: F1:155 with F2:27 and BC1:27 (Figure 5), F1:157 with F2:26 and BC1:26 (Figure 6), and F1:158 with F2:25 and BC1:25 (Figure 7).
Fruit traits were recorded for all harvested fruits as listed in Supplementary Table S1. Seed number varied in segregating F2 fruits (10–428 per fruit), while fruit length and width varied between 5 and 28.5 cm, and 4.5–20 cm, respectively. The number of loops (locules) was found to be between three and six. However, the TSS value in F1 plants exceeded the watermelon parent (7.15), reaching 10–13, while it dropped to 8.8 in segregating F2 fruits. Likewise, LYC in F1 plants exceeded the watermelon parent (1.65), reaching 3.14–3.54, while it surpassed the parents and F1 to 5.22 in segregating F2 fruits (Supplementary Table S1).
Likewise, for the reciprocal crosses, fruit traits were recorded for all harvested fruits from the reciprocal crosses as listed in Supplementary Table S2. Seed number varied in segregating F2 fruits (20-377 per fruit), while fruit length and width varied between 11 and 30 cm, and 7.5–27 cm, respectively. The number of loops (locules) was between three and six. However, the TSS in F2 plants exceeded that of the watermelon parent (7.15), reaching nine. Likewise, LYC in F2 plants exceeded the watermelon parent (1.65), reaching 5.74, while it surpassed the parents and F1 to 5.22 in segregating F2 fruits (Supplementary Table S2).
The data were collectively presented as box plots for all measured fruit traits among the generated progeny lines (Figure 8). The plots were grouped for F1 lines generated by CC × CL, for F1 lines generated by CL × CC, and for F2 lines generated by selfing F1 plants. The box plot for fruit dimensions (Figure 8A,C,E) revealed smaller fruits resulting from the introgression cross CC × CL than from the reciprocal cross CL × CC, which was also smaller than those generated in F2 lines, with the broadest size distribution evident among the segregating F2 lines. The fixed locule count of six in fruits resulting from the introgression cross CC × CL was variable in the reciprocal cross and in segregating F2 lines (Figure 8G). Likewise, the narrow seed count around 150 in fruits resulting from the introgression cross CC × CL was variable in the reciprocal cross and in segregating F2 lines (Figure 8B). This was also evident for the biochemical traits in segregating F2 lines: TSS (Figure 8D), pH (Figure 8F), and lycopene content (Figure 8H).
The pair correlation analysis between all measured fruit traits showed either weak or strong correlations (Figure 9). The strongest correlation was achieved for fruit dimensions including fruit width, fruit length, FSI and pith, where the highest calculated correlation was found between fruit width and fruit length (93%), while biochemical-analysis-related traits, including lycopene content, TSS, and pH, revealed weak correlations, such as the correlation between lycopene content (absorbance) and pH (32%).
Hierarchical clustering analysis was applied to relative fruit characteristics measured in F2:23 lines generated from (watermelon accession 184 as female X bitter apple accession Y25 as male) as compared to the female parent (watermelon accession 184) (Figure 10). The analysis showed a first-level cluster between loops and pH, followed by fruit width and LYC. Another cluster grouped fruit length with TSS. However, when the same analysis was applied to relative fruit characteristics measured in F2:23 lines as compared to the male parent (bitter apple accession Y25), the results showed a different clustering (Figure 11). In this case, three nice clusters were achieved: TSS and LYC (first cluster); length, width, and grid (second cluster); and loops, seed number, and pH (third cluster).

4. Discussion

4.1. Novel Watermelon Breeding Progenies

Introgression lines have been widely employed in breeding programs across various crop species to transfer novel genes from wild progenitors into cultivated varieties [33,34,35,36,37]. Watermelon cultivars exhibit low genetic diversity, highlighting the need to expand their allelic foundation by incorporating wild relatives or subspecies obtained from natural populations, which possess extensive genetic variation [33]. The breeding lines presented in this study offer valuable potential for transferring chloroplast and mitochondrial genomes of wild relatives into cultivated watermelon lines. These lines can facilitate the investigation of the impacts of foreign cytoplasmic genomes, specifically from Citrullus colocynthis, on physiological traits such as photosynthesis, respiration, flower development, fruit quality, alongside resistance to biotic stresses [38,39,40]. Additionally, they may contribute to the improvement of watermelon cultivars by promoting early development of globular fruits characterized by a thick green rind and firm, solid flesh, devoid of hollow heart formation, as these are desired by watermelon growers.
The generated populations herein serve as the foundation for subsequent analyses and the development of introgression lines. Such lines are pivotal for broadening the limited genetic diversity present within both national and international commercial cultivars, as well as facilitating detailed investigations and identification of genes associated with disease resistance and fruit quality traits in watermelon.

