Total Chromosome Length Research Articles

Repatterning of the Karyotype in a Polyploid Dioecious <i>Rumex</i>

The basic karyotype of the hexaploid R. acetocella (2n=42) were compared with its progenitor diploid R. angiocarpus (2n=14). Both the species are dioecious. The sex chromosomes of R. acetosella were in active evolutionary state and a little differentiation had taken place in the autosomes. On the contrary, the sex chromosomes of R. angiocarpus have been reported to be stable while the autosomes show considerable repatterning.The basic karyotype of the two species resembled closely. The corresponding chromosome by chromosome and total chromosome length comparisons of the diploid and the hexaploid species revealed that chromatin loss, decrease of 19.62 precent from the expected, has occured during the evolution of R. acetosella subsequent to polyploidization. The chromatin loss had caused higher degree of karyotypic asymmetry in the polyploid. The evolutionary significances of the karyotype repatterning in R. acetocella have been discussed.

The somatic chromosomes of a wild population of rats: numerical polymorphism

The karyotype and quantitative characteristics of a wild population of rats, Rattus rattus, were studied. Individuals of the population were classified into three distinct groups, each with a characteristic chromosome number of 38, 42 and ± 54 respectively. The frequency distribution of the three groups of rats in the sample studied was as follow: group I with 38 chromosomes formed 14%, group II with 42 chromosomes formed 54% and group III rats have had chromosome numbers varying between 50–60 formed 32%. The rats with 38 chromosomes had two pairs of marker chromosomes (2 long metacentric pairs). Those of group III were characterised by having a marked decrease or complete absence of short metacentric chromosomes with a simultaneous increase in the frequency of short telocentric chromosomes. Group II rats had more or less the chromosomal characteristics established for laboratory rats studied by previous workers. The total chromosomal length of somatic cells in either group I and II were found to be similar. The notable chromosomal polymorphism in number was explained in terms of centromeric fusion or dissociation.

Medicago murex with 2n=16 and 2n=14 chromosome complements

Contrary to the generally held view, M. murex Willd. was found to be of 2n=14 constitution in the great majority of accessions. The 2n= 16 type, though distributed sparsely over a wide area, is most likely a relic. Chromosome studies at pachytene indicate that the 2n=14 type probably arose from the 2n=16 type by translocation of the chromosomal material of the smallest chromosome in the 2n=16 complement to one of the longer chromosomes, with loss of the centromere of the small chromosome. The total chromosome lengths of the two types thus remained not significantly different, which is in line with the few small morphological differences which were detectable between the two types. Of considerably phylogenetic interest was the finding of complete inter-sterility of the two M. murex types. Neither M. aculeata 2n= 16 nor M. constricta 2n=14 could be crossed with M. murex of the same chromosome number. Inviable hybrids were obtained on crossing M. murex 2n= 16 with M. turbinata 2n=16. The spiny vs. spineless pod character was investigated in the 2n=16 type and found to be inherited on a monofactorial basis.

Karyotypic analyses of accessions of Kersting’s groundnut

Kersting’s groundnut (Macrotyloma geocarpa) is an orphan crop of significant food and household income in sub-Saharan Africa. The objective of this study was to evaluate the chromosomal variability among the accessions of the species through analysis of the karyotypes. Six accessions of Kersting’s groundnut were subjected to conventional cytological techniques, using root tip meristems. The somatic chromosome number was 2n = 20 in all the accessions. The highest total chromosome length was obtained in Tkg 1 (1.739 + 0.047); while the least was in Tkg 12 (1.014 + 0.029). All the accessions had metacentric, submetacentric and subtelocentric chromosomes. However, there was a preponderance towards metacentric chromosomes, suggesting a symmetrical karyotype, which is known to be a primitive condition. Five of the accessions were grouped in karyotype 2A while one accession, Tkg 9 was grouped in karyotype 2B. The 2B karyotypes are known to have greater chromosomal asymmetry than 2A; thus Tkg 9 may be advancing evolutionarily faster than the other accessions. Hierarchical cluster analysis divided the accessions into three clusters. Cluster 1 comprised of four accessions (Tkg 1, Tkg 6, Tkg 9 and Tkg 11); while Cluster 2 comprised of one accession (Tkg 6); and cluster 3 had one accession (Tkg 12). Tkg 6 and Tkg 12 were observed to be distantly related to the other accessions; as such could serve as parents in hybridisation programmes to generate wider variability for the improvement of the species.

