Bimodal Karyotype Research Articles

Chromosome Cytology of the African Genus Lapeirousia (Iridaceae-Ixioideae)

Chromosome counts, now available for 33 of the approximately 40 species of Lapeirousia, show the genus to be remarkably variable cytologically. Excluding polyploidy, numbers range from n = 10 to 9, 8, 7, 6, 5, 4, and 3 in what we believe to be a descending dysploid series. A cycle of polyploidy and secondary descending dysploidy appears to have occurred in the tropical African section Paniculata. In L. erythrantha of the section there are both diploid populations with n = 4 and polyploid populations with n = 6, the latter karyotype having about twice as much chromosome material as the n = 4 karyotype. Although chromosome numbers vary among the species, the single large chromosome pair and total amount of chromosome material appear to be conserved, indicating the progressive fusion of small chromosomes, from a putatively ancestral bimodal karyotype with one long and nine short chromosome pairs

Chromosome evolution in Himalayan Impatiens (Balsaminaceae)

AKIYAMA, S., WAKABAYASHI, M. & OHBA, H., 1992. Chromosome evolution in Himalayan Impatiens (Balsaminaceae). Chromosome numbers and karyotypes have been investigated in species of Himalayan Impatiens. In addition to confirming previous chromosome counts, the presence of a tetraploid taxon (I. exilis) is revealed. In central and east Nepal species with x= 9 are more common than those with other basic numbers and this number is shown to be one of the most frequent numbers in the genus. Most species with x= 9 have a bimodal karyotype. The species relationships are discussed.

Genomic in situ hybridization reveals the allopolyploid nature ofMilium montianum (Gramineae)

Molecular techniques that “paint” chromosomes offer exciting new opportunities for testing genome relationships.Milium montianum (2n=22) is a grass whose distinctive bimodal karyotype comprises 8 large (L-) and 14 smaller (S-) chromosomes. The proposal thatM. montianum is an allotetraploid, with diploidMilium vernale (2n=8) as the L-chromosome genome donor, has been impossible to confirm by classical means. To test this hypothesis, biotinylated total genomic DNA of diploidM. vernale (2n=8) was hybridized in situ to root tip chromosomes ofM. montianum. TheM. vernale probe hybridized preferentially to all L-chromosomes, but not to the S-chromosomes. These results (i) confirm the allopolyploid nature ofM. montianum, (ii) strongly support the theory that the L-chromosomes ofM. montianum were donated byM. vernale, or a closely related genotype and (iii) show that subsequently the L-chromosomes have largely retained their genomic integrity in the new allopolyploid backgroud. Clearly, genomic in situ hybridization (GISH) is a potentially powerful tool for studying genome evolution and biosystematics. It will often be useful for investigating the origins of wild and cultivated polyploid plant species, especially where conventional methods have failed, for studying introgression, and for understanding the mechanism(s) of origin of bimodal karyotypes.

Karyological analysis and genome size in Milium (Gramineae) with special reference to polyploidy and chromosomal evolution

Karyotypes, nuclear DNA amounts, and meiotic behaviour are presented for Milium effusum L. (2n = 28), Milium montianum Parl. (2n = 22), and two cytotypes of Milium vernale Bieb. (2n = 8, 10). The bimodal karyotype of M. montianum (8 large and 14 small chromosomes) is described for the first time. Evidence from C-banding and geographical distribution suggests an ancient interracial allopolyploid origin for M. effusum (2n = 28). Although M. montianum is undoubtedly allopolyploid, its parentage is unconfirmed. A strong resemblance between the M. vernale (2n = 8) karyotype and the eight large chromosomes in M. montianum suggests a common ancestry. It is possible that a diploid form of M. effusum contributed the remaining 14 chromosomes. A selective loss of DNA sequences from the smaller chromosomes during the subsequent reorganization of the allopolyploid genome may have enhanced the bimodality of the karyotype. Geographical distribution and a change in the breeding system support the direction of the change x = 5 to x = 4 in M. vernale. Allopolyploidy appears to have played a central role in the chromosome evolution and speciation of Milium.Key words: Milium (Gramineae), karyotype analysis, genome size, polyploidy, chromosome evolution.

