Extensive Amplification and Transposition of a Novel Repetitive Element, Xstir, Together with Its Terminal Inverted Repeat in the Evolution of Xenopus

A DNA fragment containing short tandem repeat sequences (approximately 86-bp repeat) was isolated from a Xenopus laevis cDNA library. Southern blot and in situ hybridization analyses revealed that the repeat was highly dispersed in the genome and was present at approximately 1 million copies per haploid genome. We named this element Xstir (Xenopus short tandemly and invertedly repeating element) after its arrangement in the genome. The majority of the genomic Xstir sequences were digested to monomer and dimer sizes with several restriction enzymes. Their sequences were found to be highly homogeneous and organized into tandem arrays in the genome. Alignment analyses of several known sequences showed that some of the Xstir-like sequences were also organized into interspersed inverted repeats. The inverted repeats consisted of an inverted pair of two differently modified Xstirs separated by a short insert. In addition, these were framed by another novel inverted repeat (Xstir-TIR). The Xstir-TIR sequence was also found at the ends of tandem Xstir arrays. Furthermore, we found that Xstir-TIR was linked to a motif characterizing the T2 family which belonged to a vertebrate MITE (miniature inverted-repeat transposable element) family, suggesting the importance of Xstir-TIR for their amplification and transposition. The present study of 11 anuran and 2 urodele species revealed that Xstir or Xstir-like sequences were extensively amplified in the three Xenopus species. Genomic Xstir populations of X. borealis and X. laevis were mutually indistinguishable but significantly different from that of X. tropicalis. Received: 5 April 2000 / Accepted: 3 August 2000     

Globin evolution in the genusXenopus: Comparative analysis of cDNAs coding for adult globin polypeptides ofXenopus borealis andXenopus tropicalis

Summary Globin mRNAs ofXenopus borealis andXenopus tropicalis have been cloned and sequenced. The nucleotide and derived amino acid sequences were compared with each other and with already available data fromXenopus laevis. This analysis rendered clear evidence that the common ancestor ofX. laevis andX. borealis, but not ofX. tropicalis, had lost one amino acid of the -globins prior to a genome duplication event that preceded the segregation of the former two species. Replacement-site substitutions were used to calculate a rough time scale of genome duplication and species segregation. The results suggest an ancient separation between theX. laevis and theX. tropicalis groups occurring approximately 110–120 million years ago. Analysis of the amino acid chains demonstrated various alterations. However, some functional domains, like heme-binding sites and₁₂ contact sites, were subject to a high degree of conservation, indicating the existence of functional constraints on them also in the genusXenopus.     

Elucidating the functional evolution of heat sensors among Xenopus species adapted to different thermal niches by ancestral sequence reconstruction

Ambient temperature fluctuations are detected via the thermosensory system which allows animals to seek preferable thermal conditions or escape from harmful temperatures. Evolutionary changes in thermal perception have thus potentially played crucial roles in niche selection. The genus Xenopus (clawed frog) is suitable for investigating the relationship between thermal perception and niche selection due to their diverse latitudinal and altitudinal distributions. Here we performed comparative analyses of the neuronal heat sensors TRPV1 and TRPA1 among closely related Xenopus species (X. borealis, X. muelleri, X. laevis, and X. tropicalis) to elucidate their functional evolution and to assess whether their functional differences correlate with thermal niche selection among the species. Comparison of TRPV1 among four extant Xenopus species and reconstruction of the ancestral TRPV1 revealed that TRPV1 responses to repeated heat stimulation were specifically altered in the lineage leading to X. tropicalis which inhabits warmer niches. Moreover, the thermal sensitivity of TRPA1 was lower in X. tropicalis than the other species, although the thermal sensitivity of TRPV1 and TRPA1 was not always lower in species that inhabit warmer niches than the species inhabit cooler niches. However, a clear correlation was found in species differences in TRPA1 activity. Heat‐evoked activity of TRPA1 in X. borealis and X. laevis, which are adapted to cooler niches, was significantly higher than in X. tropicalis and X. muelleri which are adapted to warmer niches. These findings suggest that the functional properties of heat sensors changed during Xenopus evolution, potentially altering the preferred temperature ranges among species.     

