Heteromorphic sex chromosomes of lizardfish (Synodontidae): focus on the ZZ-ZW1W2 system in Trachinocephalus myops

The karyotype and DNA content of four lizardfish species (family Synodontidae), that is, Saurida elongata, Synodus ulae, Synodus hoshinonis and Trachinocephalus myops, were analyzed. The karyotype of T. myops significantly differed from that of the other three species having diploid chromosome number of 48 with mainly acrocentric chromosomes and the ZZ-ZW sex chromosome system. The chromosome number of male T. myops was 2n=26, while that of female T. myops was 2n=27. The karyotype consisted of 11 pairs of metacentrics, one pair of acrocentrics and, in addition, two large metacentrics in the male and a single large metacentric, a distinctly small subtelocentric and a microchromosome in the female. C-banding demonstrated that in the female the subtelocentric chromosome and the microchromosome were heterochromatic. The karyotype of T. myops was thought to be derived from a 48 chromosome type synodontid fish through the involvement of Robertsonian rearrangement; the rearrangement of the sex chromosomes proceeded during karyotype evolution. Among the chromosomes, the large metacentrics were determined to be neo-Z (a fusion of the original Z and an autosome), the microchromosomes the W₁ (originally W), and the subtelocentric chromosomes the W₂ (derived from an autosome pair). The miniaturization of W₁ and W₂ chromosomes and their heterochromatinization suggested that sex chromosomes in this species have been already highly differentiated. The findings on DNA content implied that the karyotype of T. myops evolved by centric fusion events without loss in DNA amount.     

Chromosome and nucleotide sequence differentiation in genomes of polyploid Triticum species

Summary The nature of genome change during polyploid evolution was studied by analysing selected species within the tribe Triticeae. The levels of genome changes examined included structural alterations (translocations, inversions), heterochromatinization, and nucleotide sequence change in the rDNA regions. These analyses provided data for evaluating models of genome evolution in polyploids in the genus Triticum, postulated on the basis of chromosome pairing at metaphase I in interspecies hybrids.The significance of structural chromosome alterations with respect to reduced MI chromosome pairing in interspecific hybrids was assayed by determining the incidence of heterozygosity for translocations and paracentric inversions in the A and B genomes of T. timopheevii ssp. araraticum (referred to as T. araraticum) represented by two lines, 1760 and 2541, and T. aestivum cv. Chinese Spring. Line 1760 differed from Chinese Spring by translocations in chromosomes 1A, 3A, 4A, 6A, 7A, 3B, 4B, 7B and possibly 2B. Line 2541 differed from Chinese Spring by translocations in chromosomes 3A, 6A, 6B and possibly 2B. Line 1760 also differed from Chinese Spring by paracentric inversions in arms 1AL and 4AL whereas line 2541 differed by inversions in 1BL and 4AL (not all chromosomes arms were assayed). The incidence of structural changes in the A and B genomes did not coincide with the more extensive differentiation of the B genomes relative to the A genomes as reflected by chromosome pairing studies.To assay changing degrees of heterochromatinization among species of the genus Triticum, all the diploid and polyploid species were C-banded. No general agreement was observed between the amount of heterochromatin and the ability of the respective chromosomes to pair with chromosomes of the ancestral species. Marked changes in the amount of heterochromatin were found to have occurred during the evolution of some of the polyploids.The analysis of the rDNA region provided evidence for rapid fixation of new repeated sequences at two levels, namely, among the 130 bp repeated sequences of the spacer and at the level of the repeated arrays of the 9 kb rDNA units. These occurred both within a given rDNA region and between rDNA regions on nonhomologous chromosomes. The levels of change in the rDNA regions provided good precedent for expecting extensive nucleotide sequence changes associated with differentiation of Triticum genomes and these processes are argued to be the principal cause of genome differentiation as revealed by chromosome pairing studies.     

Position-specific effects in the mutagenic action of mitomycin C on the chromosomes of Hordeum vulgare L.

