Comparative physical mapping of the 5S and 18S-25S rDNA in nine wild Hordeum species and cytotypes

Absract  The physical locations of the 5S and 18S-25S rDNA sequences were examined in nine wild Hordeum species and cytotypes by double-target in situ hybridization using digoxigenin-labelled 5S rDNA and biotin-labelled 18S-25S rDNA as probes. H. vulgare ssp. spontaneum (2n=2x=14; I-genome) had a similar composition of 5S and 18S-25S rDNA to cultivated barley (H. vulgare ssp. vulgare, I-genome), with two major 18S-25S rDNA sites and minor sites on four of the other five chromosomes; three chromosomes had 5S rDNA sites. The closely related H. bulbosum (2x; also I-genome) showed only one pair of 5S rDNA sites and one pair of 18S-25S rDNA sites on different chromosomes. Four wild diploid species, H. marinum (X-genome), H. glaucum and H. murinum (Y-genomes) and H. chilense (H-genome), differed in the number (2–3 pairs), location, and relative order of 5S and the one or two major 18S-25S rDNA sites, but no minor 18S-25S rDNA sites were observed. H. murinum 4x had three chromosome pairs carrying 5S rDNA, while the diploid had only a single pair. Two other tetraploid species, H. brachyantherum 4x and H. brevisubulatum 4x (both considered to have H-type genomes), had minor 18S-25S rDNA sites, as well as the major sites. Unusual double 5S rDNA sites – two sites on one chromosome arm separated by a short distance – were found in the American H-genome species, H. chilense and H. brachyantherum 4x. The results indicate that the species H. brachyantherum 4x and H. brevisubulatum 4x have a complex evolutionary history, probably involving the multiplication of minor rDNA sites (as in H. vulgare sensu lato), or the incorporation of both I and H types of genome. The rDNA markers are useful for an investigation of chromosome evolution and phylogeny. Received: 9 February 1998 / Accepted: 14 July 1998     

Localization of 5S rRNA Loci in Three Coregonid Species (Salmonidae)

In the present study the chromosome distribution of the 5S rDNA loci and its relation to the major rDNA genes were investigated in three Coregonid species (Salmonidae): Coregonus lavaretus, Coregonus peled and Coregonus albula, a family which has experienced large karyotype rearrangements along its evolution starting from a tetraploid ancestor. 5S PRINS/CMA₃ sequential staining together with previous data enabled us to locate 5S rRNA genes and nucleolar organizer regions (NORs) in the three species analyzed. PRINS revealed the 5S rDNA cluster at the distal part of the long arm of a similar submetacentric chromosome pair in the three species. Our data indicate that 5S rDNA clusters have probably conserved chromosomal location in the genus Coregonus, whereas 45S rDNA (NOR) sites are clearly differentiated, from a single locus in C. peled, to multiple loci in C. lavaretus and highly polymorphic multichromosomal location in C. albula.     

Chromosomal localization of 18S and 5S rDNA using FISH in the genus <Emphasis Type="Italic">Tor</Emphasis> (Pisces,Cyprinidae)

Dual color fluorescence in situ hybridization (FISH) was performed to study the simultaneous chromosomal localization of 18S and 5S ribosomal genes in the genus Tor for the first time. The 18S and 5S rDNAs in four Tor species were amplified, sequenced and mapped on the metaphase chromosomes. The number and distribution of 18S and 5S rDNA clusters were examined on metaphase chromosome spreads using FISH. The specimens of T. chelynoides, T. putitora and T. progeneius showed six bright fluorescent signals of 18S rDNA and T. tor exhibited ten such signals. The 5S rDNA signals were present only on one pair of chromosomes in all the four Tor species. Ag-NORs were observed on two pairs of chromosomes in T. chelynoides, T. putitora, T. progeneius and four pairs in T. tor. Comparison of the observed 18S rDNA FISH signals and Ag-NORs strongly suggested a possible inactivation of NORs localized at the telomeres of a subtelocentric and telocentric chromosome pairs in all four species. The 5S rDNA contained an identical 120 bp long coding region and 81 bp long highly divergent non-transcribed spacers in all species examined. 18S and 5S rDNA sequencing and chromosomal localization can be a useful genetic marker in species identification as well as phylogenetic and evolutionary studies.     