Evolutionary relationships,host range and geographical distribution of Camallanus Railliet & Henry, 1915 species (Nematoda: Camallaninae) from clawed toads of the genus Xenopus (Anura: Pipidae)

Representatives of the genus Camallanus Railliet & Henry, 1915 occur mainly in teleost fishes, although a significant number of species have also been recorded from anuran amphibians. The taxonomy, host range, geographical distribution and phylogenetic relationships of Camallanus spp. from African clawed toads (Xenopus spp.) are reviewed. Besides C. kaapstaadi Southwell & Kirshner, 1937, which shows a widespread distribution in sub-Saharan Africa and occurs in X. laevis subspecies, X. wittei, X. fraseri-like toads, X. borealis and X. muelleri, three new species were found: C. siluranae n. sp. from X. tropicalis in west Africa, C. macrocephalus n. sp. from X. borealis in Kenya, and C. xenopodis n. sp. from X. laevis laevis in South Africa and X. borealis in Kenya. C. johni Yeh, 1960 described from Xenopus sp. in Tanzania is considered a species inquirenda. C. kaapstaadi and C. macrocephalus are very closely related and both occur in the oesophagus of their hosts, unlike other Camallanus spp. which are found in the intestine or more rarely the stomach. Some of the unusual morphological features of these species may be an adaptation to attachment in the oesophagus. The host of C. siluranae, X. tropicalis, belongs to a separate species group (as has been established by recent molecular and cytological studies) to those of C. kaapstaadi, C. macrocephalus and C. xenopodis. Morphological affinities suggest that Camallanus spp. from clawed toads are not monophyletic with those from other amphibians and that C. siluranae is distantly related to, and probably not monophyletic with the remaining species from clawed toads. The Camallanus fauna of Xenopus spp. may thus be derived from at least two independent colonisations, of different host clades, by parasite lineages occurring in teleost fishes.     

Origin and Evolution of Tandem Repeats in the Mitochondrial DNA Control Region of Shrikes (<Emphasis Type="BoldItalic">Lanius</Emphasis> spp.)

The origin and evolution of a 128-bp tandem repeat in the mtDNA control region of shrikes (Lanius: Aves) were investigated. The tandem repeat is present in only two species, L. excubitor and L. ludovicianus. In contrast to the variation in repeat number in L. ludovicianus, all individuals of three subspecies of L. excubitor had three repeats. Comparative analysis suggests that a short direct repeat, and a secondary structure including the tandem repeat and a downstream inverted repeat, may be important in the origin of the tandem repeat by slipped-strand mispairing and its subsequent turnover. Homogenization of repeat sequences is most simply explained by expansion and contraction of the repeat array. Surprisingly, mtDNA sequences from L. excubitor were found to be paraphyletic with respect to L. ludovicianus. These results show the utility of a comparative analysis for insights into the evolutionary dynamics of mtDNA tandem repeats.[Reviewing Editor: Martin Kreitman]     

Repeats and subrepeats in the intergenic spacer of rDNA from the nematode Meloidogyne arenaria

Summary Ribosomal DNA (rDNA) repeats of the plant-parasitic nematode Meloidogyne arenaria are heterogeneous in size and appear to contain 5S rRNA gene sequences. Moreover, in a recA ⁺ bacterial host, plasmid clones of a 9 kb rDNA repeat show deletion events within a 2 kb intergenic spacer (IGS), between 28S and 5S DNA sequences. These deletions appear to result from a reduction in the number of tandem 129 by repeats in the IGS. The loss of such repeats might explain how rDNA length heterogeneity, observed in the Meloidogyne genome, could have arisen. Each 129 by repeat also contains three copies of an 8 by subrepeat, which has sequence similarity to an element found in the IGS repeats of some plant rDNAs.     

