Karyotyping in Melon (Cucumis melo L.) by Cross-Species Fosmid Fluorescence in situ Hybridization

Abstract

Chromosome identification is critical for cytogenetic research and will accelerate studies on genetic variation and breeding, especially for those species with relatively little sequence information. So far, no reliable cytological landmarks have been developed to distinguish individual chromosomes in melon. In this study, using FISH (fluorescence in situ hybridization) combined with comparative genome information, we selected 21 cucumber fosmids anchored by SSR markers as chromosome-specific cytological markers for melon chromosomes. Moreover, with the help of melon centromeric satellite DNA repeats CentM, 45S rDNA and 5S rDNA, sequential FISH with 3 sets of multi-fosmid cocktails were conducted on the same metaphase cell, which allowed us to simultaneously identify each of the 12 metaphase chromosomes of melon and a standardized melon karyotype of somatic metaphase chromosomes was constructed. Finally, we compared the distribution of 21 FISH-mapped fosmids between melon and cucumber chromosomes, which allows a better understanding of the evolutionary process shaping these 2 species. Our study provides a basis for cytological characterization of the melon genome and comparative genomics of Cucurbitaceae.

Divergence between<b><i> C. melo</i></b> and African <b><i>Cucumis</i></b> Species Identified by Chromosome Painting and rDNA Distribution Pattern

“…Consist with the previous results [Liu et al, 2010], the C. melo genome contained one 5S rDNA site (green) on chromosome 12 and two 45S rDNA sites (red) on chromosomes 4 and 10 ( Fig. 1 a2, a4).…”

“…Previous studies showed that C. melo had one 5S rDNA locus on chromosome 12 and two 45S rDNA loci on chromosomes 4 and 10, and C. hystrix had three 45S loci on chromosomes 8, 10, and 12, respectively [Liu et al, 2010;Yang et al, 2014]. To identify chromosomes with rDNA loci in C. melo and C. hystrix , 2 oligo libraries were synthesized suitable for chromosome painting.…”

“…Chromosomes 8, 10, and 12 in C. melo and C. hystrix can be distinguished easily with the 2 libraries. We did not select oligos from chromosome 4 carrying the 45S rDNA locus in C. melo because this chromosome can easily be identified as it is the longest chromosome, with 45S rDNA signals close to the centromere, and there is an obvious satellite on the short arm [Liu et al, 2010]. First, we performed FISH with the 2 oligo painting probes on metaphase chromosomes of C. melo .…”

“…To date, the distribution of 5S and 45S rDNA loci has only been reported in C. sativus and C. melo of the genus Cucumis [Han et al, 2008;Liu et al, 2010]. The distribution of 45S rDNA loci has also been analyzed in C. hystrix, C. anguria , and C. metuliferus [Yang et al, 2014;Zhang et al, 2015].…”

Comparative chromosomal localization of 45S and 5S rDNAs and implications for genome evolution inCucumis

“…In Cucumis, studies involving molecular cytogenetics were mainly focused on important cultivated crops (e.g., cucumbers, melons) (Chen et al 1998;Han et al 2008;Liu et al 2010). Few relevant studies have been reported concerning the genome organization and chromosomal structure of wild species, except for some concerning chromosome morphology and karyotype (Chen et al 1999;Zhao et al 2011;Yagi et al 2014Yagi et al , 2015.…”