Generic relationships in the Alooideae (Asphodelaceae)

SummarySmith, G. F. & Van Wyk, B‐E.: Generic relationships in the Alooideae (Asphodelaceae). ‐ Taxon 40: 557–581. 1991. ‐ ISSN 0040‐0262.Within the Asphodelaceae the subfamily Alooideae is a natural (monophyletic) group. All the genera in this subfamily are therefore considered to have evolved from a common ancestor, based on the possession of character states which distinguish this complex from all other genera of the Asphodelaceae. However, no comprehensive interpretation of phylogenetic relationships within the entire subfamily based on comparative morphological data and phytogeographical considerations has yet been accomplished. The present paper investigates phylogenetic relationships of the genera Aloe, Astroloba, Chortolirion, Gasteria, Haworthia, Lomatophyllum and Poellnitzia using cladistic methodology. The most obvious similarities amongst these genera are the succulent leaf consistency, crescentiform or cymbiform leaf outline in cross‐section and the markedly bimodal karyotype of 2n = 14 chromosomes. For the cladistic analysis 16 characters were used and the genus Kniphofia was selected as outgroup on the basis of two synapomorphies with Alooideae, namely tubular flowers and fused perianth segments. The characters are discussed in detail, polarized into plesiomorphic and apomorphic states and then employed in the generation of a phylogenetic hypothesis using the “Hennig86” software package.

The chromosomes of <i>Chortolirion</i> and <i>Poellnitzia</i> (Asphodelaceae: Alooideae)

The somatic chromosomes of Chortolirion angolense (Baker) A. Berger and Poellnitzia rubriflora (L. Bol.) Uitewaal have been studied. Both taxa are monotypic genera in the subfamily Alooideae of the Asphodelaceae. Prior to this study Chor­tolirion had not been cytologically examined, while karyograms and idiograms have never been presented for Poellnitzia. Chortolirion and Poellnitzia are diploid with 2n = 14 chromosomes and with a bimodal karyotype typical of the entire subfamily, comprising four pairs of long chromosomes and three pairs of short chromosomes. For Poellnitzia minute structural differences in karyotype morphology were noted between observed and published data, especially with regard to the presence of satellites.

Meiotic chromosome behaviour reflects levels of sequence divergence inSus scrofa domestica satellite DNA

We present a general model for the evolution of chromosome-specific satellite DNA subfamilies. Sus scrofa domestica has a bimodal karyotype with two autosomal subsets of 12 meta-/submetacentric (Mc) and 6 acrocentric (Ac) chromosome types (Mc and Ac "subgenomes"). We show that the centromeric heterochromatin is characterised by two distinct satellite DNA families designed Mc1 and Ac2. Mc1 is a diverse satellite family of the Mc subgenome of which certain members with a 100 bp repeat unit are found to occur at the pericentromeric regions of each Mc autosome, while others are chromosome-specific, e.g. clone Mc pAv1.5, a higher order repeat variant, which hybridises specifically to chromosome 1. Ac2 is a homogeneous satellite occurring at the subterminal pericentromeric regions of all Ac autosomes. DNA sequence analyses showed that all clones investigated are built up from a 14 bp repeat unit which is highly conserved. In situ hybridisation to meiotic pachytene nuclei revealed a distinct spatial arrangement of the Ac2 centromeric satellite.

Chromosome Studies in the Southern African Flora. 58–94. Chromosome Evolution in the GenusGasteriaDuval

SUMMARYA cytological analysis of thirty-seven accessions of Gasteria has shown that this genus is very uniform chromosomally. All the species have a bimodal karyotype of eight large and six small chromosomes. Karyotype variability between the species is no more than that found between individuals of a single species showing that Gasteria is composed of taxa more closely related than in other genera.

Structure and behaviour of chromosomes in four different species of Agave.

Cytological studies have been carried out on four different species of Agave, distributed in different parts of India. They revealed triploid to hexaploid numbers ranging from 2n=90 to 180. A distinct bimodal karyotype with very long and very short chromosomes in a definite proportion of 5:25 ratio and a constant base number of n=30 chromosomes are the most significant characteristics. However, structural alterations, principally involving the nucleolar chromosomes of both long and short types, have been recorded. Clear absence of distinct multiples in the karyotypes rules out the absence of autopolypoloidy. Total chromosome length and volume do not necessarily represent direct multiplication along with the multiplication of the chromosome number. All these factors suggest the role of structural alterations along with differential degrees of coiling of chromosomes and allopolyploidy rather than autopolyploidy in the evolution of the different taxa.

Structural differences of chromosomes in diploid Agave.

Cytological studies have been carried out on five different species of Agave, collected from different parts of India. All of them revealed diploidy having 2n=60 chromosomes. A distinct bimodal karyotype with very long and very short chromosomes in a definite proportion of 5:25 ratio is the most significant characteristic of this genus. However, the karyotype formulae, as brought out in the present study differ quite distinctly amongst the species in-vestigated. Such variations principally involve the nucleolar chromosomes of both long and short types. The total chromosome lengths and volumes also differ at the interspecific level. Structural alterations along with differential degrees of coiling of chromosomes are the suggested factors principally responsible for such variations.