4.2. Crossability Between Watermelon and Bitter Apple

Citrullus lanatus var. lanatus (CLL) and Citrullus lanatus var. citron (CLC) are considered to have evolved from a common ancestral lineage, with subsequent haplotype fixation, resulting in a genomic divergence at a similarity level of 58.8%. In comparison, Citrullus colocynthis (CC) showed genetic divergence at a 38.9% similarity level with CLL [23]. The close genetic relationship between CLL and CLC is reflected in their reproductive compatibility, as they readily hybridize and yield fertile progeny through conventional breeding approaches. In contrast, crosses of CLL and CC combinations demonstrate directional tendencies and commonly result in substantially decreased fertility of the offspring [41]. In the present study, however, no signs of reproductive barriers were observed, as we were able to successfully generate F1, BC1, and sufficient F2 populations (Figure 3 and Figure 4). These results differ from earlier findings [42], which indicated that CLL × CC crosses produced inconsistent numbers of F1 and BC1 progeny and often yielded very few or no F2 seeds, suggesting the presence of partial incompatibility between these two Citrullus species. Our findings, therefore, highlight the feasibility of using CC as a genetic donor for introgression breeding, offering new opportunities to broaden the genetic base of cultivated watermelon and strengthen its resistance against both biotic and abiotic stresses.