Physical mapping of 5S and 45S rDNA loci in pufferfishes (Tetraodontiformes)

Chromosomal features, location and variation of the major and minor rDNA genes cluster were studied in three pufferfish species: Sphoeroides greeleyi and Sphoeroides testudineus (Tetraodontidae) and Cyclichthys spinosus (Diodontidae). The location of the major rDNA was revealed with an 18S probe in two loci for all species. The minor rDNA loci (5S rDNA) was found in one chromosome pair in tetraodontid fishes and four sites located on two distinct chromosomal pairs in C. spinosus. A syntenical organization was not observed among the ribosomal genes. Signal homogeneity for GC/AT-DNA specific fluorochromes was observed in diodontid fish except in the NORs regions, which were CMA₃-positive. Giemsa karyotypes of tetraodontid species presents 2n = 46, having the same diploid value of other Sphoeroides species that have been investigated. On the other hand, the karyotype of C. spinosus, described for the first time, shows 2n = 50 chromosomes (4m + 18sm + 12st + 16a). The foreknowledge of the karyotypic structure of this group and also the physical mapping of certain genes could be very helpful for further DNA sequence analysis.     

Chromosomal location and reorganization of the 45S and 5S rDNA in theBrachyscome lineariloba complex (Asteraceae)

The chromosomal locations of the 45S (18S-5.8S-26S) and 5S ribosomal DNA in theBrachyscome lineariloba complex and two related species have been determined by the use of multicolor fluorescencein situ hybridization (McFISH). TheBrachyscome lineariloba complex includes five cytodemes with 2n=4, 8, 10, 12 and 16. Each of the 5S and 45S rDNA loci occurs at two sites on chromosomes in cytodemes with 2n=4. While in cytodemes with 2n=8, 10, 12 and 16, the number of 5S rDNA sites increases from four to eight paralleled to the genomic addition of diploid (4 chromosomes) or haploid (2 chromosomes) dosage. Of the 5S rDNA sites, only one pair is major, except for the cytodeme with 2n=10. The remaining 5S rDNA sites are minor and seem to have reduced the unit number of the 5S rDNA during the successive genomic additions. The 45S rDNA site is detected only at two nucleolar organizing regions in all cytodemes regardless of successive genomic addition. The loss or diminution of 45S rDNA sequences seem to have proceeded more rapidly than 5S rDNA sequences in theB. lineariloba complex.     

Chromosomal location of 18S and 5S rDNA sites in Triportheus fish species (Characiformes, Characidae)

The location of 18S and 5S rDNA sites was determined in eight species and populations of the fish genus Triportheus by using fluorescent in situ hybridization (FISH). The males and females of all species had 2n = 52 chromosomes and a ZZ/ZW sex chromosome system. A single 18S rDNA site that was roughly equivalent to an Ag-NOR was detected on the short arms of a submetacentric pair in nearly all species, and up to two additional sites were also observed in some species. In addition, another 18S rDNA cluster was identified in a distal region on the long arms of the W chromosome; this finding corroborated previous evidence that this cluster would be a shared feature amongst Triportheus species. In T. angulatus, a heterozygotic paracentric inversion involving the short arms of one homolog of a metacentric pair was associated with NORs. The 5S rDNA sites were located on the short arms of a single submetacentric chromosomal pair, close to the centromeres, except in T. auritus, which had up to ten 5S rDNA sites. The 18S and 5S rDNA sites were co-localized and adjacent on the short arms of a chromosomal pair in two populations of T. nematurus. Although all Triportheus species have a similar karyotypic macrostructure, the results of this work show that in some species ribosomal genes may serve as species-specific markers when used in conjunction with other putatively synapomorphic features.     

Distribution of 45S and 5S rDNA sites in 23 species of Eleocharis (Cyperaceae)

Studies of rDNA location in holocentric chromosomes of the Cyperaceae are scarce, but a few reports have indicated the occurrence of multiple 45S rDNA sites at terminal positions, and in the decondensed state of these regions in prometaphase/metaphase. To extend our knowledge of the number 45S and 5S rDNA sites and distribution in holocentric chromosomes of the Cyperaceae, 23 Brazilian species of Eleocharis were studied. FISH showed 45S rDNA signals always located in terminal regions, which varied from two (E. bonariensis with 2n = 20) to ten (E. flavescens with 2n = 10 and E. laeviglumis with 2n = 60). 5S rDNA showed less variation, with 16 species exhibiting two sites and 7 species four sites, preferentially at terminal positions, except for four species (E. subarticulata, E. flavescens, E. sellowiana and E. geniculata) that showed interstitial sites. The results are discussed in order to understand the predominance of terminal rDNA sites, the mechanisms involved in the interstitial positioning of 5S rDNA sites in some species, and the events of amplification and dispersion of 45S rDNA terminal sites.     