Karyotype Analysis inTriticum: II — Analysis ofT. AraraticumJakubz. andT. TimopheeviZhuk. and Their Relationships with Other Tetraploid Wheats

SUMMARYAnalysis of karyotypes of T. timopheevi and T. araraticum (respectively a cultivated and a wild tetraploid wheat belonging to the same species) demonstrated an identical chromosome morphology. Both are quite different from the karyotypes presented by other tetraploid wheats belonging to T. turgidum group. Differences in total chromosome length and arm ratio are evident, particularly in satellited chromosomes. Moreover, chromosomes with subterminal and median centromere are more frequent in timopheevi than in turgidum lines. Differences in chromosome morphology support the hypothesis of a distinct origin of the two groups of tetraploid wheats or, at least, give the evidence of occurrence, during differentiation, of large chromosome restructurations. Current hypotheses of origin and speciation of tetraploid wheats are discussed in the light of phytogeographical, genetical, cytogenetical, cytological and biochemical data available.

Heterochromatin and chromosome aberrations.

The chromosome breaking effect of mitomycin C, methyl methanesulfonate, maleic hydrazide, 8-ethoxycaffeine and gamma rays on the primary root meristematic cells of Nigella damascena was studied. All the agents tested except 8-ethoxycaffeine, produced relatively fewer aberrations, when compared to Vicia faba cells, though both the species have nearly similar total chromosomal length. Test for the presence of heterochromatin in Nigella gave negative results and it is interpreted that the observed differences between Vicia and Nigella are due to the presence and absence of heterochromatin in their chromosome complements respectively. The role of heterochromatin in the production of chromosome aberrations and its significance in evolution are briefly discussed.

Effects on genetic recombination of Escherichia coli K 12 produced by P-32 decay in the Hfr male.

The effects of P32disintegration in the Hfr donor cell on the genetic recombination ofEscherichia coliK 12 have been studied. The dose-response relationship for the transfer of markers at a distancexfrom the chromosomal origin,N/N0=e−αxD, which summarizes the findings of Fuerstet al.(1956) for early markers of Hfr H, has been found to hold true to a good approximation for more distal selected markers in this strain, as well as for selected markers of Hfr CS, which transfers its chromosome in the opposite direction. The inheritance by recombinants of non-selected male markers located between the origin and the selected marker is not affected at all by prior P32dose to the Hfr. The sensitivity of each Hfr strain studied to the lethal effects of P32decay corresponds to the sensitivity to transfer effects of a male marker located about one-third of the total chromosomal length away from the origin. Genetic and lethal effects in Hfr H showed the same dependence on storage temperature during P32decay. Our data, therefore, supports the concept of a common origin for the lethal and genetic effects of P32decay.The quantitative predictions of current models for chromosome transfer with regard to radiation mating experiments have been examined. A simple model, involving no DNA synthesis, as well as the replicon model of Jacobet al.(1963) predict exponential marker inactivation curves of the kind found experimentally. The model of Bouck & Adelberg (1963) predicts a more complex dose dependence, but one not nearly so different from that of the other two models as was anticipated by Jacobet al.It appears unjustified to attempt to exclude any of the three models on the basis of presently available data from radiation mating experiments.

CHROMOSOMAL POLYMORPHISM IN RATTUS RATTUS (L.) COLLECTED IN KUSUDOMARI AND MISIMA.

Chromosomes of Rattus rattus (L.), collected in Kusudomari (Nagasaki) and Misima (Sizuoka) were examined. The karyotype revealed a remarkable heteromorphism in chromosome no. 1. The homozygotic, i.e. standard type, was characterized by 13 pairs of telocentric and 7 pairs of metacentric chromosomes. Chromosome pair no. 1 was telocentric. X and Y chromosomes were also telocentrics. 18.4 per cent of rats from Kusudomari and 40 per cent from Misima showed heteromorphic pair in chromosome no. 1. One chromosome of the heteromorphic pair is conspicuous by the subtelocentric centromere. Total length of the telocentric chromosome of no. 1 is almost the same as of its subtelocentric partner. These facts indicate that the subtelocentric no. 1 chromosome might have arisen by a centromeric inversion of the telocentric chromosome. Individuals homozygous for the subtelocentric no. 1 chromosome could not be found in either population. The difference in the frequency of the dimorphics collected in Kusudomari and Misima was statistically significant. Possible causes of the difference are discussed.

Studies on some nuclear factors controlling radiation sensitivity and the induced mutation rate in eu- and para-sorghum species