4.3. Fruit Evaluation Introgression Lines

Introgression of beneficial alleles from Citrullus colocynthis (CC) into cultivated watermelon is often hindered by linkage drag, as favorable loci are frequently associated with undesirable fruit traits. This problem is most evident in F2 populations, where chromosomal structural differences restrict recombination and limit allele transfer. Nevertheless, our successful development of F1, BC1, and F2 progeny indicates that these barriers can be at least partially overcome, and modern tools such as marker-assisted selection and genome editing can hold promise for reducing linkage drag and accelerating the effective use of CC alleles in watermelon breeding [42]. Our results agreed with this theory, as none of the characteristics that we obtained in our study agreed with Mendel’s ratios [42]. However, we noticed that F1 generation characteristics are affected by the maternal parent. The F1 produced fruits from crossing 25 CC (female) and 184 CL (Charleston Grey) (male) were similar to CC (Supplementary Table S1). In contrast, while F1 fruits produced from the reciprocal cross were closer to CL with spherical fruits, between their parents, with bitter taste and white flesh color in the first cross between (25CC (female) and 184 CL (Male)). On the other hand, F1 fruits from the reciprocal cross were bigger than those from the first cross, and exceeded their parents in pH and lycopene content, showing a transgressive segregation effect; however, they still had a bitter taste (Supplementary Table S2).
Transgressive segregation is a phenomenon that occurs when the traits of the hybrid progeny display phenotypic variation exceeding that of both parents. It is different from heterosis, which is characterized by increased vigor in the F1 generation. Transgressive segregation, on the other hand, is more common in the F2 generation and can persist in subsequent generations. The success of selection in self-pollinated crops largely depends on the ability of breeders to identify and stabilize transgressive segregants during the early generations. By accurately predicting heterotic crosses, breeders are able to develop superior lines that exhibit transgressive segregation in advanced generations, highlighting the close relationship between heterosis and transgressive variation. It has been proposed that the genetic distance between parental lines plays a central role in determining the magnitude of heterosis and is positively correlated with the frequency of transgressive segregants [43,44,45]. While hybrids produced earlier [46,47] showed F1 fruits with irregular appearances rather than spherical ones, such as CC or CL, it has been claimed that they could reflect the reduction in the number of produced seeds caused by abnormal chromosomal association [46].
Distortion refers to the degenerational distortion that describes the deviation of observed genotype frequencies from the ratios predicted by Mendelian segregation. This phenomenon can be influenced by multiple factors, such as biased genetic transmission, gametic and zygotic selection, non-homologous recombination, horizontal gene transfer, transposable elements, and environmental pressures. These factors can affect the mapping population and lead to deviations from the expected Mendelian inheritance patterns. This phenomenon is observed in several crops such as rice, peas, and citrus [48,49,50,51].
BC1 generations from both crosses exhibited intermediate traits relative to their parents in terms of size, shape, TSS, and pH, with all displaying bitterness and white flesh. These characteristics resembled more or less the wild parent (CC) than the cultivated parent (CL), supporting the maternal inheritance of chloroplasts and mitochondria. In crosses between cultivated watermelon and its wild relative, maternal cytoplasmic inheritance is maintained across successive backcrosses when the cultivated watermelon functions as the recurrent male parent and the backcross progeny are evaluated, retaining the wild species organelle genomes from the female parent [52].
F2 siblings became closer to CL than to CC; they have a very wide segregation range, and the segregation ratio does not match Medels’ ratio, as we need more generations to fit the ratio. Tremendous variations in fruit flesh color were observed among the F2 plants, exhibiting distinct phenotypes: white, resembling the wild parent, and other colorations, corresponding to the cultivated parent. However, they were absent in both F1 and BC1 lines. The color distribution was typically localized either around the seed tissues or throughout the flesh. Some fruits exhibited multiple colors in distinct zones, and noticeable variation in color and other traits occurred among fruits from the same plant. These findings were noticed in similar work [53]. Regarding F2 flesh color, such variation was noted here in BC1 rather than in F2 populations. Earlier studies have indicated that one to two genes with epistatic interactions, including LYCB, PDS, and PSY, regulate color development across different fruit regions such as the margins, rind vicinity, and seed area (Figure 3) [8].
Hierarchical clustering of fruit traits plays a significant role in watermelon breeding because it helps breeders make data-driven decisions to enhance desirable traits, maintain genetic diversity, and improve overall breeding efficiency [54]. Two hierarchical clustering heat maps were achieved by applying relative fruit characteristics measured in F2:23 lines generated from (watermelon accession 184 as female X bitter apple accession Y25 as male), as compared to the female parent (watermelon accession 184) (Figure 10) or as compared to the male parent (bitter apple accession Y25) (Figure 11). They resulted in different heat maps, stressing the importance of the base parent in the analysis. These data need further analysis, including watermelon yield components [55], after multiple generations of selfing to achieve enough homozygosity in advanced lines.

5. Conclusions

Crop wild relatives (CWRs) serve as crucial reservoirs of important traits, including tolerance to biotic and abiotic stresses, along with key quality and yield-related traits, making them valuable sources of novel genes for crop improvement. Globally, there is a growing focus on utilizing CWRs to introgress these genes into cultivated varieties. In this study, successful hybridization between watermelon and bitter apple yielded viable F1, BC1, and F2 generations, enabling the transfer of traits from the wild desert species C. colocynthis to C. lanatus. These findings offer important insights for future functional validation of candidate genes and for breeding programs aimed at enhancing fruit quality and stress tolerance in watermelon through the use of C. colocynthis.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/horticulturae11111304/s1, Table S1: Fruit features of the family cross (Watermelon accession 184 as female × Bitter apple accession Y25 as male); Table S2: Fruit features of the family cross (Bitter apple accession Y25 as female × watermelon accession 184 as male).

Author Contributions

Conceptualization, L.W.A.-Q. and M.T.S.; methodology, K.B.-Y.; software, B.A.; validation, K.B.-Y.; formal analysis, L.W.A.-Q.; investigation, L.W.A.-Q. and M.T.S.; resources, S.Y.S.; data curation, M.S.S.; writing—original draft preparation, L.W.A.-Q.; writing—review and editing, and M.T.S.; visualization, M.T.S.; supervision, M.T.S.; project administration, M.T.S. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

The original contributions presented in this study are included in the article/supplementary material. Further inquiries can be directed to the corresponding author.

