The genus Lycoris (Amaryllidaceae) comprises ≈30 taxa (Kurita and Hsu, 1998), most of which have been grown for centuries as ornamentals and medicinal plants in China, Japan, and Korea. Lycoris taxa vary in flower shape and color as well as chromosome number and karyotypes, posing a considerable challenge in studying their phylogenetic relationships (Kurita and Hsu, 1998). In natural habitats, interspecific hybridization is frequent and has been proven to be an important mode of speciation in the genus Lycoris (Kurita, 1988; Kurita and Hsu, 1996). The interspecific hybrids can be propagated asexually through bulblets, and some of them have become widely dispersed. Several chromosome variations have been found in 38 populations of Lycoris sanguinea (Kurita, 1989). The heterozygous nature of Lycoris taxa was also shown in five diploid progenitor species in which genetic segregation of several allozyme loci was observed (Ma et al., 2004). On the other hand, a Lycoris taxon may show a genetic constancy among widely distributed populations (Kurita and Hsu, 1998). A completely sterile triploid Lycoris radiata var. radiata (2n = 33A), which is widely dispersed in Japan and Korea, shares the same karyotype (Kurita, 1987b), allozyme loci (Chung, 1999), and sequences of two genes: the lectin gene in the nuclear genome and the maturase gene in the chloroplast genome (Hayashi et al., 2005). These populations could have been derived from only one or a few bulblets originally introduced from China ≈3000 years ago.
Nishiyama (1928) was the first to count chromosome numbers of Lycoris species and gave n = 11. A large number of cytogenetic studies of Lycoris have been published since then (review by Jones, 1998). Lycoris taxa are classified into three groups based on chromosome complements: acrocentrics (A-type), metacentrics (M-type), and telocentrics (T-type) (Kurita, 1986). Plants with an A or MT karyotype are fertile diploids, whereas those with an MT-A karyotype are sterile (Kurita and Hsu, 1998). Recently, numerous investigations of Lycoris taxa were focused on cytological analyses, taxon identifications, and molecular phylogenetic relationships (Hori et al., 2006; Hsu et al., 1994; Kim and Lee, 1991; Kurita, 1987a, 1987b, 1988; Lee and Kim, 1987; Liu and Hsu, 1989; Shi et al., 2006; Yuan et al., 1998). However, it remains difficult to differentiate species with the same karyotype or to discriminate T-type from A-type chromosomes in some MT-A dikaryotype hybrids (Kurita, 1986). Furthermore, little knowledge on the ancestral karyotype as well as the mechanism of chromosome evolution in Lycoris species is available (Inariyama, 1951; Kurita, 1988). To address these questions, efficient and reliable landmarks for chromosome or even for chromosomal arm identification are urgently needed.
Ribosomal RNA genes (rDNAs) have been proven to be reliable landmarks in karyotype studies in Aegilops (Badaeva et al., 1996; Castilho and Heslop-Harrison, 1995), Arabidopsis (Fransz et al., 1998), Hordeum (Taketa et al., 1999), and Trifolium (Ansari et al., 1999). Karyotyping with rDNA loci can reflect the phylogenetic relationship among closely related species. In higher eukaryotes, rDNAs are organized into two distinct multigene families, one coding for 45S rRNA and the other coding for 5S rRNA (Suzuki et al., 1996). In plants, 45S rDNAs are highly repeated and arranged in tandem at one or a few chromosomal loci. A chromosome locus where 45S rDNAs are active in transcription is often associated with a nucleolus. This locus is called a nucleolar organizing region (McClintock, 1934). A 5S rDNA locus also contains a tandem array of hundreds or even thousands of repeats, which is usually located separately from 45S rDNA locus (review by Drouin and Moniz de Sa, 1995). The number and chromosomal position of 5S rDNA loci as well as those of 45S rDNA loci may vary among closely related plant species (references in Chung et al., 2008) [e.g., Phaseolus vulgaris (Pedrosa-Harand et al., 2006) and certain Oryza species (Chung et al., 2008)].