Summary The mutagenic action of mitomycin C (MMC) on the chromosomes of two reconstructed karyotypes of barley was studied. MMC-induced chromatid aberrations were found to be distributed non-randomly along the chromosomes. The regions situated next to the secondary constrictions of chromosomes 6 and 7 appeared to be clearly pronounced aberration hot spots. In these segments, intercalary deletions and duplication-deletions were the most frequently induced aberration types. The comparative analysis of the frequency and localization of MMC-induced aberrations in the chromosomes of the two karyotype variants, which differ from each other by the position of the hot spot segments, provided new evidence about the influence of the segment transposition on the hot spot expressivity. The most remarkable finding obtained in the study is that the size of the segment involved in both intercalary deletions and duplication-deletions proved to be strongly affected by the structural peculiarity of the reconstructed chromosome. The possible reasons underlying this finding are discussed.     

The Effect of W Chromosome Origin on Sex-Chromosome Pairing in ZZWW Tetraploid Females of the Domesticated Silkworm, Bombyx Mori, and the Congenic Wild Silkworm, Bombyx Mandarina

To analyze the degree of pairing of the Z and W chromosomes in ZZWW tetraploid female silkworms that have the W chromosomes of the domesticated silkworm, Bombyx mori, and those of the wild silkworm, Bombyx mandarina, we induced two types of ZZWW tetraploid female silkworms (Cr4n, Wr4n) through cold treatment of the eggs. The Wr4n female is congenic to the Cr4n female for W chromosomes; namely, the W chromosomes of the Wr4n female are derived from those of B. mandarina. Each of the sex ratios (/) in filial triploids from the Cr4n females was shown to be in the range of 3.9–5.3 (4.6 as an average of six cases). On the other hand, each of the sex ratios (/) in filial triploids from the Wr4n females was shown to be in the range of 6.2–9.0 (6.9 as an average of nine cases). The results of a t-test indicated that the difference in sex ratios in the two groups is highly significant (at the 0.1% level). These results suggest that, in the meiosis of the ZZWW tetraploid female, the frequency of pairing of the W chromosome of B. mandarina and the Z chromosome of B. mori is lower than that of the pairing of the W and Z chromosomes of B. mori. Furthermore, the t-test results are evidence that the W chromosomes have undergone significant evolutional change.     

Modification of nuclear gene expression by inhibition of mitochondrial translation during sporulation in MAT alpha/MATa diploids of Saccharomyces cerevisiae

Summary Sporulation of S. cerevisiae MAT/MATa was accompanied by a novel pattern of protein synthesis as shown by the disappearance of some mitotic polypeptides and by the appearance of a new set of meiotic polypeptides. Inhibition of mitochondrial protein synthesis by erythromycin within the 1st h caused the disappearance of several meiotic polypeptides. These meiotic polypeptides were also sensitive to cycloheximide and were localized in the cytosol, demonstrating that they were not mitochondrial translational products. Since erythromycin affected neither protein synthesis nor sporulation in a mitochondrially inherited er y mutant, we conclude that mitochondrial protein synthesis is needed for the expression of some nuclear genes during sporulation.     

The single DR β gene of the DRw8 haplotype is closely related to the DR β 3III gene encoding DRw52

In most individuals two HLA-DR genes are expressed from each chromosome. One of these genes encodes one of the classical DR specificities, while the other encodes either of the supertypic DRw52/DRw53 specificities. In addition to these genes usually one or two DR pseudogenes are present. In contrast, the DRw8 chromosomal region only contains a single DR gene. To determine the relationship of this single gene to the multiple DR genes of other DR specificities, comparisons of Southern genomic blots were carried out. In this analysis genomic clones for each individual DR chain locus were included. The DR w8 gene was indistinguishable from the DR III gene of DR3 cells (encoding DRw52), suggesting that it is closely related to the latter gene. The functional implications of this finding are discussed.     

