The identification of the nucleolus organiser chromosomes of diploid wheat

The two nucleolus organiser chromosomes of diploid wheat are identified as 1A and 5A by the combination of in situ hybridisation and cytological markers.

Interspecific variation in C-banded chromosomes of diploid Aegilops species

Based on an improved C-banding technique, the C-banding patterns of all 11 diploid Aegilops species were described and compared. All diploid species exhibit characteristically different patterns which enable the chromosomes of any complement to be identified individually. These patterns confirm existing genome symbols and provide further evidence for the suggested changes in genome symbols of Ae. umbellulata and Ae. sharonensis, U and S(sh) respectively. Furthermore, Ae. uniaristata should be given a separate symbol, probably N. Aegilops speltoides and Ae. sharonensis could be possible donors to the B genome of wheat. Interspecific divergence in these diploid species has been accompanied by either amplification or deletion as well as massive repatterning of heterochromatin from the centromere to the telomere.

The nucleolar organisers of tetraploid and hexaploid wheats revealed by in situ hybridisation

The technique of in situ hybridisation of cloned ribosomal DNA has been used to establish the numbers of nucleolar organising sites in a range of tetraploid and hexaploid wheats.

Investigation of a preferentially transmitted Aegilops sharonensis chromosome in wheat

An attempt to produce a set of addition lines of Aegilops sharonensis to the wheat variety 'Chinese Spring' produced only one addition line. This was due to preferential transmission of one chromosome from Ae. sharonensis. This chromosome was studied in detail by established cytological methods of chromosome observation and by the newer techniques of C-banding and in situ hybridization of a cloned DNA sequence. The chromosome was found to be partially homologous to an Ae. sharonensis chromosome of similar behaviour in another wheat addition line. The incomplete homology of the two Ae. sharonensis chromosomes was due to the presence of a translocated segment of a wheat chromosome. - Substitution lines of the Ae. sharonensis chromosome for wheat homoeologous group 4 were produced and the Ae. sharonensis chromosome thereby designated 4 S (l) .

Comparison of the chromosomes of Triticum timopheevi with related wheats using the techniques of C-banding and in situ hybridization

The chromosomes of the tetraploid wheats Triticum timopheevi (Genome AAGG) and T. araraticum (Genome AAGG) were C-banded at mitosis. The identity of the banded and unbanded chromosomes was then established by firstly making comparisons with the hexaploid species T. zhukovskyi which has the genome formula AAAAGG. Secondly, the meiotic pairing in F1 hybrids between T. timopheevi and diploid wheats was examined by means of C-banding. The results showed that the banded chromosomes belonged to the G genome, while the unbanded chromosomes belonged to the A genome. Only one of the two pairs of satellited chromosomes had strong heterochromatic bands. The relationship between the genomes of T. timopheevi and T. dicoccum (Genome AABB) was then assessed at meiosis in hybrids between these species, using the techniques of C-banding and in situ hybridisation of a cloned ribosomal RNA gene probe. It was concluded that there were differences both in the amount and distribution of heterochromatin and also translocation differences between the species.

Constraints upon the composition of supplementary DNA

Speciation in eukaryotes is often accompanied by massive changes in nuclear DNA amount resulting from the accumulation or deletion of DNA base sequences within chromosomes. Our evidence shows that, among related species, the DNA lost or gained during divergence is of a remarkably consistent and restricted composition in respect of the ratio of repetitive to non-repetitive base sequences. It is argued that the restriction may in part at least be imposed by natural selection, in the sense that the only changes tolerable are confined to DNA fractions of particular composition and organisation.

The effects of three levels of auditory masking on selected vocal characteristics and the frequency of disfluency of adult stutterers

This study was designed to test the hypothesis that the growth function of the masking effect in stuttering behavior is promoted by systematic increases in vocal intensity. Sixteen normal-hearing adult stutterers read aloud in four conditions (quiet and 10-, 50-, and 90-dB sensation levels of continuous white noise). The oral readings were tape recorded. The recorded data were analyzed later to quantify subjects' vocal intensity, stuttering frequency, and reading time in each condition. The results showed that under various levels of masking, there is an inverse relationship between vocal intensity and stuttering frequency. The results of reading time comparisons between conditions were affected by the absolute frequency of stuttering in quiet or the magnitude of the masking effect or both. Finally, changes in vocal intensity were translated into physiological terms and then interpreted in light of how they might promote fluency and reduce stuttering.