Constitutive heterochromatin, 5S and 18S rDNA genes in Apareiodon sp. (Characiformes, Parodontidae) with a ZZ/ZW sex chromosome system

Karyotype, sex chromosome system and cytogenetics characteristics of an unidentified species of the genus Apareiodon originating from Piquiri River (Paraná State, Brazil) were investigated using differential staining techniques (C-banding and Ag-staining) and fluorescent in situ hybridization (FISH) with 5S and 18S rDNA probes. The diploid chromosome number was 2n = 54 with 25 pairs of meta- (m) to submetacentric (sm) and 2 pairs of subtelocentric (st) chromosomes. The major ribosomal rDNA sites as revealed by Ag-staining and FISH with 18S rDNA probe were found in distal region of longer arm of st chromosome pair 26, while minor 5S sites were observed in the interstitial sites on chromosome pairs 2 (smaller cluster) and 7 (larger one). The C-positive heterochromatin had pericentromeric and telomeric distribution. The heteromorphic sex chromosome system consisted of male ZZ (pair 21) and female middle-sized m/st Z/W chromosomes. The pericentric inversion of heterochromatinized short arm of ancestral Z followed by multiplication of heterochromatin segments is hypothesized for origin of W chromosome. The observed karyotype and chromosomal markers corresponded to those found in other species of the genus.     

Chromosome Localization of 5S and 45S Ribosomal DNA in the Genomes of Linum L. Species of the Section Linum (Syn. Protolinum and Adenolinum)

Fluorescence in situ hybridization (FISH) was for the first time used to study the chromosomal location of the 45S (18S–5.8S–26S) and 5S ribosomal genes in the genomes of five flax species of the section Linum (syn. Protolinum and Adenolinum). In L. usitatissimum L. (2n = 30), L. angustifolium Huds. (2n = 30), and L. bienne Mill. (2n = 30), a major hybridization site of 45S rDNA was observed in the pericentric region of a large metacentric chromosome. A polymorphic minor locus of 45S rDNA was found on one of the small chromosomes. Sites of 5S rDNA were colocalized with those of 45S rDNA, but direct correlation between signal intensities from the 45S and 5S rDNA sites was observed only in some cases. Other 5S rDNA sites mapped to two chromosomes in these flax species. In L. grandiflorum Desf. (2n = 16) and L. austriacum L. (2n = 18), large regions of 45S and 5S rDNA were similarly located on a pair of homologous satellite-bearing chromosomes. An additional large polymorphic site of 45S and 5S rDNA was found in the proximal region of one arm of a small chromosome in the L. usitatissimum, L. angustifolium, and L. bienne karyotypes. The other arm of this chromosome contained a large 5S rDNA cluster. A similar location of the ribosomal genes in the pericentric region of the pair of satellite-bearing metacentrics confirmed the close relationships of the species examined. The difference in chromosomal location of the ribosomal genes between flax species with 2n = 30 and those with 2n = 16 or 18 testified to their assignment to different sections. The use of ribosomal genes as chromosome markers was assumed to be of importance for comparative genomic studies in cultivated flax, a valuable crop species of Russia, and in its wild relatives.     

Distribution of 5S and 35S rRNA gene sites in 34 Chenopodium species (Amaranthaceae)

Studies on Chenopodium chromosomes are scarce and restricted mainly to chromosome number estimation. To extend our knowledge on karyotype structure of the genus, the organization of 5S and 35S rRNA genes in Chenopodium chromosomes was studied. The rDNA sites were predominantly located at chromosomal termini, except in a few species where 5S rDNA sites were interstitial. The majority of the diploid species possessed one pair each of 35S and 5S rDNA sites located on separate chromosomes. Slightly higher diversity in rDNA site number was observed in polyploid accessions. One or two pairs of 35S rDNA sites were observed in tetraploids and hexaploids. Tetraploid species had two, four or six sites and hexaploid species had six or eight sites of 5S rDNA, respectively. These data indicate that, in the evolution of some polyploid species, there has been a tendency to reduce the number of rDNA sites. Additionally, polymorphism in rDNA site number was observed. Possible mechanisms of rDNA locus evolution are discussed. © 2012 The Linnean Society of London, Botanical Journal of the Linnean Society, 2012, ??, ??–??.     