Chromosome replication in four species of snakes

Chromosome measurements were performed in four species of snakes related at the level of suborder (Boa constrictor amarali, Xenodon merremii, Philodryas patagoniensis, Bothrops jararaca). The data obtained point out that pairs 1–3 were common to the four snakes and probably inherited from the ancestor of the suborder Serpentes. Pairs 5–8-W were characteristic of each snake; hence, it is possible to assume that they followed evolving after the appearing of the suborder Serpentes. Z-chromosomes were metacentric in B. constrictor amarali, X. merremii and B. jararaca and slightly submetacentric in P. patagoniensis. Area of these chromosomes varied from 8.6–10.6% of the haploid set in the four species studied.-The study of chromosome replication at the end of the S period points out that shared chromosomes have similar patterns of labeling. Therefore, it is proposed that the distribution of late replicating regions and heterochromatin in the genome is phylogenetically transmitted and probably genetically determined.—The analysis of the ending-sequence of chromosome replication shows that sex chromosomes finish earlier than macroautosomes. It is concluded that snakes probably have no mechanism of sex chromosome heterochromatinization in either sex. The absence of late replicating Z-chromosome in the males, favours the hypothesis that no mechanism of sex dosage compensation is acting in the suborder Serpentes.This work was partially supported by Public Health Service Research grant No. GM-14577-03 from the National Institute of General Medical Sciences and by the Fundo de Pesquisas do Instituto Butantan.     

A high-resolution karyotype of <Emphasis Type="Italic">Brassica rapa</Emphasis> ssp. <Emphasis Type="Italic">pekinensis</Emphasis> revealed by pachytene analysis and multicolor fluorescence in situ hybridization

A molecular cytogenetic map of Chinese cabbage (Brassica rapa ssp. pekinensis, 2n=20) was constructed based on the 4-6-diamino-2-phenylindole dihydrochloride-stained mitotic metaphase and pachytene chromosomes and multicolor fluorescence in situ hybridization (McFISH), using three repetitive DNA sequences, 5S rDNA, 45S rDNA, and C11-350H. The lengths of mitotic metaphase chromosomes ranged from 1.46 m to 3.30 m. Five 45S and three 5S rDNA loci identified were assigned to different chromosomes. The C11-350H loci were located on all the mitotic metaphase chromosomes, except chromosomes 2 and 4. The pachytene karyotype consisted of two metacentric (chromosomes 1 and 6), five submetacentric (chromosomes 3, 4, 5, 9 and 10), two subtelocentric (chromosomes 7 and 8), and one acrocentric (chromosome 2) chromosome(s). The mean lengths of ten pachytene chromosomes ranged from 23.7 m to 51.3 m, with a total of 385.3 m, which is 17.5-fold longer than that of the mitotic metaphase chromosomes. In the proposed pachytene karyotype, all the chromosomes of B. rapa ssp. pekinensis can be identified on the basis of chromosome length, centromere position, heterochromatin pattern, and the location of the three repetitive sequences. Moreover, the precise locations of the earlier reported loci of 5S rDNA, 45S rDNA, and Chinese cabbage tandem DNA repeat C11-350H were established using McFISH analysis. We also identified a 5S rDNA locus on the long arm of pachytene bivalent 7, which could not be detected in the mitotic metaphase chromosomes in the present and earlier studies. The deduced karyotype will be useful for structural and functional genomic studies in B. rapa.     

Population cytogenetics of the genus Caledia (Orthoptera: Acridinae)

The genus Caledia contains two species. C. species nova 1 is restricted to the Oriomo Plateau of S.W. Papua and has a complement of twelve telocentric chromosomes. The second species C. captiva has a much wider distribution pattern—from S.W. Papua in the North, down the entire Eastern seaboard of Australia to Southern Victoria. It is also found in the Northern Territory. Although the chromosome number is the same as C. species nova 1, four major and distinct chromosomal races can be distinguished in C. captiva. — The basic ancestral race is found in Tropical North Queensland at the base of the Cape York Peninsula. All twelve chromosomes are telocentric and the karyotypic organization is similar to that found in C. species nova 1 and in other Acridines. A second, general purpose karyotypic race has a wide distribution between S.W. Papua, Arnhem Land and the East Australian coast as far South as Brisbane. It is considered a derivative form of the ancestral type and is fixed for small pericentric inversions on seven pairs of chromosomes. In the South-Eastern Queensland region there exists a further race which carries large pericentric inversions on all the autosomes and the X chromosome. The situation here is confounded since the basic chromosomes can be represented as either acro or telocentrics. Various levels of polymorphism for the inversions exist between different chromosomes in different populations indicating considerable differentiation within this zone. This race is almost completely surrounded by the general purpose karyotype where the races are contiguous in certain parts of the range. — The South-Eastern corner of Australia is characterised by a chromosome race quite different from those found further North. Here a complex pericentric inversion system exists involving a series of seven small inversions and larger inversions on chromosomes 1, 2, 4 and 10. Chromosomes 2 and 4, in particular, are highly polymorphic. — The presence and persistence of these 4 chromosomal races can be accounted for in terms of the known climatic changes which have occurred in this region in the recent past.     