The interaction between the length and number of chromosomes, DNA content and nuclear volume in relation to radio-sensitivity was studied in S. nitidum and S. purpureosericeum, two species with 2n=10 belonging to the Para-Sorghum group and S. vulgare, a cultivated Eu-Sorghum species with 2n=20. The LD 50 dosages for Co 60 gamma- and 50 kV X-irradiation of seeds of S. nitidum were respectively 26,060 and 16,000 r and for S. vulgare were 65,160 and 63,680 r. With regard to radiosensitivity, S. vulgare behaves like a polyploid. The total length of the chromosome complement varied in the ratio 1 : ·16 : 1·33 in S. vulgare, S. nitidum and S. purpureo-sericeum, the ratio of nuclear DNA contents being 1 : 1·70 :1·62. The absence in these three species of any consistent relationship between chromosome length and DNA content may be due to the presence of variable amounts of heterochromatin. S. vulgare and S. purpureo-sericeum had a similar nuclear volume but no well defined proprtionality existed between total chromosome length and nuclear volume. Our results suggest that in these three Sorghum species (i) no direct relationship may exist between chromatin length and DNA content in species characterized by chromosomes having distinct eu- and heterochromatic segments, (ii) no well defined relationship may exist between nuclear volume, chromatin length and DNA content. Nuclear volume alone, therefore, may not not always be a reliable index of radiation sensitivity and (iii) ‘tetraploid’ Sorghum vulgare with smaller chromosomes and lesser DNA content is more resistant to radiation than the diploid Para-Sorghum species with much longer chromosomes and higher DNA value. Phenotypic mutations occurred in S. vulgare but were absent in the Para-Sorghum species. It is suggested that diplontic selection effects may be responsible for this.

Further studies on pachytene pairing failure in maize.

Pairing failure in pachytene chromosomes was studied against a genetically constant background. Very significant negative correlations were found between total chromosome length per cell on the one hand and number of terminal pairing failures and their total extent (per cent length) on the other, and between number of terminal failures and their average extent (per cent length). No significant correlation was found, however, between total chromosome length per cell and the average extent (per cent length) of pairing failure. If the pairing failures are dissociations increasing in extent and number during the pachytene stage studied, the simplest reconciliation of the results requires that the average rate of their extension be roughly proportional to total chromosome length or that certain constants be related to each other as described. An alternate interpretation, that the pairing failures were present before pachytene and that chromosomes are shortening faster in cells containing them, is favored by the findings that heterogeneity is low between anthers in number of failures per cell and that no significant correlation was found between anthers in total chromosome length and number of pairing failures. The possibility is also discussed that the pairing failures are a complex combination of both initial synaptic failure and later dissociation.

Chromosome diminution and evolution of polyploid species in Triticum.

1. The length of the chromosome complements of T. monococcum, Ae. speltoides var. ligustica and Ae. squarrosa, the donors of the A, B and D genomes respectively of T. aestivum, varied in the ratio 1.0 ∶ 1.24 ∶ 0.76. The proportion of the total lengths of the chromosomes belonging to the A, B and D genomes in the bread wheat variety Chinese Spring, as calculated from the data of Sears (1954), also falls into a similar ratio (1.0 ∶ 1.28 ∶ 0.84). 2. T. monococcum, T. dicoccum and T. aestivum differed in the proportion 1.0 ∶ 1.5 ∶ 2.0 as regards chromosome length and not as 1.0 ∶ 2.24 ∶ 3. 0 as would be expected from the additive value of the total chromosome length of the concerned genome donors. 3. The DNA content of individual nuclei in the species studied varied in proportion to chromosome length, thus indicating that the DNA content per unit length of chromosome is constant in the material studied. 4. The type of karyotype asymmetry in the different Triticum and Aegilops species was classified according to the scheme of Stebbins (1958). The diploid species had the most symmetrical and the hexaploids, the most asymmetrical karyotype. No prominent heterochromatic segment could be detected in any of the species. 5. The data indicate that a considerable degree of elimination of chromosomal material has taken place in tetraploid and hexaploid wheats subsequent to their origin. It is suggested that this might have been an important factor in the conversion of tetraploid and hexaploid wheats into functional diploids. A combination of the multivalent gene suppressor system and chromosome elimination appears to have led to a synthesis of the advantageous features of auto- and allo-polyploidy in tetraploid and hexaploid wheats.

The desoxyribonucleic acid content of the nucleus as a cytotaxonomic character in mantids (Orthorptera: Mantoidea).

1. The relative amount of DNA per spermatid nucleus has been determined by photometric microscopic measurement in eight species of mantids, and the results are discussed in relation to their cytology and the cytotaxonomic problems they present. The mantids studied are the following: Subfamily Liturgousinae—Liturgousa maya S. & Z., L. actuosa Rehn, L. cursor Rehn; Subfamily Choeradodinae —Choeradodis rhombicollis Latr.; Subfamily Manteinae—Tauromantis championi S. & Z., Stagmomantis carolina Johann, S. heterogamia S. & Z.; and Subfamily Pseudomiopteriginae—Pseudomiopteryx infuscata S. & Z.2. Of the three closely similar species of Liturgousa tested, L. maya with 16 autosomes and L. cursor with 32 have the same content of DNA in the spermatid nucleus, confirming the cytological evidence that no polyploid relation exists between them. The greater diameter and greater total length of metaphase chromosomes in L. maya as compared with L. cursor remain unexplained.3. Liturgousa actuosa has approximately one and ...