Comparative analysis of rDNA location in five Neotropical gomphocerine grasshopper species

We report here, for the first time, the chromosome complement, number and location of the nucleolar organizer regions (NORs) revealed by silver staining (AgNO₃) and fluorescent in situ hybridization (FISH) in five Neotropical gomphocerine species: Rhammatocerus brasiliensis, R. brunneri, R. palustris, R. pictus and Amblytropidia sp. The objective of this study was to summarize available data and propose a model of chromosome evolution in Neotropical gomphocerines. All five species studied showed chromosome numbers consisting of 2n = 23,X0 in males and 2n = 24,XX in females. Amblytropidia sp. was the only species showing a bivalent (M₈) with megameric behavior during meiosis. The rDNA sites were restricted to autosomal pairs, i.e. the pericentromeric region of the S₉ chromosome, the consensus NOR location in all five species. R. brasiliensis was the only species showing additional NORs on M₄ and M₆ pairs which, likewise the S₉ NOR, were active in all cells analyzed. Comparison of these results with those reported previously in Palearctic gomphocerine species suggests higher resemblance of Neotropical species with the Old World species also possessing 23/24 chromosomes. Evolutionary mechanisms responsible for the observed interspecific variation in NOR location in this group are discussed.     

Cytogenetic characterization and mapping of rDNAs,core histone genes and telomeric sequences in <Emphasis Type="Italic">Venerupis aurea</Emphasis> and <Emphasis Type="Italic">Tapes rhomboides</Emphasis> (Bivalvia: Veneridae)

We describe the chromosomal location of GC-rich regions, 28S and 5S rDNA, core histone genes, and telomeric sequences in the veneroid bivalve species Venerupis aurea and Tapes (Venerupis) rhomboides, using fluorochrome staining with propidium iodide, DAPI and chromomycin A3 (CMA) and fluorescent in situ hybridization (FISH). DAPI dull/CMA bright bands were coincident with the chromosomal location of 28S rDNA in both species. The major rDNA was interstitially clustered at a single locus on the short arms of the metacentric chromosome pair 5 in V. aurea, whereas in T. rhomboides it was subtelomerically clustered on the long arms of the subtelocentric chromosome pair 17. 5S rDNA also was a single subtelomeric cluster on the long arms of subtelocentric pair 17 in V. aurea and on the short arms of the metacentric pair 9 in T. rhomboides. Furthermore, V. aurea showed four telomeric histone gene clusters on three metacentric pairs, at both ends of chromosome 2 and on the long arms of chromosomes 3 and 8, whereas histone genes in T. rhomboides clustered interstitially on the long arms of the metacentric pair 5 and proximally on the long arms of the subtelocentric pair 12. Double and triple FISH experiments demonstrated that rDNA and H3 histone genes localized on different chromosome pairs in the two clam species. Telomeric signals were found at both ends of every single chromosome in both species. Chromosomal location of these three gene families in two species of Veneridae provides a clue to karyotype evolution in this commercially important bivalve family.     

Nuclear DNA content,base composition,heterochromatin and rDNA in Picea omorika and Picea abies

Two closely related spruces, Picea abies and Picea omorika, a Balkan paleoendemic species, often share habitats, yet never hybridize in nature. The present study adresses their characteristics such as nuclear DNA content, base composition, heterochromatin and rDNA pattern. The genome size of P. abies was 10% larger than that of P. omorika when assessed by flow cytometry, respectively 2C=37.2 pg and 33.8 pg; although when estimated as total chromosome length it was virtually the same. The heterochromatin Chromomycin-A (CMA)/ DAPI fluorochrome banding patterns of both P. abies and P. omorika are given here for the first time. Simultaneous FISH (fluorescent in situ hybridization) using 18S-26S and 5S rDNA probes revealed 16 18S rDNA sites in P. omorika, 12 18S rDNA sites in P. abies, and a single 5S rDNA locus in both species. The genomes have about 41% GC. The number and position of CMA/DAPI bands and rDNA loci provide good chromosome markers to clarify the karyotypes of the two species. Received: 18 October 2000 / 14 June 2001     

Phylogenetic and genomic relationships in Setaria italica and its close relatives based on the molecular diversity and chromosomal organization of 5S and 18S-5.8S-25S rDNA genes