Cytogenetics of the chinese giant salamander,Andrias davidianus (Blanchard): the evolutionary significance of cryptobranchoid karyotypes

Cytogenetic aspects of the cryptobranchid salamander Andrias davidianus of western China have been studied, including chromosome number and morphology, C-band patterns, meiosis, and the chromosomal localization of ribosomal 5S RNA genes. Our data regarding chromosome number (2n=60) and general chromosome morphology largely confirm the results of Morescalchi et al. (1977). The karyotype consists of 16 pairs of macrochromosomes that decrease gradually in relative length to 14 pairs of microchromosomes. Telocentric chromosomes are a conspicuous feature of the karyotype, representing more than half the genome. Differential staining reveals that all of the chromosomes, except four pairs of microchromosomes, have C-band heterochromatin in their centromeric regions, the amount varying irrespective of chromosome size. Faint bands of interstitial and telomeric C-band heterochromatin are found in mitotic chromosomes but are not seen in meiotic preparations. In C-banded mitotic preparations from a female, one of the smallest macrochromosome pairs is heteromorphic in respect to C-band heterochromatin and centromere position. In situ hybridization of an iodinated 5S RNA probe to meiotic chromosome preparations reveals that this repeated gene is clustered near the telomeric region of chromosome 7, a medium size telocentric, a location corresponding to a band of heterochromatin. Studies of spermatocytes indicate that the process of meiosis in A. davidianus closely resembles that of more advanced salamanders, and that the microchromosomes are meiotically stable. The significance of microchromosomes and chromosome morphology in the reorganization of salamander genomes during evolution is discussed on the basis of cytogenetic data available for A. davidianus and various other primitive and advanced salamanders.     

Identification of two Drosophila TGF-β family members in the grasshopper Schistocerca americana

Intercellular signaling molecules of the transforming growth factor- (TGF-) superfamily are required for pattern formation in many multicellular organisms. The decapentaplegic (dpp) gene of Drosophila melanogaster has several developmental roles. To improve our understanding of the evolutionary diversification of this large family we identified dpp in the grasshopper Schistocerca americana. S. americana diverged from D. melanogaster approximately 350 million years ago, utilizes a distinct developmental program, and has a 60-fold-larger genome than D. melanogaster. Our analyses indicate a single dpp locus in D. melanogaster and S. americana, suggesting that dpp copy number does not correlate with increasing genome size. Another TGF- superfamily member, the D. melanogaster gene 60A, is also present in only one copy in each species. Comparison of homologous sequences from D. melanogaster, S. americana, and H. sapiens, representing roughly 900 million years of evolutionary distance, reveals significant constraint on sequence divergence for both dpp and 60A. In the signaling portion of the dpp protein, the amino acid identity between these species exceeds 74%. Our results for the TGF- superfamily are consistent with current hypotheses describing gene duplication and diversification as a frequent response to high levels of selective pressure on individual family members.     

