Phylogeny of Cucumis based on isozyme variability and its comparison with plastome phylogeny

Molecular Cytogenetic Analysis of Cucumis Wild Species Distributed in Southern Africa: Physical Mapping of 5S and 45S rDNA with DAPI

Wild Cucumis species have been divided into Australian/Asian and African groups using morphological and phylogenetic characteristics, and new species have been described recently. No molecular cytogenetic information is available for most of these species. The crossability between 5 southern African Cucumis species (C. africanus, C. anguria, C. myriocarpus, C. zeyheri, and C. heptadactylus) has been reported; however, the evolutionary relationship among them is still unclear. Here, a molecular cytogenetic analysis using FISH with 5S and 45 S ribosomal DNA (rDNA) was used to investigate these Cucumis species based on sets of rDNA-bearing chromosomes (rch) types I, II and III. The molecular cytogenetic and phylogenetic results suggested that at least 2 steps of chromosomal rearrangements may have occurred during the evolution of tetraploid C. heptadactylus. In step 1, an additional 45 S rDNA site was observed in the chromosome (type III). In particular, C. myriocarpus had a variety of rch sets. Our results suggest that chromosomal rearrangements may have occurred in the 45 S rDNA sites. We propose that polyploid evolution occurred in step 2. This study provides insights into the chromosomal characteristics of African Cucumis species and contributes to the understanding of chromosomal evolution in this genus.

Diversification and genetic differentiation of cultivated melon inferred from sequence polymorphism in the chloroplast genome

Molecular analysis encouraged discovery of genetic diversity and relationships of cultivated melon (Cucumis melo L.). We sequenced nine inter- and intra-genic regions of the chloroplast genome, about 5500 bp, using 60 melon accessions and six reference accessions of wild species of Cucumis to show intra-specific variation of the chloroplast genome. Sequence polymorphisms were detected among melon accessions and other Cucumis species, indicating intra-specific diversification of the chloroplast genome. Melon accessions were classified into three subclusters by cytoplasm type and then into 12 subgroups. Geographical origin and seed size also differed between the three subclusters. Subcluster Ia contained small-seed melon from Southern Africa and South and East Asia and subcluster Ib mainly consisted of large-seed melon from northern Africa, Europe and USA. Melon accessions of subcluster Ic were only found in West, Central and Southern Africa. Our results indicated that European melon groups and Asian melon groups diversified independently and shared the same maternal lineage with northern African large-seed melon and Southern African small-seed melon, respectively. Cultivated melon of subcluster Ic may have been domesticated independently in Africa. The presence of 11 cytoplasm types in Africa strongly supported African origin of cultivated melon and indicated the importance of germplasm from Africa.

Phylogenetics of Cucumis (Cucurbitaceae): cucumber (C. sativus) belongs in an Asian/Australian clade far from melon (C. melo)

Background

Melon, Cucumis melo, and cucumber, C. sativus, are among the most widely cultivated crops worldwide. Cucumis, as traditionally conceived, is geographically centered in Africa, with C. sativus and C. hystrix thought to be the only Cucumis species in Asia. This taxonomy forms the basis for all ongoing Cucumis breeding and genomics efforts. We tested relationships among Cucumis and related genera based on DNA sequences from chloroplast gene, intron, and spacer regions (rbcL, matK, rpl20-rps12, trnL, and trnL-F), adding nuclear internal transcribed spacer sequences to resolve relationships within Cucumis.

Results

Analyses of combined chloroplast sequences (4,375 aligned nucleotides) for 123 of the 130 genera of Cucurbitaceae indicate that the genera Cucumella, Dicaelospermum, Mukia, Myrmecosicyos, and Oreosyce are embedded within Cucumis. Phylogenetic trees from nuclear sequences for these taxa are congruent, and the combined data yield a well-supported phylogeny. The nesting of the five genera in Cucumis greatly changes the natural geographic range of the genus, extending it throughout the Malesian region and into Australia. The closest relative of Cucumis is Muellerargia, with one species in Australia and Indonesia, the other in Madagascar. Cucumber and its sister species, C. hystrix, are nested among Australian, Malaysian, and Western Indian species placed in Mukia or Dicaelospermum and in one case not yet formally described. Cucumis melo is sister to this Australian/Asian clade, rather than being close to African species as previously thought. Molecular clocks indicate that the deepest divergences in Cucumis, including the split between C. melo and its Australian/Asian sister clade, go back to the mid-Eocene.