We have analyzed the phylogenetic and genomic relationships in the genus Setaria Beauv. including diploid and tetraploid species, by means of the molecular diversity of the 5S rDNA spacer and chromosomal organization of the 5S and 18S-5.8S-25S rDNA genes. PCR amplification of the 5S rDNA sequences gave specific patterns. All the species studied here share a common band of about 340 bp. An additional band of an approximately 300-bp repeat unit was found for Setaria verticillata and the Chinese accessions of Setaria italica and Setaria viridis. An additional band of 450 bp was found in the sole species Setaria faberii. Fluorescent in situ hybridization was used for physical mapping of the 5S and 18S-5.8S-25S rDNA genes and showed that they are localized at two separate loci with no polymorphism of chromosome location among species. Two chromosome pairs carrying the 5S and 18S-5.8S-25S rDNA clusters can now be unambiguously identified using FISH. Phylogenetic trees based on the variation of the amplified 5S rDNA sequences showed a clear separation into four groups. The clustering was dependent on the genomic composition (genome A versus genome B) and confirmed the closest relationship of S. italica and S. viridis accessions from the same geographical region. Our results confirm previous hypotheses on the domestication centers of S. italica. They also show the wide difference between the A and B genomes, and even clarify the taxonomic position of S. verticillata. Received: 28 August 2000 / Accepted: 27 January 2001     

Genomic dispersion of 28S rDNA during karyotypic evolution in the ant genusMyrmecia (Formicidae)

The chromosomal localization of 28S rDNA was investigated in 16 speices of the Australian ant genus Myrmecia, with 2n numbers ranging from 4 to 76, using the fluorescence in situ hybridization method and karyographic analysis. A unique phenomenon was observed: the number of chromosomes carrying 28S rDNA increases from 2 in species with low chromosome numbers to 19 in species with high chromosome numbers. This is termed rDNA dispersion. Centric fission and a reciprocal translocation that occurs in C-bands were detected as the major mechanisms involved in rDNA dispersion. Received: 22 March 1996; in revised form: 3 June 1996 / Accepted: 4 June 1996     

Chromosomal localization of 45S and 5S rDNA in 14 species and the implications for genome evolution of genus Epimedium

Studying the genome structure of Epimedium has been hindered by the large genomes and uniform karyotypes. Consequently our understanding of the genome organization and evolutionary changes of Epimedium is extremely limited. In the present study, the 45S and 5S rDNA loci of 14 Epimedium species were physically mapped by double-probe FISH for the first time. Results showed the following: (1) Chromosomes I and II of all 14 species examined, except for E. shuichengense, hosted one pair of 45S rDNA sites, respectively. Most of the 45S rDNA sites gave clear signals and were positioned in the distal regions of the short arms. (2) All species studied of section Diphyllon were found to have one pair of 5S rDNA sites localized in the interstitial regions of the long arm of chromosome IV, and the two species of section Epimedium, E. alpinum and E. pubigerum, had two pairs of 5S rDNA sites localized in the interstitial regions of the long arm of chromosomes IV and V, respectively. (3) In section Diphyllon, all species of small flower taxa, except E. shuichengense, had three pairs of 45S rDNA sites, clearly more than species of big flower taxa, except E. davidii, with two pairs of 45S rDNA sites. Based on the 45S and 5S rDNA distribution patterns and other chromosomal morphological characteristics, six pairs of chromosomes can be unambiguously identified in all 14 Epimedium species. The stable differentiation in 45S and 5S rDNA FISH patterns between the two sections suggests that chromosomal rearrangements and transpositional events played a role in the splitting of the two sections, and section Diphyllon may be more primitive than section Epimedium. In the same way, big flower taxa may be more primitive than small flower taxa in section Diphyllon.     

Chromosomal Characteristics of rDNA in European Grayling <Emphasis Type="Italic">Thymallus thymallus</Emphasis> (Salmonidae)

The chromosomal characteristics, locations and variations of two classes of ribosomal DNA (5S and 18S) were studied in European grayling karyotype (Thymallus thymallus, Salmonidae). Major rDNA sites as revealed by sequential CMA₃/Ag staining and confirmed by in situ hybridization with a 18S rDNA probe were situated in two loci and were found to be polymorphic in size and displaying several distinct forms. The 5S rDNA was located by PRINS on three pairs of subtelocentric chromosomes, additional minor signal was present at the centromere of one metacentric element. 5S sites were not associated with NORs. The dosage compensation mechanism was proposed as an explanation of high frequency of lethal rDNA-deleted forms of the NOR-bearing chromosomes. Double variable pattern in the number and location of NORs supported the bi-directional evolution of salmonid rDNA loci.     