Effect of time delay in specific growth rate on the periodic operation of bioreactors with input multiplicities

The effect of time delay in specific growth rate () on the periodic operation of bioreactors with input multiplicities is theoretically analyzed for productivity improvement. A periodic rectangular pulse is applied either in feed substrate concentration (S_f) or in dilution rate (D). Periodic operation under feed substrate concentration cycling gives improvement in productivity at lower value of ¯S_f of the two steady-state multiplicities of S_f only when the time delay in is larger. Whereas the larger value of ¯S_f gives improvement in average productivity for all values of time delay. Dilution rate (D) cycling gives an improvement in average productivity particularly for larger time delay in . This improvement in average productivity is obtained only at smaller value of dilution rate out of the two steady-state input multiplicities of D.List of Symbols D 1/h dilution rate - F memory function - g dummy variable - K_i g/l substrate inhibition constant - K_m g/l substrate saturation constant - P g/l product concentration - P_m g/l product saturation constant - Q g/(hl) product cell produced per unit time - S g/l substrate concentration - S_f g/l feed substrate concentration - S_f,p g/l feed substrate concentration during fraction of a period - X g/l biomass concentration - Y_X/S g/g cell mass yield - w variable either S or Z - Z g/l weighted average of substrate concentration Greek Letters 1/h time delay parameter - ₁ , ₂ product yield parameters, g/g and 1/h - pulse width expressed as a fraction of a period - 1/h specific growth rate - _m 1/h maximum specific growth rate - h period of oscillation - – average value     

The distribution of oocyte 5S,somatic 5S and 18S + 28S rDNA sequences in the lampbrush chromosomes of Xenopus laevis

The nucleolus organizer locus of Xenopus laevis lampbrush chromosomes was identified by in situ hybridization of a ³H-labelled probe complementary to 18S + 28S rDNA. The nucleolus organizer is an axial granule on chromosome III that lies four-fifths the way down this chromosome reading from its larger (left) telomere, just within an exploded region that extends to its right end, where the lateral loops are exceptionally long. By in situ hybridization of ³H-labelled oocyte and somatic 5S spacer cRNA probes to similarly RNase-treated and denatured lampbrush chromosomes, the multiple sites of oocyte and somatic 5S gene families were identified. Oocyte 5S genes lie at the larger telomeres of the 15 chromosomes that possess these structures; that is, all but chromosomes X, XVII and XVIII. There are a further four sites, all peripheral, and in three of these, on chromosomes VII, X and XI, the sequences lie on lateral loops that are resolvable with the light microscope. By contrast all of the somatic 5S gene clusters occupy peripheral sites. There are two sites on chromosome III, one of which may be shared with oocyte 5S sequences; one on chromosome VII, which is very likely shared with oocyte 5S sequences; one terminal site on chromosome X; one site on chromosome XI that lies on a single pair of long loops which are inserted in a conspicuous and recognizable axial granule, loops which certainly carry oocyte 5S sequences too; two nearly terminal sites alongside the larger telomeres on chromosomes XII and XIV; and single interstitial sites on all three of the sphere-bearing chromosomes, VIII, IX and XVI. We suggest that 5S sequences on resolvable loops are transcribed by readthrough from upstream promoters, probably by polymerase II.     

Molecular cloning of the β-cyclodextrin synthetase gene from an alkalophilic Bacillus and its expression in Escherichia coli and Bacillus subtilis

Summary The gene for -CGTase from an alkalophilic bacterium, Bacillus sp. #1011, was cloned in an Escherichia coli phage D69 and recloned in an E. coli plasmid pBR322 and a B. subtilis plasmid pUB110. An E. coli recombinant plasmid pTUE202 and a B. subtilis plasmid pTUB703 were selected from ten plasmids, because the transformants by each of the two plasmids produced the highest amount of extracellular -CGTase in each strain. The plasmids were stably maintained and expressed in each bacterial strain. A common DNA region of approximately 2.5 kb was defined in the ten plasmids, and the enzymatic activity was lost when a part of the common region was deleted. The major product of hydrolysis from starch by the -CGTases of E. coli [pTUB202] and B. subtilis [pTUB703] was -CD as in the case of the enzyme of the parental Bacillus sp. #1011.Abbreviations -CGTase -cyclodextrin synthetase - -CD -cyclodextrin - -CD -cyclodextrin - -CD -cyclodextrin - [] designates plasmid-carrier state     