Conclusion

Based on congruent nuclear and chloroplast phylogenies we conclude that Cucumis comprises an old Australian/Asian component that was heretofore unsuspected. Cucumis sativus evolved within this Australian/Asian clade and is phylogenetically far more distant from C. melo than implied by the current morphological classification.

Molecular phylogeny of Cucumis species as revealed by consensus chloroplast SSR marker length and sequence variation

To investigate phylogenetic relationships in the genus Cucumis, 9 consensus chloroplast simple sequence repeat (ccSSR) primer pairs (ccSSR3, 9, 11, 13, 14, 17, 20, 21, and 23) were employed for DNA fragment length variation and 5 amplified fragments, ccSSR4, 12, 13, 19, and 20, were sequenced using total DNA from 13 accessions representing 7 African Cucumis species (x = 12), 3 Cucumis melo L. (x = 12) accessions, 2 Cucumis sativus L. (x = 7) accessions, and 1 Cucumis hystrix Chakr. (x = 12) accession. A Citrullus lanatus (Thunb.) Matsum. & Nakai (x = 11) accession was used as an outgroup. While fragment length analysis revealed the existence of 3 major species clusters (i.e., a group of African Cucumis species, a group composed of C. melo accessions, and a group containing C. sativus and C. hystrix species), sequence variation analysis identified 2 major species clusters (i.e., a group of African Cucumis species and a group composed of C. melo, C. sativus, and C. hystrix species). Comparative analysis using nuclear DNA (previous studies) and cpDNA sequence substitution data resulted in the placement of C. melo and C. sativus in different cluster groupings. Thus, both nuclear and cytoplasmic DNA should be employed and compared when a putative progenitor or specimens of an ancestral Cucumis species lineage is investigated. In addition, C. ficifolius (2x) and C. aculeatus (4x) of the African Cucumis species clustered together in this study. This result does not agree with reported isozyme analyses, but does agree with previously characterized chromosome homologies between these 2 species. Although African Cucumis species and C. hystrix do not share a close relationship, genetic affinities between C. sativus and C. hystrix are considerable. Combined evidence from previously published studies and data presented herein lend support to the hypothesis that C. hystrix is either a progenitor species of C. sativus or that they at least share a common ancestral lineage.

Comparative mapping of ZYMV resistances in cucumber (Cucumis sativus L.) and melon (Cucumis melo L.)

Zucchini yellow mosaic virus (ZYMV) routinely causes significant losses in cucumber ( Cucumis sativus L.) and melon ( Cucumis melo L.). ZYMV resistances from the cucumber population 'TMG1' and the melon plant introduction (PI) 414723 show different modes of inheritance and their genetic relationships are unknown. We used molecular markers tightly linked to ZYMV resistances from cucumber and melon for comparative mapping. A 5-kb genomic region (YCZ-5) cosegregating with the zym locus of cucumber was cloned and sequenced to reveal single nucleotide polymorphisms and indels distinguishing alleles from ZYMV-resistant (TMG1) and susceptible (Straight 8) cucumbers. A low-copy region of the YCZ-5 clone was hybridized to bacterial artificial chromosome (BAC) clones of melon and a 180-kb contig assembled. One end of this melon contig was mapped in cucumber and cosegregated with ZYMV resistance, demonstrating that physically linked regions in melon show genetic linkage in cucumber. However the YCZ-5 region segregated independently of ZYMV resistance loci in two melon families. These results establish that these sources of ZYMV resistances from cucumber TMG1 and melon PI414723 are likely non-syntenic.