EVOLUTIONARY INFERENCES FROM RESTRICTION MAPS OF MITOCHONDRIAL DNA FROM NINE TAXA OF XENOPUS FROGS

Restriction endonuclease cleavage maps were prepared by the double digestion method for mitochondrial DNAs (mtDNAs) purified from Xenopus borealis, X. clivii, X. fraseri, X. muelleri, X. ruwenzoriensis, X. vestitus, X. laevis victorianus, X. l. laevis, and a variant of X. laevis designated X. laevis “davis.” An average of 21 cleavage sites per genome were mapped with 11 restriction endonucleases. Among the four invariant sites found are three conserved not only among the Xenopus mtDNAs tested but also among nearly all vertebrate mtDNAs examined to date. Two of these are Sac II sites in the 12S and 16S ribosomal RNA genes, and one is a Hpa I site in the gene for asparagine transfer RNA. These three sites permit the alignment and comparison of mtDNAs from different vertebrate classes. Although most of the differences observed among the Xenopus maps are attributable to point mutations causing gain or loss of restriction sites, the maps also differ by three large length mutations in or near the displacement loop. Phylogenetic analysis of 30 informative sites suggests that those members of the laevis species-group that have 36 chromosomes per somatic cell can be divided into three subgroups: 1) X. borealis, X. clivii, and perhaps X. fraseri (the “borealis” subgroup), 2) X. muelleri, and 3) the subspecies of X, laevis. The mtDNA of the hexaploid (2n = 108) species, X. ruwenzoriensis, is most similar to that of taxa in the latter two subgroups, which contrasts with the morphological similarity of this species to X. fraseri. X. ruwenzoriensis may be an allopolyploid with a mother (the contributor of the cytoplasmic mtDNA genome) on the X. laevis or X. muelleri lineage and a father on the X. fraseri lineage. We present a model showing how mtDNA and nuclear genomes can yield contrasting phytogenies for species-groups that have undergone several rounds of interspecific hybridization. Comparison of mitochondrial and nuclear sequence divergences suggests that Xenopus mtDNA, like that of mammals and birds, evolves faster than nuclear DNA. Genetic distances among mtDNAs of Xenopus species are very large, generally approaching or exceeding one substitution per nucleotide.     

A candidate autonomous version of the wheat MITE <Emphasis Type="Italic">Hikkoshi</Emphasis> is present in the rice genome

A miniature inverted-repeat transposable element (MITE), designated as Hikkoshi, was previously identified in the null Wx-A1 allele of Turkish bread wheat lines. This MITE is 165 bp in size and has 12-bp terminal inverted repeats (TIRs) flanked by 8-bp target site duplications (TSDs). Southern and PCR analyses demonstrated the presence of multiple copies of Hikkoshi in the wheat genome. Database searches indicated that Hikkoshi MITEs are also present in barley, rice and maize. A 3.4-kb element that has Hikkoshi-like TIRs flanked by 8-bp TSDs has now been identified in the rice genome. This element shows high similarity to the 5 subterminal region of the wheat Hikkoshi MITE and contains a transposase (TPase) coding region. The TPase has two conserved domains, ZnF_TTF and hATC, and its amino acid sequence shows a high degree of homology to TPases encoded by Tip100 transposable elements belonging to the hAT superfamily. We designated the 3.4-kb element as OsHikkoshi. Several wheat clones deposited in EST databases showed sequence similarity to the TPase ORF of OsHikkoshi. The sequence information from the TPase of OsHikkoshi will thus be useful in isolating the autonomous element of the Hikkoshi system from wheat.     

Primary structure and evolutionary relationship between the adult α-globin genes and their 5′-flanking regions ofXenopus laevis andXenopus tropicalis

Summary To investigate the evolution of globin genes in the genusXenopus, we have determined the primary structure of the related adult ₁- and _II genes ofX. laevis and of the adult -globin gene ofX. tropicalis, including their 5-flanking regions. All three genes are comprised of three exons and two introns at homologous positions. The exons are highly conserved and code for 141 amino acids. By contrast, the corresponding introns vary in length and show considerable divergence. Comparison of 900 bp of the 5-flanking region revealed that theX. tropicalis gene contains a conserved proximal 310-bp promoter sequence, comprised of the canonical TATA and CCAAT motifs at homologous positions, and five conserved elements in the same order and at similar positions as previously shown for the corresponding genes ofX. laevis. We therefore conclude that these conserved upstream elements may represent regulatory sequences for cell-specific regulation of the adultXenopus globin genes.     

