Meiotic behavior and fertility of Capsicum interspecific hybrids

ABSTRACT This study was performed to evaluate the meiotic behavior and fertility of four hybrids obtained from the crossing of Capsicum baccatum and its botanical forms with Capsicum chinense and Capsicum frutescens. We aimed to identify the causes of the low fertility in interspecific hybrids involving species from different gene pools of Capsicum. Hybrid flower buds were fixed in fixative solution and the slides were prepared using 1%-acetic carmine solution. Meiosis was observed in all phases and the main meiotic abnormality observed was the complete or partial lack of chromosome pairing, indicating that the species are either genetically distant or that some asynaptic gene was present in the hybrids. Meiotic index (MI), the frequency of unreduced gametes (type 2n), and pollen viability were estimated. MI and pollen viability of the hybrids were low. We concluded that due to the complete or partial lack of chromosome pairing, the species used in the hybrid combinations are genetically distant. We also noticed that the low-fertility hybrids are a consequence of the lack of chromosome homology between the two genomes involved. The hybrids were considered partially sterile since their pollen viability percentage was lower than 50%.

A new meiotic abnormality in Zea mays: multiple spindles associated with abnormal cytokinesis in both divisions

We report here a new meiotic abnormality recorded in one plant of an inbred line of Zea mays. After an apparently normal prophase I, chromosomes did not congregate in a single metaphase plate. Bivalents remained scattered in the cytoplasm, giving rise to several spindles. Despite the occurrence of multiple spindles, meiosis I proceeded normally, forming a varied number of nuclei at telophase I. The presence of one or a few chromosomes in the nucleus was enough to induce local cytokinesis, which was evident from metaphase I. Each cell resulting from meiosis I expressed its own program and progressed through the cell cycle. Therefore, failure of chromosome congregation on a single plate also occurred at meiosis II, where further irregular cytokinesis was observed. As a consequence of the two abnormalities, polyads occurred, resulting in pollen grains of different sizes and in sterility at a frequency of up to 93.5%.Key words: maize, meiosis, cytokinesis, multiple spindles, polyads.

Inheritance of a meiotic abnormality that causes the ovulation of primary oocytes and the production of digynic triploid mice

SummaryPrevious studies have demonstrated that the LT/SvKau strain of mice ovulates a high proportion of oocytes as diploid primary oocytes rather than secondary oocytes. These ovulated primary oocytes are arrested at meiotic metaphase I but may be fertilized to produce digynic triploid embryos. In the present study, 40·4% of eggs analysed from LT/SvKau females were ovulated as primary oocytes, compared to 1·2% from control C57BL/Ws strain mothers. These two inbred strains were intercrossed to produce eight sets of Fl, F2 and backcross females and the frequency of triploidy was investigated. The results are compatible with segregation of a co-dominant, autosomal gene that has a major influence on the incidence of triploidy. We suggest that the provisional gene symbol Poo (primary oocyte ovulation) be assigned to this gene, with alleles Pool (the ‘mutant’ allele present in the LT/SvKau strain) and Poob (the normal allele present in C57BL/Ws mice). Poo is incompletely penetrant and has variable expressivity because the proportion of oocytes ovulated as primary oocytes by LT/SvKau mice was variable and, in some cases, nil. In putative Pool/Poob heterozygotes the frequency of ovulated primary oocytes was increased only marginally (from 1·2% to 66%) by the presence of one copy of the Pool allele, but this increase was found consistently (in two reciprocal Fl crosses) and was statistically significant. No evidence was found for tight genetic linkage between Poo and two Mendelian loci (brown on chromosome 4 and glucose phosphate isomerase on chromosome 7), that were segregating in the crosses. The Pool mutant in the LT/SvKau strain of mice provides a valuable resource to study the cell and molecular biology of mammalian oocyte maturation and the control of meiosis.

Use of 4x–2x crosses to determine gene–centromere map distances of isozyme loci in Solanum species

With the meiotic abnormality described as the parallel spindles mechanism operating during anaphase II, first division type restitution 2n gametes (FDR) were identified in selected diploid Solanum clones. Generating 4x–2x FDR testcrosses of the general form aaaa × Aa with these diploid parents provided a means of estimating gene–centromere linkages with the application of half-tetrad analysis to the segregating tetraploid progenies. In this study, gene–centromere map distances were obtained for 10 isozyme loci and the yellow tuber flesh locus (Y). Electrophoretic assay of the tetraploid progenies yielded pooled gene–centromere linkage estimates for Got-1 (0.9 map units, mu), Pgm-2 (2.0 mu), Sdh-1 (8.3 mu), Aps-1 (13.5 mu), Adh-1 (15.8 mu), Prx-3 (18.0 mu), Idh-1 (18.4 mu), Pgi-1 (26.0 mu), 6-Pgdh-3 (30.1 mu), Mdh-1 (33.5 mu), and y (16.8 mu). These linkage data were combined with previously reported diploid testcross linkage data to show that these 11 markers are distributed over at least 8 of the 24 chromosome arms in the potato genome. In addition, gene order for the Idh-1/Sdh-1 linkage block (36.8 mu) was predicted to be of the sequence Id1-1 –centromere–Sdh-1, which is similarly found in Lycopersicon. Because of the codominant nature of these enzyme-coding loci, the theoretical expectations for 4x–2x FDR segregation were tested in this crop. Tests for reciprocity of exchange products of pooled data for 6-Pgdh-3, Mdh-1, Pgi-1, Adh-1, and Sdh-1 families yielded good fits to the expected 1:1 ratio. With these defined electrophoretic markers available, more critical genetic analyses can be extended to a number of genetic and breeding problems concerning the potato. Key words: half-tetrad analysis, 4x–2x, isozymes, Solanum, linkages.