Genetic analysis of Spanish melon ( Cucumis melo L.) germplasm using a standardized molecular-marker array and geographically diverse reference accessions

Genetic relationships among 125 Spanish melon ( Cucumis melo L.) accessions from a Spanish germplasm collection were assessed using a standard molecular-marker array consisting of 34 random amplified polymorphic DNA (RAPD) markers bands (19 primers) and 72 reference accessions drawn from previous studies. The reference accession array consisted of a broad range [Japanese (19) Crete (17), African (15), and USA and Europe (US/EU, 21)] of horticultural groupings (Group Cantalupensis, Group Conomon, Group Inodorus, Group Flexuosus, and Group Chito), and of melon market classes (e.g., Charentais, U.S. Western and European Shipper types, Ogen, and Galia, Honeydew, and Casaba). Spanish melon accessions (largely Casaba, Group Inodorus) were genetically distinct from the reference accessions and other Group Inodorus melons of different origins. Most African accessions showed common genetic affinities, and grouped with the Group Chito and the Group Conomon accessions examined. Those accession groupings were distinct from all other accessions belonging to Group Cantalupensis, Flexuosus, and Inodorus accessions originating from Crete, Japan, Europe, and the U.S. Genetic diversity was highest in accessions of African origin and lowest in accessions of Spanish origin. Additional RAPD markers (49 primers, 141 bands) and 22 selected agronomic traits (quantitative and qualitative) were then used to assess the genetic diversity among Spanish accessions. While cluster analysis using fruit characteristics grouped accessions into cultivars, RAPD-based genetic-distance estimate did not provide consistent accession groupings either by cultivar or geographic origin. While the highest level of polymorphism was detected among melons originating from the central region of Spain, and in the Rochet cultivar, accessions from the Andalucía region and Green cultivars were comparatively less diverse. These results indicate that the Spanish melon accessions could be used to broaden the genetic base of local and foreign Casaba germplasm, to enhance the genetic diversity of U.S and European commercial melon germplasm, and to delineate collection strategies for acquisition of additional Spanish landraces.

Simple sequence repeats in Cucumis mapping and map merging

Thirty-four polymorphic simple-sequence repeats (SSRs) were evaluated for length polymorphism in melon (Cucumis melo L.) and cucumber (Cucumis sativus L.). SSR markers were located on three melon maps (18 on the map of 'Vedrantais' and PI 161375, 23 on the map of 'Piel de Sapo' and PI 161375, and 16 on the map of PI 414723 and 'Dulce'). In addition, 14 of the markers were located on the cucumber map of GY14 and PI 183967. SSRs proved to be randomly distributed throughout the melon and cucumber genomes. Mapping of the SSRs in the different maps led to the cross-identification of seven linkage groups in all melon maps. In addition, nine SSRs were common to both melon and cucumber maps. The potential of SSR markers as anchor points for melon-map merging and for comparative mapping with cucumber was demonstrated.Key words: microsatellites, Cucumis melo, melon, Cucumis sativus, cucumber, comparative mapping.

Molecular evolution of the internal transcribed spacers (ITS1 and ITS2) and phylogenetic relationships among species of the family Cucurbitaceae

Phylogenetic relationships of different members of the family Cucurbitaceae were estimated from sequences of the internal transcribed spacer (ITS1 and ITS2) regions of the nuclear ribosomal RNA genes. Twenty-six species of different genera belonging to different tribes and several subtribes were analyzed. The whole ITS regions were amplified by PCR technique and cloned, and three to five different clones of each species were sequenced; for some species PCR products were sequenced directly. ITS1 and ITS2 regions are slightly variable in length, with each length appearing genus-specific. A substitution rate of 3.62 x 10(-9) substitutions per site per year was calculated assuming 40 MYA separation time. Phylogenetic relationships inferred from ITS sequences of some species is in agreement with morphological data, but deviations to the taxonomic classification were also observed. A polyphyletic origin of the New World species must be considered. In the genus Cucurbita different "types" of ITS sequences within one species exist, possibly due to the high frequency of introgression during domestication or due to polyploidization events; in contrast, low intraspecific variability was detectable in the genus Cucumis, indicating different stages of speciation.