Acknowledgments

The authors would like to thank the University of Jordan for its support. The authors would like to extend their thanks to Saja Allail, Sabreen, Selien, Mohammed Alsakarneh, and Zaied Mahanoh for helping with the sample collection.

Conflicts of Interest

The authors declare no conflicts of interest.

Abbreviations

The following abbreviations are used in this manuscript:
BCBackcross
CCCitrullus colocynthis
CLCitrullus lanatus
CLLCitrullus lanatus var. lanatus
CLCCitrullus lanatus var. citron
CWRCrop wild relative
FFilial generation
TSSTotal soluble solids

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Figure 1. (A) Outline of introgression cross between watermelon (184 CL) and bitter apple (LY 25 CC), including generated F1 introgression lines, F2 line, and backcross lines BC1. (B) Outline of reciprocal cross between bitter apple (LY 25 CC) and watermelon (184 CL), including generated F1 introgression lines, F2 lines, and backcross lines BC1.
Figure 1. (A) Outline of introgression cross between watermelon (184 CL) and bitter apple (LY 25 CC), including generated F1 introgression lines, F2 line, and backcross lines BC1. (B) Outline of reciprocal cross between bitter apple (LY 25 CC) and watermelon (184 CL), including generated F1 introgression lines, F2 lines, and backcross lines BC1.
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Figure 2. (A) Established parental plants in the greenhouse. (B,C) fruits of successful crosses.
Figure 2. (A) Established parental plants in the greenhouse. (B,C) fruits of successful crosses.
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Figure 3. Introgression population generated from (watermelon accession 184 as female × bitter apple accession Y25 as male) cross. For each line (parents, F1, F2, and BC1), representative fruits (closed and cross-section) are shown with considerable diversity for all fruit characteristics. Scale bar equals 5 cm.
Figure 3. Introgression population generated from (watermelon accession 184 as female × bitter apple accession Y25 as male) cross. For each line (parents, F1, F2, and BC1), representative fruits (closed and cross-section) are shown with considerable diversity for all fruit characteristics. Scale bar equals 5 cm.
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Figure 4. Introgression population generated from (bitter apple accession Y25 as female × watermelon accession 184 as male) cross. Two families are illustrated: left side (F1:154 with F2:20 and BC1:20) and right side (F1:156 with F2:18 and BC1:18). For each line (parents, F1, F2, and BC1), representative fruits (closed and cross-sectioned) are shown with great diversity for all fruit characteristics. Scale bar equals 5 cm.
Figure 4. Introgression population generated from (bitter apple accession Y25 as female × watermelon accession 184 as male) cross. Two families are illustrated: left side (F1:154 with F2:20 and BC1:20) and right side (F1:156 with F2:18 and BC1:18). For each line (parents, F1, F2, and BC1), representative fruits (closed and cross-sectioned) are shown with great diversity for all fruit characteristics. Scale bar equals 5 cm.
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Figure 5. Introgression population generated from (bitter apple accession Y25 as female × watermelon accession 184 as male) cross. One family is outlined (F1:155 with F2:27 and BC1:27). For each line (parents, F1, F2, and BC1), representative fruits (closed and cross-sectioned) are shown with high diversity for all fruit characteristics. Scale bar equals 5 cm.
Figure 5. Introgression population generated from (bitter apple accession Y25 as female × watermelon accession 184 as male) cross. One family is outlined (F1:155 with F2:27 and BC1:27). For each line (parents, F1, F2, and BC1), representative fruits (closed and cross-sectioned) are shown with high diversity for all fruit characteristics. Scale bar equals 5 cm.
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Figure 6. Introgression population generated from (bitter apple accession Y25 as female × watermelon accession 184 as male) cross. One family is outlined (F1:157 with F2:26 and BC1:26). For each line (parents, F1, F2, and BC1), representative fruits (closed and cross-sectioned) are shown with substantial diversity for all fruit characteristics. Scale bar equals 5 cm.