Comparative Analysis of the Proteins with Tandem Repeats from 8 Microsporidia and Characterization of a Novel Endospore Wall Protein Colocalizing with Polar Tube from Nosema bombycis

As a common feature of eukaryotic proteins, tandem amino acid repeat has been studied extensively in both animal and plant proteins. Here, a comparative analysis focusing on the proteins having tandem repeats was conducted in eight microsporidia, including four mammal‐infecting microsporidia (Encephalitozoon cuniculi, Encephalitozoon intestinalis, Encephalitozoon hellem and Encephalitozoon bieneusi) and four insect‐infecting microsporidia (Nosema apis, Nosema ceranae, Vavraia culicis and Nosema bombycis). We found that the proteins with tandem repeats were abundant in these species. The quantity of these proteins in insect‐infecting microsporidia was larger than that of mammal‐infecting microsporidia. Additionally, the hydrophilic residues were overrepresented in the tandem repeats of these eight microsporidian proteins and the amino acids residues in these tandem repeat sequences tend to be encoded by GC‐rich codons. The tandem repeat position within proteins of insect‐infecting microsporidia was randomly distributed, whereas the tandem repeats within proteins of mammal‐infecting microsporidia rarely tend to be present in the N terminal regions, when compared with those present in the C terminal and middle regions. Finally, a hypothetical protein EOB14572 possessing four tandem repeats was successfully characterized as a novel endospore wall protein, which colocalized with polar tube of N. bombycis. Our study provided useful insight for the study of the proteins with tandem repeats in N. bombycis, but also further enriched the spore wall components of this obligate unicellular eukaryotic parasite.     

Retinal stem/progenitor cells in the ciliary marginal zone complete retinal regeneration: A study of retinal regeneration in a novel animal model

Our research group has extensively studied retinal regeneration in adult Xenopus laevis. However, X. laevis does not represent a suitable model for multigenerational genetics and genomic approaches. Instead, Xenopus tropicalis is considered as the ideal model for these studies, although little is known about retinal regeneration in X. tropicalis. In the present study, we showed that a complete retina regenerates at approximately 30 days after whole retinal removal. The regenerating retina was derived from the stem/progenitor cells in the ciliary marginal zone (CMZ), indicating a novel mode of vertebrate retinal regeneration, which has not been previously reported. In a previous study, we showed that in X. laevis, retinal regeneration occurs primarily through the transdifferentiation of retinal pigmented epithelial (RPE) cells. RPE cells migrate to the retinal vascular membrane and reform a new epithelium, which then differentiates into the retina. In X. tropicalis, RPE cells also migrated to the vascular membrane, but transdifferentiation was not evident. Using two tissue culture models of RPE tissues, it was shown that in X. laevis RPE culture neuronal differentiation and reconstruction of the retinal three‐dimensional (3‐D) structure were clearly observed, while in X. tropicalis RPE culture neither ßIII tubulin‐positive cells nor 3‐D retinal structure were seen. These results indicate that the two Xenopus species are excellent models to clarify the cellular and molecular mechanisms of retinal regeneration, as these animals have contrasting modes of regeneration; one mode primarily involves RPE cells and the other mode involves stem/progenitor cells in the CMZ. © 2014 Wiley Periodicals, Inc. Develop Neurobiol 74: 739–756, 2014     

Homoeologous chromosomes of Xenopus laevis are highly conserved after whole-genome duplication

It has been suggested that whole-genome duplication (WGD) occurred twice during the evolutionary process of vertebrates around 450 and 500 million years ago, which contributed to an increase in the genomic and phenotypic complexities of vertebrates. However, little is still known about the evolutionary process of homoeologous chromosomes after WGD because many duplicate genes have been lost. Therefore, Xenopus laevis (2n=36) and Xenopus (Silurana) tropicalis (2n=20) are good animal models for studying the process of genomic and chromosomal reorganization after WGD because X. laevis is an allotetraploid species that resulted from WGD after the interspecific hybridization of diploid species closely related to X. tropicalis. We constructed a comparative cytogenetic map of X. laevis using 60 complimentary DNA clones that covered the entire chromosomal regions of 10 pairs of X. tropicalis chromosomes. We consequently identified all nine homoeologous chromosome groups of X. laevis. Hybridization signals on two pairs of X. laevis homoeologous chromosomes were detected for 50 of 60 (83%) genes, and the genetic linkage is highly conserved between X. tropicalis and X. laevis chromosomes except for one fusion and one inversion and also between X. laevis homoeologous chromosomes except for two inversions. These results indicate that the loss of duplicated genes and inter- and/or intrachromosomal rearrangements occurred much less frequently in this lineage, suggesting that these events were not essential for diploidization of the allotetraploid genome in X. laevis after WGD.     