The fluorescent in situ hybridization (FISH) technique allows physical mapping of rDNA locus regardless of chromosomal feature and transcriptional activities of rDNA repeats (Cabrero and Camacho, 2008; Chung et al., 1993, 2008; Linde-Laursen et al., 1992). The locations of 45S rDNA loci have been determined by FISH for Lycoris chinensis var. sinuolata (2n = 16, 6M + 2SM + 8T) and some Korean endemic Lycoris species (Lee and Kim, 2000, 2004). The results of 5S and 45S rDNA FISH analyses suggested that L. radiata var. radiata (2n = 3x = 33) was derived from a diploid botanical variety, L. radiata var. pumila [2n = 22A (Hayashi et al., 2005)]. Also, rDNA loci were used as reliable landmarks for checking the formation of unreduced gametes of three interspecific hybrids and a selfed progeny of Lycoris sprengeri (Ogawa et al., 2006).
In this study, we applied rDNA FISH to investigate the variations of 5S and 45S rDNA loci on chromosomes of 12 Lycoris taxa, including nine species and three artificial hybrids. Distributions of rDNA loci on chromosomes of most taxa are reported for the first time except for L. radiata var. pumila and L. sprengeri (Ogawa et al., 2006). We showed that the number and locations of 5S and 45S rDNA loci varied among taxa of three karyotype groups and even among taxa within a karyotype group. This study is also the first to observe 5S rDNA loci were colocalized with 45S rDNA loci at the end of p-arm of T-type chromosomes in Lycoris taxa.
Abd El-TwabM.H.KondoK.2006FISH physical mapping of 5S, 45S and Arabidopsis-type telomere sequence repeats in Chrysanthemun zawadskii showing intra-chromosomal variation and complexity in natureChromosome Bot.115
Abd El-TwabM.H.KondoK.2007FISH physical mapping of 5S rDNA and telomere sequence repeats identified a peculiar chromosome mapping and mutation in Leucanthemella linearis and Nipponanthemum nipponicum in Chrysanthemum sensu latoChromosome Bot.21117
AnsariH.A.EllisonN.W.ReaderS.M.BadaevaE.D.FriebeB.MillerT.E.WilliamsW.M.1999Molecular cytogenetic organization of 5S and 18S–26S rDNA loci in white clover (Trifolium repens L.) and related speciesAnn. Bot. (Lond.)83199206
BadaevaE.D.FriebeB.GillB.S.1996Genome differentiation in Aegilops. 1. Distribution of highly repetitive DNA sequences on chromosomes of diploid speciesGenome39293306
CabreroJ.CamachoJ.P.M.2008Location and expression of ribosomal RNA genes in grasshoppers: Abundance of silent and cryptic lociChromosome Res.16595607
CastilhoA.Heslop-HarrisonJ.S.1995Physical mapping of 5S and 18S-25S rDNA and repetitive DNA sequences in Aegilops umbellulataGenome389196
ClarksonJ.J.LimK.Y.KovarikA.ChaseM.W.KnappS.LeitchA.R.2005Long-term genome diploidization in allopolyploid Nicotiana section Repandae (Solanaceae)New Phytol.168241252
DobignyG.Ozouf-CostazC.BonilloC.VolobouevV.2003Evolution of rRNA gene clusters and telomeric repeats during explosive genome repatterning in Taterillus × (Rodentia, Gerbillinae)Cytogenet. Genome Res.10394103
DrouinG.Moniz de SaM.1995The concerted evolution of 5S ribosomal genes linked to the repeat units of other multigene familiesMol. Biol. Evol.12481493
FranszP.ArmstrongS.Alonso-BlancoC.FisherT.C.Torres-RuizR.JonesG.1998Cytogenetics for the model system Arabidopsis thalianaPlant J.13867876
GarciaS.GarnatjeT.HidalgoO.McArthurE.D.Siljak-YakovlevS.VallesJ.2007Extensive ribosomal DNA (18S-5.8S-26S and 5S) colocalization in the North American endemic sagebrushes (subgenus Tridentatae, Artemisia, Asteraceae) revealed by FISHPlant Syst. Evol.2677992
GerlachW.L.DyerT.A.1980Sequence organization of the repeated units in the nucleus of wheat which contain 5S rRNA genesNucleic Acids Res.848514865
