Spontaneous structural rearrangements in Solanum phureja Juz. et Buk. 2. Meiotic behaviour and identification of interchange chromosomes using primary trisomics

Meiosis was studied in two diploid (2n = 2x = 24) siblings of Solanum phureja Juz. et Buk. and in 11 disomic and 2 trisomic descendants. The diploid siblings carry the same heterozygous interchange and either one or two inversions. The frequency of quadrivalents at diakinesis/metaphase I in these clones was 0.56 and 0.62 per pollen mother cell. In two plants from the first inbred generation (I1) this frequency was about the same but in some other I1 plants and a full sib the frequency was substantially lower, varying from 0.00 to 0.16. Most quadrivalents, 78–83%, were rings. A variety of quadrivalent configurations at diakinesis and metaphase I was observed, giving rise to balanced and unbalanced gametes. The absence of ring quadrivalents in trisomic descendants of one of the siblings implied that tertiary trisomics or primaries being homozygous for the interchange were present in the I1 generation. Regular chromosome distribution (12–12) at anaphase I occurred in 46.5 and 73.2% of the pollen mother cells studied in the two original clones. Irregularities, such as 11–13 distribution, lagging chromosomes, and a bridge and fragment, were detected on average in 2.7, 3.3, and 32.5%, respectively, of the anaphase I cells analysed. In hybrids from crosses between 6 primary trisomics as females with the interchange heterozygote, the involvement in the interchange of chromosomes 3 and 12 was clearly demonstrated.Key words: Solanum phureja, interchange heterozygote, chromosome identification, interchange trisomic, meiosis.

Cytogenetical studies in Rhoeo spathacea (Commelinaceae). I. A desynaptic and second division restitution mutant

A mutant causing partial desynapsis (dissociation of paired chromosomes), and consequently a high frequency of univalents at metaphase 1, was found in Rhoeo spathacea among the selfed progeny of a wild-collected ring-forming complex interchange heterozygote. All the plants were diploid, 2n = 2x = 12. The mutant formed univalents in all microsporocytes (range 2–12; average 7.56/cell), and 23.69% of the pollen mother cells contained the maximum of 12 univalents at metaphase I. There was no significant difference in pollen fertility between the mutant and parental plants. The desynaptic mutant produced microspores with chromosome numbers of n = 6–14 in the same anther. Unreduced pollen was formed at telophase II by second division restitution and comprised 52.74% of the pollen grains. By selfing, diplandrogynous tetraploid progeny (2n = 4x = 24) of 12 seedlings were obtained. The results demonstrate that both male and female gametes are unreduced.Key words: Rhoeo, desynapsis, univalent, second division restitution, tetraploid.

Differential mitotic behaviour of genetically balanced and unbalanced microspores of an interchange heterozygote of Allium triquetrum

Genetically balanced and unbalanced microspores of an interchange heterozygote of Allium triquetrum are shown to behave differently with respect to their first pollen grain mitosis (PGMI). Two types of unbalanced microspores abort immediately prior to PGMI, while another two types are delayed in their progression through PGMI. The data presented have relevance to transmission potentials of unbalanced postmeiotic cells and estimates of their frequencies in interchange (reciprocal translocation) heterozygotes.Key words: differential behaviour, unbalanced cells, pollen grain mitosis, interchange heterozygosity.

Metaphase position and orientation of an interchange quadrivalent of Allium triquetrum: II. Changes in position with changes in percentage alternate orientation

SummaryThe position and orientation of an interchange quadrivalent in flattened lateral views of metaphase I were studied in inflorescences of an interchange heterozygote of Allium triquetrum showing high, medium and low alternate orientation frequencies. As the overall frequency of alternate orientation changes in these different inflorescences so the positioning of the alternately oriented quadrivalents changes, in that it becomes more like that of the adjacent quadrivalents. The present data argue for the biological significance of the position/orientation data, and imply the presence of two components of alternately oriented quadrivalents with different positional characteristics.

Cytogenetics of a semidwarf phenotype in pearl millet, Pennisetum americanum (L.) Leeke

A semidwarf phenotype designated TLD (translocation dwarf) was isolated as a segregant in the selfed progeny of a spontaneously produced interchange heterozygote in the line IP 457 of pearl millet. The mutant is characterised by dark green and erect narrow leaves, early maturity, and a sterile spike tip. Chlorophyll content per unit area of leaf in the mutant is about 29% more than in the control. Results of cytogenetic evaluation of the semidwarf showed that the mutant phenotype is an interchange homozygote. The homozygosity for the interchanged chromosomes and the mutant phenotype are tightly linked; the chromosomes involved are identified as chromosomes 3 and 6. The phenotype is inherited as monogenic recessive. There is normal viability of gametes carrying interchange chromosomes attended by normal viability of zygotes of the three genotypic classes. The potential use of this phenotypic marker as a tester in genetic and breeding experiments concerning the improvement of this crop is stressed.Key words: Pennisetum, pearl millet, translocation, dwarf.

Position and orientation in the metaphase equator of an interchange quadrivalent of Allium triquetrum

SUMMARYThe position and orientation of an interchange quadrivalent in flattened, lateral views of metaphase I were studied in pollen-mother-cells of an interchange heterozygote of Allium triquetrum. The quadrivalent is most often located in marginal rather than central positions in the equator. Moreover, when positioned marginally the quadrivalent is more often than expected found in adjacent orientation, whilst when positioned centrally it is more often than expected found in alternate orientation. Consequently, the frequency of alternate (genetically balanced) orientation in the quadrivalent varies sharply according to whether data are obtained from more marginal or more central positions in the metaphase plate.