Evolution Analysis of Simple Sequence Repeats in Plant Genome

Simple sequence repeats (SSRs) are widespread units on genome sequences, and play many important roles in plants. In order to reveal the evolution of plant genomes, we investigated the evolutionary regularities of SSRs during the evolution of plant species and the plant kingdom by analysis of twelve sequenced plant genome sequences. First, in the twelve studied plant genomes, the main SSRs were those which contain repeats of 1–3 nucleotides combination. Second, in mononucleotide SSRs, the A/T percentage gradually increased along with the evolution of plants (except for P. patens). With the increase of SSRs repeat number the percentage of A/T in C. reinhardtii had no significant change, while the percentage of A/T in terrestrial plants species gradually declined. Third, in dinucleotide SSRs, the percentage of AT/TA increased along with the evolution of plant kingdom and the repeat number increased in terrestrial plants species. This trend was more obvious in dicotyledon than monocotyledon. The percentage of CG/GC showed the opposite pattern to the AT/TA. Forth, in trinucleotide SSRs, the percentages of combinations including two or three A/T were in a rising trend along with the evolution of plant kingdom; meanwhile with the increase of SSRs repeat number in plants species, different species chose different combinations as dominant SSRs. SSRs in C. reinhardtii, P. patens, Z. mays and A. thaliana showed their specific patterns related to evolutionary position or specific changes of genome sequences. The results showed that, SSRs not only had the general pattern in the evolution of plant kingdom, but also were associated with the evolution of the specific genome sequence. The study of the evolutionary regularities of SSRs provided new insights for the analysis of the plant genome evolution.     

The Sunflower High-oleic Mutant Ol Carries Variable Tandem Repeats of FAD2-1, a Seed-specific Oleoyl-phosphatidyl Choline Desaturase

Ol, a chemically induced, incompletely dominant mutation, greatly increases oleic acid and is correlated with greatly reduced expression of a seed-specific oleoyl-phosphatidyl choline desaturase (FAD2-1) in developing seeds of sunflower (Helianthus annuus L.). FAD2-1 is duplicated in high-oleic (mutant) strains and cosegregates with Ol. Codominant RFLP markers have been developed for FAD2-1 and are diagnositic for the Ol mutation; however, the structure of the mutant FAD2-1 locus is unknown and polymorphic sequence-tagged-site (STS) DNA markers have not been developed for FAD2-1. The mutant was discovered to carry tandem repeats of FAD2-1 separated by a 2.67 kb intergenic region. The upstream repeat (FAD2-1U) carries a 1.69 kb intron in the 5′UTR, whereas the downstream repeat (FAD2-1D) is missing the first 1.54 kb of the 5′UTR and intron. Other than the deletion in FAD2-1D, no DNA polymorphisms were identified between wildtype and mutant FAD2-1 alleles among elite oilseed inbred lines. We developed dominant INDEL markers diagnostic for presence or absence of the Ol mutation (tandem FAD2-1 repeats) by targeting DNA sequences upstream of FAD2-1D, identified 49 SNPs and five INDELs (two haplotypes) in DNA sequences downstream of FAD2-1 in the wildtype and FAD2-1U in the mutant, identified polymorphic [AT]_n and [GT]_n repeats in the 3′UTR of FAD2-1, and developed codominant SSR and INDEL markers for FAD2-1. Novel FAD2-1 alleles found in exotic low-oleic genotypes could be introgressed into elite low-oleic genotypes to facilitate marker-assisted selection of Ol in mid- and high-oleic sunflower breeding programs.     

Resources and transgenesis techniques for functional genomics in Xenopus

Recent developments in genomic resources and high‐throughput transgenesis techniques have allowed Xenopus to ‘metamorphose’ from a classic model for embryology to a leading‐edge experimental system for functional genomics. This process has incorporated the fast‐breeding diploid frog, Xenopus tropicalis, as a new model‐system for vertebrate genomics and genetics. Sequencing of the X. tropicalis genome is nearly complete, and its comparison with mammalian sequences offers a reliable guide for the genome‐wide prediction of cis‐regulatory elements. Unique cDNA sets have been generated for both X. tropicalis and X. laevis, which have facilitated non‐redundant, systematic gene expression screening and comprehensive gene expression analysis. A variety of transgenesis techniques are available for both X. laevis and X. tropicalis, and the appropriate procedure may be chosen depending on the purpose for which it is required. Effective use of these resources and techniques will help to reveal the overall picture of the complex wiring of gene regulatory networks that control vertebrate development.     