HoriT.HayashiA.SasanumaT.KuritaS.2006Genetic variations in the chloroplast genome and phylogenetic clustering of Lycoris speciesGenes Genet. Syst.81243253
KaoF.I.ChengY.Y.ChowT.Y.ChenH.H.LiuS.M.ChengC.H.ChungM.C.2006An integrated map of Oryza sativa L. chromosome 5Theor. Appl. Genet.112891902
KuritaS.1986Variation and evolution in the karyotype of Lycoris, Amaryllidaceae. 1. General karyomorphological characteristics of the genusCytologia (Tokyo)51803815
KuritaS.1987aVariation and evolution on the karyotype of Lycoris, Amaryllidaceae. II. Karyotype analysis of ten taxa among which seven are native in ChinaCytologia (Tokyo)521940
KuritaS.1987bVariation and evolution in the karyotype of Lycoris, Amaryllidaceae. IV. Interspecific variation in the karyotype of L. rediata (L'Herit.) Herb. and the origin of this triploid speciesCytologia (Tokyo)52137149
KuritaS.1988Variation and evolution in the karyotype of Lycoris, Amaryllidaceae. VII. Modes of karyotype alteration within species and probable trend of karyotype evolution in the genusCytologia (Tokyo)53323335
KuritaS.1989Variation and evolution in the karyotype of Lycoris (Amaryllidaceae). V. Chromosomal variation in L. sanguinea MaximPlant Species Biol.44760
KuritaS.HsuP.S.1998Cytological patterns in the Sino-Japanese flora. Hybrid complexes in Lycoris, Amaryllidaceae171180BouffordD.E.OhbaH.Sino-Japanese flora its characteristics and diversification. Bul. No. 37University Museum, Univ. TokyoTokyo, Japan
LeeB.KimM.2000Chromosomal localization of rDNA genes in the Korean endemic Lycoris flavescens M. Kim et S. Lee and its related species (Amaryllidaceae)Korean J. Genet.22117
LeeB.KimM.2004Genome characterization of a Korean endemic species Lycoris chejuensis (Amaryllidaceae) by in situ hybridizationKorean J. Genet.268389
Linde-LaursenI.IbsenE.Von BothmerR.GieseH.1992Physical localization of active and inactive rRNA gene loci in Hordeum marinum ssp. gussoneanum (4x) by in situ hybridizationGenome3510321036
MaB.OgawaT.TarumotoI.2004Genetic segregation of allozymes in selfed progenies of diploid Lycoris species (Amaryllidaceae)Sci. Rpt. Graduate School Agr. Biol. Sci. Osaka Prefecture. Univ.561722
McClintockB.1934The relationship of a particular chromosomal element to the development of the nucleoli in Zea maysZeitschrift fur Zellforschung und microskopische Anatomie21294328
OgawaT.TarumotoI.MaB.UenoM.MorikawaT.2006A study on interspecific hybrids and selfed progeny of Lycoris by means of fluorescence in situ hybridizationBreed. Sci.56209212
Pedrosa-HarandA.de AlmeidaC.C.S.MosiolekM.BlairM.W.SchweizerD.GuerraM.2006Extensive ribosomal DNA amplification during Andean common bean (Phaseolus vulgaris L.) evolutionTheor. Appl. Genet.112924933
RaskinaO.BelyayevA.NevoE.2004Quantum speciation in Aegilops: Molecular cytogenetic evidence from rDNA cluster variability in natural populationsProc. Natl. Acad. Sci. USA1011481814823
ShiS.QsiuY.LiE.WuL.FuC.2006Phylogenetic relationships and possible hybrid origin of Lycoris species (Amaryllidaceae) revealed by ITS sequencesBiochem. Genet.44198208
SuzukiH.SakuraiS.MatsudaY.1996Rat rDNA spacer sequences and chromosomal segment of the genes to the extreme terminal region of chromosome 19Cytogenet. Cell Genet.7214
TaketaS.HarrisonG.E.Heslop-HarrisonJ.S.1999Comparative physical mapping of the 5S and 18S–25S rDNA in nine wild Hordeum species and cytotypesTheor. Appl. Genet.9819
ZhouS.B.YuB.Q.LuoQ.HuJ.R.BiD.2007Karyotypes of six populations of Lycoris radiata and discovery of the tetraploidActa Phytotaxon. Sinica45513522