Watermelon fruit texture and quality are determined by flesh firmness. As a quality trait, flesh firmness is controlled by multigenes. Defining the key regulatory factors of watermelon flesh firmness is of great significance for watermelon genetic breeding. In this study, the hard-flesh egusi seed watermelon PI186490 was used as the male parent, the soft-flesh cultivated watermelon W1-1 was used as the female parent, and 175 F 2 generations were obtained from selfing F 1 . Primary mapping of the major genes controlling center flesh firmness was achieved by bulked-segregant analysis (BSA)-Seq analysis and molecular marker technology. Finally, major genes were delimited in the physical interval between 6,210,787 and 7,742,559 bp on chromosome 2 and between 207,553 and 403,137 bp on chromosome 8. The content of each cell wall component and hormone was measured, and comparative transcriptome analysis was performed during fruit development in watermelon. The protopectin, cellulose, hemicellulose, indole-3-acetic acid (IAA) and abscisic acid (ABA) contents were measured, and paraffin sections were made during the three fruit developmental stages. The results revealed that protopectin, celluloses, and hemicelluloses exhibited similar trends for flesh firmness, while the IAA and ABA concentrations continued to decrease with fruit ripening. Paraffin sections showed that PI186490 cells were more numerous, were more tightly packed, had clearer cell wall edges and had thicker cell walls than W1-1 cells at every developmental stage. Comparative transcriptome analysis was conducted on RNA samples of flesh during fruit development and ripening in W1-1 and PI186490. The results from the localization interval transcriptome analysis showed that Cla016033 (DUF579 family member), which may influence the cell wall component contents to adjust the flesh firmness in watermelon fruit, was different in W1-1 and PI186490 and that Cla012507 (MADS-box transcription factor) may be involved in the regulation of fruit ripening and affect the hardness of watermelon fruit.
During plant evolution, there is genetic communication between organelle and nuclear genomes. A comparative analysis was performed on the organelle and nuclear genomes of the watermelon and melon. In the watermelon, chloroplast-derived sequences accounted for 7.6% of the total length of the mitochondrial genome. In the melon, chloroplast-derived sequences accounted for approximately 2.73% of the total mitochondrial genome. In watermelon and melon, the chloroplast-derived small-fragment sequences are either a subset of large-fragment sequences or appeared multiple times in the mitochondrial genome, indicating that these fragments may have undergone multiple independent migration integrations or emerged in the mitochondrial genome after migration, replication, and reorganization. There was no evidence of migration from the mitochondria to chloroplast genome. A sequence with a total length of about 73 kb (47%) in the watermelon chloroplast genome was homologous to a sequence of about 313 kb in the nuclear genome. About 33% of sequences in the watermelon mitochondrial genome was homologous with a 260 kb sequence in the nuclear genome. A sequence with a total length of about 38 kb (25%) in the melon chloroplast genome was homologous with 461 sequences in the nuclear genome, with a total length of about 301 kb. A 3.4 Mb sequence in the nuclear genome was homologous with a melon mitochondrial sequence. These results indicate that, during the evolution of watermelon and melon, a large amount of genetic material was exchanged between the nuclear genome and the two organelle genomes in the cytoplasm.
The plant compact and dwarf growth habit is an important agronomic trait when breeding watermelon (Citrullus lanatus) cultivars because of their reduced vine length, high-density planting, and better land utilization; however, the genetic basis of the dwarf growth habit is not well-known. In this study, the plant population of six generations, P1, P2, F1, F2, BC1P1, and BC1P2, were studied. A genetic segregation analysis demonstrated that dwarfism is mainly controlled by a single recessive Cldw gene. Furthermore, whole-genome sequencing of two distinct watermelon cultivars, W1-1 (P1) and 812 (P2), was performed and preliminarily mapped through a bulked segregant analysis of F2 individuals that revealed the Cldw gene locus on chromosome 9. Two candidate genes, Cla015407 and Cla015408, were discovered at the delimited region of 43.2 kb by fine mapping, and gene annotation exposed that the Cla015407 gene encodes gibberellic acid 3β-hydroxylase protein. In addition, a comparative analysis of gene sequence and cultivars sequences across the reference genome of watermelon revealed the splice site mutation in the intron region of the Cldw gene in dwarf-type cultivar 812. The quantitative real-time polymerase chain reaction exhibited a significantly higher expression of the Cla015407 gene in cultivar W1-1 compared with 812. There was no significant difference in the vine length of both cultivars after gibberellic acid treatment. In brief, our fine mapping demonstrated that Cla015407 is a candidate gene controlling dwarfism of watermelon plants.
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