Figure 6. Introgression population generated from (bitter apple accession Y25 as female × watermelon accession 184 as male) cross. One family is outlined (F1:157 with F2:26 and BC1:26). For each line (parents, F1, F2, and BC1), representative fruits (closed and cross-sectioned) are shown with substantial diversity for all fruit characteristics. Scale bar equals 5 cm.
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Figure 7. Introgression population generated from (bitter apple accession Y25 as female × watermelon accession 184 as male) cross. One family is outlined (F1:158 with F2:25 and BC1:25). For each line (parents, F1, F2, and BC1), representative fruits (closed and cross-sectioned) are shown with high diversity for all fruit characteristics. Scale bar equals 5 cm.
Figure 7. Introgression population generated from (bitter apple accession Y25 as female × watermelon accession 184 as male) cross. One family is outlined (F1:158 with F2:25 and BC1:25). For each line (parents, F1, F2, and BC1), representative fruits (closed and cross-sectioned) are shown with high diversity for all fruit characteristics. Scale bar equals 5 cm.
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Figure 8. Box plots for all measured fruit traits among the generated progeny lines. (A) fruit length, (B) seed count, (C) fruit width, (D) total soluble solids, (E) fruit shape index, (F) pH, (G) locule count, and (H) lycopene content. Data represent F1 lines generated by CC × CL or CL × CC and F2 lines generated by selfing F1 plants.
Figure 8. Box plots for all measured fruit traits among the generated progeny lines. (A) fruit length, (B) seed count, (C) fruit width, (D) total soluble solids, (E) fruit shape index, (F) pH, (G) locule count, and (H) lycopene content. Data represent F1 lines generated by CC × CL or CL × CC and F2 lines generated by selfing F1 plants.
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Figure 9. Correlation values between recorded fruit traits among generated progeny lines (FSI: fruit shape index, TSS: total soluble solids).
Figure 9. Correlation values between recorded fruit traits among generated progeny lines (FSI: fruit shape index, TSS: total soluble solids).
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Figure 10. Heat map of hierarchical clustering of relative fruit features for F2:23 lines generated from the cross (watermelon accession 184 as female × bitter apple accession Y25 as male) as compared to the female parent (watermelon accession 184).
Figure 10. Heat map of hierarchical clustering of relative fruit features for F2:23 lines generated from the cross (watermelon accession 184 as female × bitter apple accession Y25 as male) as compared to the female parent (watermelon accession 184).
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Figure 11. Heat map of hierarchical clustering of relative fruit features for F2:23 lines generated from the (watermelon accession 184 as female × bitter apple accession Y25 as male) as compared to the male parent (bitter apple accession Y25).
Figure 11. Heat map of hierarchical clustering of relative fruit features for F2:23 lines generated from the (watermelon accession 184 as female × bitter apple accession Y25 as male) as compared to the male parent (bitter apple accession Y25).
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Al-Qadumii, L.W.; Sadder, M.T.; Alkharabsheh, B.; Salem, S.Y.; Salem, M.S.; Bani-Yaseen, K. Towards Introgression Between Watermelon (Citrullus lanatus) and Its Wild Relative, Bitter Apple (C. colocynthis). Horticulturae 2025, 11, 1304. https://doi.org/10.3390/horticulturae11111304

AMA Style

Al-Qadumii LW, Sadder MT, Alkharabsheh B, Salem SY, Salem MS, Bani-Yaseen K. Towards Introgression Between Watermelon (Citrullus lanatus) and Its Wild Relative, Bitter Apple (C. colocynthis). Horticulturae. 2025; 11(11):1304. https://doi.org/10.3390/horticulturae11111304

Chicago/Turabian Style

Al-Qadumii, Lana W., Monther T. Sadder, Bayan Alkharabsheh, Samih Y. Salem, Mohammad S. Salem, and Karam Bani-Yaseen. 2025. "Towards Introgression Between Watermelon (Citrullus lanatus) and Its Wild Relative, Bitter Apple (C. colocynthis)" Horticulturae 11, no. 11: 1304. https://doi.org/10.3390/horticulturae11111304

APA Style

Al-Qadumii, L. W., Sadder, M. T., Alkharabsheh, B., Salem, S. Y., Salem, M. S., & Bani-Yaseen, K. (2025). Towards Introgression Between Watermelon (Citrullus lanatus) and Its Wild Relative, Bitter Apple (C. colocynthis). Horticulturae, 11(11), 1304. https://doi.org/10.3390/horticulturae11111304

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