A genome-specific repeat sequence from kiwifruit (Actinidia deliciosa var. deliciosa)

Summary Six members of a family of moderately repetitive DNA sequences from kiwifruit (Actinidia deliciosa var. deliciosa) have been cloned and characterized. The repeat family is composed of elements that have a unit length of 463 bp, are highly methylated, occur in tandem arrays of at least 50 kb in length, and constitute about 0.5% of the kiwifruit genome. Individual elements diverge in nucleotide sequence by up to 5%, which suggests that the repeat sequence is evolving rapidly. Homologous sequences were found in A. deliciosa var. chlorocarpa. The repeat sequence was not found under low stringency hybridization conditions in the diploid A. chinensis, the species most closely related to the hexaploid kiwifruit, or in eight other Actinidia species. However, homologous repeats were detected in a tetraploid species, A. chrysantha. The results provide the first molecular evidence to suggest that kiwifruit may be an allopolyploid species.     

A systematic search and classification of T2 family miniature inverted-repeat transposable elements (MITEs) in Xenopus tropicalis suggests the existence of recently active MITE subfamilies

To reveal the genome-wide aspects of Xenopus T2 family miniature inverted-repeat transposable elements (MITEs), we performed a systematic search and classification of MITEs by a newly developed procedure. A terminal sequence motif (T2-motif: TTAAAGGRR) was retrieved from the Xenopus tropicalis genome database. We then selected 51- to 1,000-bp MITE candidates framed by an inverted pair of 2 T2-motifs. The 34,398 candidates were classified into possible clusters by a novel terminal sequence (TS)-clustering method on the basis of differences in their short terminal sequences. Finally, 19,242 MITEs were classified into 16 major MITE subfamilies (TS subfamilies), 10 of which showed apparent homologies to known T2 MITE subfamilies, and the rest were novel TS subfamilies. Intra- and inter-subfamily similarities or differences were investigated by analyses of diversity in GC content, total length, and sequence alignments. Furthermore, genome-wide conservation of the inverted pair structure of subfamily-specific TS stretches and their target site sequence (TTAA) were analyzed. The results suggested that some TS subfamilies might include active or at least recently active MITEs for transposition and/or amplification, but some others might have lost such activities a long time ago. The present methodology was efficient in identifying and classifying MITEs, thereby providing information on the evolutionary dynamics of MITEs.     

Conservation and divergence of ADAM family proteins in the <Emphasis Type="Italic">Xenopus</Emphasis> genome

Background  

Members of the disintegrin metalloproteinase (ADAM) family play important roles in cellular and developmental processes through their functions as proteases and/or binding partners for other proteins. The amphibian Xenopus has long been used as a model for early vertebrate development, but genome-wide analyses for large gene families were not possible until the recent completion of the X. tropicalis genome sequence and the availability of large scale expression sequence tag (EST) databases. In this study we carried out a systematic analysis of the X. tropicalis genome and uncovered several interesting features of ADAM genes in this species.     

Structure and genomic organization of centromeric repeats in<Emphasis Type="Italic"> Arabidopsis</Emphasis> species

Centromeric repetitive sequences were isolated from Arabidopsis halleri ssp. gemmifera and A. lyrata ssp. kawasakiana. Two novel repeat families isolated from A. gemmifera were designated pAge1 and pAge2. These repeats are 180 bp in length and are organized in a head-to-tail manner. They are similar to the pAL1 repeats of A. thaliana and the pAa units of A. arenosa. Both A. gemmifera and A. kawasakiana possess the pAa, pAge1 and pAge2 repeat families. Sequence comparisons of different centromeric repeats revealed that these families share a highly conserved region of approximately 50 bp. Within each of the four repeat families, two or three regions showed low levels of sequence variation. The average difference in nucleotide sequence was approximately 10% within families and 30% between families, which resulted in clear distinctions between families upon phylogenetic analysis. FISH analysis revealed that the localization patterns for the pAa, pAge1 and pAge2 families were chromosome specific in A. gemmifera and A. kawasakiana. In one pair of chromosomes in A. gemmifera, and three pairs of chromosomes in A. kawasakiana, two repeat families were present. The presence of three families of centromeric repeats in A. gemmifera and A. kawasakiana indicates that the first step toward homogenization of centromeric repeats occurred at the chromosome level.Communicated by W. R. McCombie