An early stage of ZW/ZZ sex chromosome differentiation in Poecilia sphenops var. melanistica (Poeciliidae,Cyprinodontiformes)

The mitotic and meiotic chromosomes of the American cyprinodont fish Poecilia sphenops var. melanistica were analysed. All 46 chromosomes are telocentric. By specific staining of the constitutive heterochromatin with C-banding and various AT-specific fluorochromes, the homomorphic chromosome pair 1 could be identified as sex chromosomes of the ZW/ZZ type. All female animals exhibit a W chromosome with a large region of telomeric heterochromatin that is not present in the Z chromosome. These sex chromosomes cannot be distinguished by conventional staining; they represent the first demonstration of sex chromosomes in fishes in an early stage of morphological differentiation. The W heterochromatin and the telomeric heterochromatin in the two autosomes 18 show a very bright fluorescence when stained with AT-specific fluorochromes. This allows the direct identification of the chromosomal sex by examining the interphase nuclei: females exhibit three, males only two brightly fluorescent heterochromatic chromocenters in their nuclei. The significance of these ZW/ ZZ sex chromosomes and their specific DNA sequences, the dose compensation of the Z-linked genes, and the experimental possibilities using sex-reversed ZW males are discussed.     

Chromosome variation in the plethodontid salamander,Aneides ferreus

Karyotype variation in the plethodontid salamander, Aneides ferreus, has been analysed. 358 individuals from 14 populations, representing the major portion of the range of this salamander, have been karyologically examined. In A. ferreus, n=14. When the chromosomes are arranged in a decreasing relative length series, the karyotype is heteromorphic with respect to chromosome number 13, which may be either telocentric (T) or subtelocentric (ST). Variation in the heteromorphism over the range of the species is sex related, and probably also reflects relative population sizes. The heteromorphism in the isolated populations of A. ferreus on Vancouver Island, British Columbia, Canada, resembles a WZfemale/ZZmale sex chromosome dimorphism, suggesting the possibility that chromosome number 13 may be involved in sex determination in this population. The possibility that chromosome number 13 is involved in sex determination in all populations of A. ferreus is discussed. Our data suggest that the ancestral A. ferreus karyotype was homomorphic for T (T/T), and that the ST was derived from the T by a pericentric inversion. In peripheral populations, only the W homologue has been affected, whereas in central populations both the W and the Z chromosomes have been rearranged. Comparisons are made with other species of Aneides for which karyological information is available, and it is concluded that chromosome rearrangements have played an important role in the evolution of the genus. In C-banded chromosomes of A. ferreus, staining is most intense at the centromere regions of the larger chromosomes and is absent only in some of the smaller chromosomes. Implications of this C-banding pattern are discussed.     

Purification and partial characterization of an acid β-fructosidase from sweet-pepper (Capsicum annuum L.) fruit

The present study describes the biochemical characteristics of an acid -fructosidase (EC 3.2.1.26) purified from the fruit of sweet pepper (Capsicum annuum L.). The soluble form, which constitutes more than 95% of the total activity at pH 4.5, hydrolyzes sucrose, raffinose, and stachyose. Its pH and temperature optima are 4.5 and 55 °C, respectively. Metal cations such as Ag⁺ and Hg²⁺ strongly inhibit its activity, suggesting the presence of at least one sulfhydryl group at the catalytic site. After purification of the enzyme by means of ammonium sulfate fractionation, gel chromatography (diethyl-aminoethyl-Sephacel, hydroxylapatite, concanavalin A-Sepharose), and preparative gel electrophoresis, the purified enzyme was shown to be a 42 kDa glycoprotein interacting specifically with concanavalin A. After complete chemical deglycosylation with trifluoromethanesulfonic acid, the molecular weight of the constitutive polypeptide was estimated to be 39 kDa. The enzyme glycans were characterized using both affino- and immunodetection. The enzyme has at least two N-linked oligosaccharide sidechains, one of the high-mannose type, and the other of the complex type. The high-mannose glycan has a low molecular weight (1 kDa), and is responsible for the interaction between the enzyme and concanavalin A. The complex-type glycan has an estimated molecular weight of 2 kDa. It contains one 1 2-linked xylose residue, probably one fucose residue 1 3-linked to the chitobiose unit, and no terminal galactose residue. The two glycans, associated to the 39 kDa polypeptide, constitute the acid -fructosidase of the sweet-pepper fruit.Abbreviations F -fructosidase - ConA concanavalin A - DEAE diethylaminoethyl - DTNB dithionitrobenzoic acid - endo F endo--N-acetylglucosamidase F - endo H endo--N-acetylglucosamidase H - NEM N-ethylmaleimide - PCMB parachloromercurobenzoate - PNGase glycopeptide-N-glycosidase - TFMS trifluoromethane sulfonic acid This work was partly supported by a grant from the Commission Permanente de Coopération Franco-Québécoise to L. Faye, and S. Yelle. D. Michaud was a recipient of a graduate scholarship from the Natural Science and Engineering Research Council of Canada.     