Isoelectric focusing in immobilized pH gradient of melon (Cucumis melo L.) seed protein: methodical and genetic aspects, and application in breeding

Genetic variability of melon seed proteins was studied by separation of seed protein by isoelectric focusing in immobilized pH gradient (IEF-IPG) under denaturing conditions. A routine procedure was developed for IEF-IPG of hundreds of individual melon seeds per day. A group of 74 accessions from 19 morphologically distinct groups and from different geographic origin were studied by IEF-IPG using pH gradients of 4-10, 4-7 and 6-10. The electrophoretic analysis of the 74 accessions showed 270 reproducible seed protein bands of which 70 were variable. Genetic evaluation led to the conclusion that at least 20 loci govern the variation found. The phylogenetic trees constructed using the protein data on one hand and the morphological data on the other hand were compared and their use was evaluated. A number of commercial Cantaloup F1 hybrid descendants derived from the F1 hybrids by diplohaploidization or single plot descent were studied by IEF-IPG using pH gradients of 4-7 and 6-10. Among the F1 hybrids and their descendants 265 reproducible protein bands could be identified of which 72 were variable as to presence versus absence. The genetic interpretation of the protein pattern as found by IEF-IPG and the use of IEF-IPG in plant breeding was discussed. It was concluded that IEP-IPG of melon seed proteins is a valuable tool in breeding.

Evaluation of restriction fragment length polymorphism in Cucumis melo

The objectives of this study were to assess the degree of restriction fragment length polymorphism (RFLP) in Cucumis melo and to determine interrelationships among cultivated varieties. Initial screening of a genomic PstI library revealed that approximately 40% of the clones were repetitive. A total of 162 unique and low-copy sequence clones were hybridized to seven diverse accesions of C. melo and a C. sativus cultivar 'Pacer' to evaluate RFLP variation. Of these, 130 probes (80%) detected a polymorphism between C. melo accessions and C. sativus, and the majority were polymorphic with more than one enzyme digest. In contrast, only 53 probes (33%) were useful in differentiating at least one of the seven accessions. Of those, only 9% were informative with more than one enzyme digest. This indicates that within C. melo, the differences among accessions are due to infrequent base substitutions, whereas between the two species, differences are mainly due to genome rearrangements such as insertions and deletions or numerous base substitutions. Of the informative probes, 34 were used in analyzing 44 C. melo lines to establish a data base of RFLP hybridization patterns. Percent similarity based on RFLP profiles was computed among lines and analyzed by principal component analysis, to visualize relationships among lines. There were clear demarcations among, but not within, muskmelon and honeydew groups.

Genetic diversity in Cucumis sativus L. assessed by variation at 18 allozyme coding loci

The genetic diversity of the U.S. Cucumis sativus L. germplasm collection [757 plant introductions (PI) representing 45 countries] was assessed using 40 enzymes which represented 74 biochemical loci. Polymorphisms were observed at 18 loci (G2dh-1, Gpi-1, Gpi-2, Gr-1, Gr-2, Idh, Mdh-1, Mdh-2, Mdh-3, Mpi-2, Pepla-2, Peppap-2, Per-4, Pgd-1, Pgd-2, Pgm-1, Pgm-3, and Skdh). Two PIs (285606 and 215589) contained alleles [G2dh-1(1) and Per-4(2), respectively] which did not occur in any other PI. Other alleles which occurred in low frequencies (in < 1% of the PIs) included Gpi-1(3), Gpi-2(3), Gr-1(3), Gr-2(1), Idh(1), Mdh-1(2), Mdh-2(1), Peppap-2(1), and Pgd-1(1). Individual loci containing more than one allele in greater than 20% of the PIs included Mpi-2, Pepla-2, Pgd-2, and Pgm-1. Multivariate analyses aided in the reduction of data (principle components), depicted relationships among PIs (cluster), and identified the most discriminating enzyme loci (Pgm-1, Pepla-2, Gr-1, Pgd-2, Mpi-2, and Skdh) (classification and regression tree).