Cytophotometric estimation of nuclear DNA in different species and varieties of Agave.

A detailed cytological and cytochemical investigation involving determination of somatic chromosome number, karyotype, total chromosome length, volume and estimation of 4C nuclear DNA were carried out in 9 different species and varieties of Agave. Of the nine species studied, five belonged to diploid level with 2n=60 chromosomes and the rest were triploid, tetraploid, pentaploid and hexaploid having 2n=90, 120, 150 and 180 chromosomes respectively. Inspite of a distinct bimodal karyotype with very long and very short chromosomes in the 5: 25 ratio and a constant base number of n=30 chromosomes featured in the detailed karyotype analysis, each species has got its own distinct karyotype, differing from the rest in minute details. The total chromosome lengths and volumes differed both within and outside the same ploidy level and the volume did not necessarily increase with the increase in chromosome number.The amount of 4C nuclear DNA, estimated through Feulgen microspectrophotometry and expressed in arbitrary unit of relative absorbances, did not show much variations in the diploid species, while the triploid species revealed slightly lesser value than that of one specific diploid species. The difference in the amounts of nuclear DNA was insignificant amongst the pentaploid and hexaploid species as compared to the difference of their chromosome numbers. No direct correlation could, however, be established between the cytological parameters and the amounts of nuclear DNA. On the basis of overall analysis, the influence of structural alterations in affecting the amounts of nuclear DNA as well as other cytological characters from diploid to hexaploid level has been suggested.

Taxonomic and Cytological Notes onBowieaHook. F. and Allied Genera (Liliaceae)

SUMMARYOn the basis of general morphology, distribution and cytology Bowiea gariepensis v. Jaarsveld is reduced to a subspecies and B. kilimandscharica Mildbraed to a synonym of the widespread B. volubilis Harv. ex Hock. The peculiar vegetative organization of Bowiea with its soft bulb and ephemeral true leaves is discussed together with its relationship to some other genera. A cytological investigation of Bowiea and of species of the allied genera Drimia, Rhadamanthus, Schizobasis and Litanthus, has revealed that they possess bimodal karyotypes generally including four large chromosomes and a varying number of small chromosomes. This is considered to be an indication of the probable common origin and evolution of the five genera. The origin of chromosome bimodality is briefly discussed.

Karyomorphology and Chromosome Behaviour of a form ofEleutherine PlicataHerb. (Iridaceae) with an Aberrant Chromosome Number and a Pericentric Inversion

SUMMARYContrary to previous reports of chromosome number variation from 2n = 12 to 2n = 15 in Eleutherine plicata, we have found regularly 2n = 12 in somatic cells and in the PMC's. Further, our plants showed a sharp centromeric heteromorphism (AC/M type) in the first chromosome pair due to inversion heterozygosity. The heterotypic or bimodal karyotype, with chromosomes ranging in length from 1.26 to 5.28 μm shows a regular bivalent formation with a mean chiasma frequency of 1.93 (± 0.03) chiasmata per bivalent and 75% pollen stainability. Thus, Eleutherine plicata appears to be a disomic with x = 6 chromosomes.

The Chromosomes of Orchids VIII. Spiranthinae and Cranichidinae

Chromosome counts of 21 species belonging to 11 genera in the Spiranthinae and Cranichidinae subtribes (Orchidaceae) are reported. The most common chromosome number found is 2n = 46, but 2n = 26, 30, 32, 60 and 92 also occur. The 2n = 26 and 46 types have unusual bimodal karyotypes, each with a single pair of chromosomes which are longer than the rest. This size difference is particularly pronounced in the 2n = 26 plants. The 2n = 92 plants also have bimodal karyotypes, but with more than one pair of long chromosomes.

Cytological studies in South AmericanIridaceae

Although the South AmericanIridaceae are cytologically diverse, three tribes are distinguishable on the basis of karyotype morphology. TheSisyrinchieae andTrimezieae have variable basic numbers, ploidy levels and chromosome sizes, while theTigridieae are characterized by a relatively uniform basic number and bimodal karyotype. Changes in chromosome size within genera may suggest fluctuations in their DNA amount with latitude and altitude, particularly inSisyrinchium. The results are considered in terms of opportunities for more detailed research.