The behavior of allocyclic chromosomes in Bloom's syndrome

The behavior of individual allocyclic chromosomes has been analyzed in lymphocytes of a sister and a brother with Bloom's syndrome. Of 4,633 diploid cells, 115 showed allocyclic chromosomes, and 74 of these had 44, 45 or 46 normal metaphase chromosomes accompanied by one or two allocyclic chromosomes. Of 56 tetraploid cells, 9 contained such chromosomes. The allocyclic chromosomes appeared pulverized or extended corresponding to S or G₂ PCC. We have proposed the hypothesis that individual allocyclic chromosomes do not, as a rule, come from micronuclei, as has often been assumed, but have been left behind in their cycle. This would be caused by a mutation or deletion of a hypothetical coiling center situated near the centromere of each chromosome arm. The following observations agree with our explanation but less well or not at all with the idea of micronuclei: (1) In only 9.6% of the cells does the allocyclic chromosome lie at the edge of the metaphase plate. (2) In 24 cells a part of a chromosome is pulverized while the rest is in metaphase. (3) Both a pulverized and an extended chromosome were present in the same cell. (4) A pulverized acrocentric is often nose-to-nose with a normal D or G chromosome. (5) No allocyclic chromosomes corresponding to G₁ PCC have been found in our material. (6) When a ring is replaced by an allocyclic chromosome, it is usually a member of a 46-chromosome complement. Furthermore, the occurrence of allocyclic chromosomes is correlated with that of other chromosome anomalies which do not follow a Poisson distribution. Allocyclic chromosomes are also more frequent (16%) in tetraploid than in diploid cells (2%).     

Cytogenetics of the parthenogenetic grasshopper Warramaba (formerly Moraba) virgo and its bisexual relatives

The distribution of late-replicating segments along the chromosomes of five clones of W. virgo is described. Some, but not all of these segments correspond to C-bands. In general, the autoradiographic profiles (histograms of linear grain density along the length of chromosomes labeled with tritiated thymidine in late S-phase) show strong resemblances throughout the five clones. However, some significant differences exist, and these are particularly marked in the case of the Boulder clone, which is anomalous in many other respects. — A similar study has also been carried out on the two bisexual species of Warramaba (P169 and P196) that gave rise, by hybridization more than half a million years ago, to the parthenogenetic W. virgo. In the case of P169, the autoradiographic profiles of the three large chromosomes (X+A, B+5, CD) which it has contributed to the W. virgo karyotype are extremely similar to those of the corresponding chromosomes in the virgo clones we have studied. In the case of P196 there is likewise, in most instances, a close resemblance of the autoradiographic profiles of the AB, X₁ and CD chromosomes to those of the same chromosomes in the virgo clones, but that of the X₁ shows no particular resemblance to the anomalous profile of the X₁ in the Boulder clone, in which the X₁ has undergone a structural reorganisation. The autoradiographic profile of the P196 CD chromosome does, however, show a much closer resemblance to that of the corresponding chromosome in the Boulder clone than to those of the CD₁₉₆ in the other four virgo clones studied. These investigations confirm the considerable evolutionary stability of DNA replication patterns.