Summary Chromosome painting based on fluorescence in situ hybridization (FISH) has played an important role in chromosome identification and research into chromosome rearrangements, diagnosis of chromosome abnormalities and evolution in human and animal species. However, it has not been applied widely in plants due to the large amounts of dispersed repetitive sequences in chromosomes. In the present work, a chromosome painting method for single‐copy gene pools in Cucumis sativus was successfully developed. Gene probes with sizes above 2 kb were detected consistently. A cucumber karyotype was constructed based on FISH using a cocktail containing chromosome‐specific gene probes. This single‐copy gene‐based chromosome painting (ScgCP) technique was performed by PCR amplification, purification, pooling, labeling and hybridization onto chromosome spreads. Gene pools containing sequential genes with an interval less than 300 kb yielded painting patterns on pachytene chromosomes. Seven gene pools corresponding to individual chromosomes unambiguously painted each chromosome pair of C. sativus. Three mis‐aligned regions on chromosome 4 were identified by the painting patterns. A probe pool comprising 133 genes covering the 8 Mb distal end of chromosome 4 was used to evaluate the potential utility of the ScgCP technique for chromosome rearrangement research through cross‐species FISH in the Cucumis genus. Distinct painting patterns of this region were observed in C. sativus, C. melo and C. metuliferus species. A comparative chromosome map of this region was constructed between cucumber and melon. With increasing sequence resources, this ScgCP technique may be applied on any other sequenced species for chromosome painting research.

BackgroundMeloidogyne incognita is a devastating nematode that causes significant losses in cucumber production worldwide. Although numerous studies have emphasized on the susceptible response of plants after nematode infection, the exact regulation mechanism of M. incognita-resistance in cucumber remains elusive. Verification of an introgression line, ‘IL10–1’, with M. incognita-resistance provides the opportunity to unravel the resistance mechanism of cucumber against M. incognita.ResultsIn the present study, analyses of physiological responses and transcriptional events between IL10–1 (resistant line) and CC3 (susceptible line) were conducted after M. incognita infection. Physiological observations showed abnormal development of giant cells and M. incognita in IL10–1, which were the primary differences compared with CC3. Furthermore, Gene ontology (GO) analysis revealed that genes encoding cell wall proteins were up-regulated in IL10–1 and that the highly expressed lipid transfer protein gene (Csa6G410090) might be the principal regulator of this up-regulation. Simultaneously, analyses of gene expression profiles revealed more auxin-related genes were suppressed in IL10–1 than in those of CC3, which corresponded with the lower level of indole acetic acid (IAA) in the roots of IL10–1 than in those of CC3. Additionally, poor nucleus development as a clear indication of abnormal giant cells in IL10–1 was related to inhibition of the cell cycle. Of those genes related to the cell cycle, the F-box domain Skp2-like genes were down-regulated in IL10–1, whereas more of these genes were up-regulated in CC3.ConclusionsAll of these findings indicate that suppressed expression of genes related to auxin and the cell cycle and highly expressed cell wall proteins play important roles in the abnormal development of giant cells, which hinders the development of M. incognita, thereby causing resistance to M. incognita in IL10–1. Knowledge from this research will provide a useful foundation for developing effective strategies in M. incognita-resistance breeding.Electronic supplementary materialThe online version of this article (10.1186/s12864-018-4979-0) contains supplementary material, which is available to authorized users.

BackgroundDifferentiation and copy number of repetitive sequences affect directly chromosome structure which contributes to reproductive isolation and speciation. Comparative cytogenetic mapping has been verified an efficient tool to elucidate the differentiation and distribution of repetitive sequences in genome. In present study, the distinct chromosomal structures of five Cucumis species were revealed through genomic in situ hybridization (GISH) technique and comparative cytogenetic mapping of major satellite repeats.ResultsChromosome structures of five Cucumis species were investigated using GISH and comparative mapping of specific satellites. Southern hybridization was employed to study the proliferation of satellites, whose structural characteristics were helpful for analyzing chromosome evolution. Preferential distribution of repetitive DNAs at the subtelomeric regions was found in C. sativus, C hystrix and C. metuliferus, while majority was positioned at the pericentromeric heterochromatin regions in C. melo and C. anguria. Further, comparative GISH (cGISH) through using genomic DNA of other species as probes revealed high homology of repeats between C. sativus and C. hystrix. Specific satellites including 45S rDNA, Type I/II, Type III, Type IV, CentM and telomeric repeat were then comparatively mapped in these species. Type I/II and Type IV produced bright signals at the subtelomeric regions of C. sativus and C. hystrix simultaneously, which might explain the significance of their amplification in the divergence of Cucumis subgenus from the ancient ancestor. Unique positioning of Type III and CentM only at the centromeric domains of C. sativus and C. melo, respectively, combining with unique southern bands, revealed rapid evolutionary patterns of centromeric DNA in Cucumis. Obvious interstitial telomeric repeats were observed in chromosomes 1 and 2 of C. sativus, which might provide evidence of the fusion hypothesis of chromosome evolution from x = 12 to x = 7 in Cucumis species. Besides, the significant correlation was found between gene density along chromosome and GISH band intensity in C. sativus and C. melo.ConclusionsIn summary, comparative cytogenetic mapping of major satellites and GISH revealed the distinct differentiation of chromosome structure during species formation. The evolution of repetitive sequences was the main force for the divergence of Cucumis species from common ancestor.Electronic supplementary materialThe online version of this article (doi:10.1186/s12864-015-1877-6) contains supplementary material, which is available to authorized users.

Ribosomal DNAs are useful cytogenetic markers for chromosome analysis. Studies investigating site numbers and distributions of rDNAs have provided important information for elucidating genome organization and chromosomal relationships of many species by fluorescence in situ hybridization. But relevant studies are scarce for species of the genus Cucumis, especially in wild species. In the present study, FISH was conducted to investigate the organization of 45S and 5S rDNA among 20 Cucumis accessions, including cultivars and wild accessions. Our results showed that the number of 45S rDNA sites varied from one to five pairs in different accessions, and most of these sites are located at the terminal regions of chromosomes. Interestingly, up to five pairs of 45S rDNA sites were observed in C. sativus var. sativus, the species which has the lowest chromosome number, i.e., 2n = 14. Only one pair of 5S rDNA sites was detected in all accessions, except for C. heptadactylus, C. sp, and C. spp that had two pairs of 5S rDNA sites. The distributions of 5S rDNA sites showed more variation than 45S rDNA sites. The phylogenetic analysis in this study showed that 45S and 5S rDNA have contrasting evolutionary patterns. We find that 5S rDNA has a polyploidizationrelated tendency towards the terminal location from an interstitial location but maintains a conserved site number, whereas the 45S rDNA showed a trend of increasing site number but a relatively conserved location.

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