Chromosome Cytology in Relation to Suprageneric Systematics of Neotropical Iridaceae

Chromosome cytology of Iridaceae in the New World is reviewed in detail, and original counts for nineteen species are presented, four of these first re- ports for genera. A tribal classification, based in part on cytology but also on mor- phological criteria, is outined in which three major groups are recognized, Sisyrin- chieae, Trimezieae, and Tigridieae, the latter divided into Cipurinae and Tigridiinae. No decision has been made on whether to recognize Sisyrinchioideae as a subfamily distinct from Iridoideae. Sisyrinchieae comprises two karyotype groups Libertia (x = 19) and Orthrosanthus (x = 9) with tiny chromosomes and Sisy- rinchium and Phaiophleps, both probably also x = 9 but with medium-sized chromo- somes. Trimezieae have medium to large chromosomes and are cytologically diverse with n = 13 and 26 in Trimezia and n = 8, 9, and 16 in Neomarica. Tigridieae have x = 7 and bimodal karyotypes of one or two long pairs and five or six much smaller pairs. Tigridiinae appear polyploid, x = 14, with two long and 12 short pairs. The first counts for ten species and four genera presented here bring to 18 the number of Neotropical genera known cytologically, more than half the total, but less than 20% of the species in this area have been counted. Of the approximately 85 genera and 1500 species of the Iridaceae, some 46 genera and about 1000 species occur in Africa south of the Sahara, mainly in southern Africa, making it the major center for Iri- daceae. However, there are in the Neotropics some 30 genera and 250 species of which about 20 genera and almost all species are endemic, making this area the second richest for the family. In contrast with the African species, the systematics and cytology of which are comparatively well understood, Neotropical Iridaceae are poorly known. There are virtually no modern generic revisions, and existing studies, many dating from the late nineteenth century, were based on very little material often of poor quality. Cytology of only some 40 species in 18 genera of the family in this area is known. The present paper is an attempt to bring together available cytological data, to present several new chromosome counts including ten new reports for species (four of which are first reports for their genera), and to draw what systematic conclusions are possible from this information. A classification below family level is out- lined in which three tribes are recognized, in one of these, Tigridieae, two subtribes are proposed. Tribes are not here assigned to subfamily.

Studies on the South Indian Compositae. III. Karyomorphology of nine species of Blumea DC.

Karyomorphology of nine species of Blumea has been presented such as B. barbata (2n=16), B. fistulosa (2n=18), B. jacquemontii (2n=18), B. lacera var. glandulosa (2n=36), B. malcolmei (2n=16), B. menzbranacea var. gardneri (2n=18), B. oxyodonta (2n=18), B. virens (2n=18) and B. wightiana (2n=18).Karyotypes of both the x=9 and x=8 species are moderately asymmetrical (2A and 2B). In finer details, the karyotypes of x=9 species are seen to be more specialized. Five x=9 species showed bimodal karyotypes and in all of them the short chromosomes greatly outnumbered the long ones.Species possessing bimodal karyotypes showed more advanced morphological characters, and this appears to indicate an association between karyotype specialization and morphological advancement of plants in the genus.

Cytogenetical investigations into some garden ornamentals. II. The genus Aloe L.

Sixteen taxa belonging to ten species of Aloe were investigated. Most of these species exist only as diploids. A. arborescens was found to be tetraploid, whereas, both diploid and tetraploid individuals were studied in A. arborescens var. natalensis, A. latifolia and A. vera. Apart from unreduced gametes and hybridization, the origin of polyploidy in the genus is attributed to mitotic system as well.Split metaphases/anaphases and/or non-orientation and ultimate exclusion of some of the chromosomes accounts for aneusomatic cells (2n=9, 10, 11, 12) in A. aristata. Non-disjunction leading to unequal anaphases explains such cells in A. humilis (2n=13, 15) and A. vera (2n=15). Somatic pairing as observed in A. ferox may also result in such unequal segregations. While, the aneusomatic cells were observed in the gametic tissue as well in A. vera, their absence in the meiotic tissue in A. aristata and A. humilis indicates that these cells are not selected in the germ line.Different species presently studied, though exhibited a gross similarity in the morphology of their bimodal karyotype, differed from one another in their total chromatin length, F% and S% ratios. Satellites were observed on the long arm of large chromosomes in A. arborescens var. natalensis, A. ferox and A. vera. In A. saponaria only one chromosome of a long pair possessed satellite.Hybrid nature of the polyploid taxa in A. latifolia and A. vera is apparent, on the basis of their karyotypes. In A. arborescens, polyploidy explains meiotic abnormalities, whereas, structural heterozygosity accounts for such aberrations in A. africana, A. ferox, A. transvaalensis and A. vera (2x).