Distribution of 18S rDNA sites and absence of the canonical TTAGG insect telomeric repeat in parasitoid Hymenoptera

Karyotypes of six species belonging to three main clades of parasitoid Hymenoptera, the superfamilies Ichneumonoidea (Ichneumonidae: Ichneumon amphibolus), Cynipoidea (Cynipidae: Diplolepis rosae) and Chalcidoidea (Eurytomidae: Eurytoma robusta, Eu. serratulae and Eu. compressa, and Torymidae: Torymus bedeguaris) were studied using FISH with 18S rDNA and telomeric (TTAGG)_n probes. Haploid karyotypes of D. rosae, Eu. robusta and Eu. serratulae carried the only 18S rDNA hybridization signal, whereas those of I. amphibolus and Eu. compressa carried three and two rDNA clusters respectively. In addition, three rDNA sites were visualized in the aneuploid female of T. bedeguaris. The number of rDNA clusters in parasitoid Hymenoptera generally correlates to the chromosome number. Apart from the overwhelming majority of the studied species of aculeate Hymenoptera, no hybridization signals were obtained from FISH with the telomeric (TTAGG)_n probe in the examined parasitoid species. These data suggest absence of the canonical (TTAGG)_n insect telomeric motif in the Ichneumonoidea, Cynipoidea and Chalcidoidea, and perhaps in parasitoid Hymenoptera in general.     

Characterization of diploid,tetraploid and hexaploid Helianthus species by chromosome banding and FISH with 45S rDNA probe

Comparative karyotype analyses of five diploid, two tetraploid, and three hexaploid species of Helianthuswere performed using Feulgen staining, Giemsa C and CMA₃ (C-CMA) staining, and FISH with 45S rDNA probe. The karyotypes are composed by a basic number of x=17 with a predominance of meta- and submetacentric chromosome types. A polyploid series is associated with the basic number. Giemsa C- and C-CMA banding revealed terminal or interstitial heterochromatin according to the species, suggesting the existence of a mechanism that may be acting in the dispersion of heterochromatic segments in Helianthus. The nucleolar organizer regions were located at terminal chromosome positions by FISH with 45S rDNA probe. Diploid species presented four, six, and eight rDNA sites, tetraploid species showed eight sites and hexaploid species presented 12 rDNA sites. Karyomorphological differences include variation in number, size and chromosome morphology, suggesting that rearrangements involving small heterochromatic and rDNA segments played a major role in karyotype evolution.     

Variability of the 5S and 45S rDNA sites in Passiflora L. species with distinct base chromosome numbers

Cytologically, the species of Passiflora with known chromosome number can be divided into four groups: (1) 2n = 12, 24, 36; (2) 2n = 24; (3) 2n = 18, 72; and (4) 2n = 20. The base chromosome number proposed for the genus is x = 6, with x = 9, x = 10 and x = 12 being considered secondary base numbers. In the present study, variability of 5S and 45S rDNA sites was investigated in 20 species of these four groups to check the reliability of this hypothesis. In the group with x = 6, five diploid species (2n = 12) exhibit two 5S rDNA sites and two (P. capsularis, P. morifolia and P. rubra) or four (P. misera 2x and P. tricuspis) 45S rDNA sites. The hexaploid cytotype of P. misera had 12 45S rDNA sites and six 5S rDNA. A tetraploid species, P. suberosa, had ten 45S rDNA sites and four 5S rDNA sites, both in the same chromosomes as the 45S rDNA sites. In the group with x = 9, P. actinia, P. amethystina, P. edmundoi, P. elegans, P. galbana, P. glandulosa and P. mucronata displayed six 45S rDNA sites, whereas P. alata, P. cincinnata, P. edulis f. flavicarpa, P. edulis var. roxo and P. laurifolia had four sites. In this group, all species were diploid (2n = 18) and had only two 5S rDNA sites. Passiflora foetida, the only species with 2n = 20, had six 45S rDNA sites and four 5S rDNA sites. The species with x = 12 (2n = 24), P. haematostigma and P. pentagona, showed four 45S rDNA sites and two 5S rDNA. In general, the number and location of 5S and 45S rDNA sites were consistent with the hypothesis of x = 6 as the probable ancestral genome for the genus, while the groups of species with x = 9, x = 10 and x = 12 were considered to be of tetraploid origin with descending dysploidy and gene silencing of some redundant gene sites, mainly those of 5S rDNA.