Somatic pairing, endomitosis and chromosome aberrations in snakes (Viperidae and Colubridae)

The positioning of macrochromosomes of Bothrops jararaca and Bothrops insularis (Viperidae) was studied in undistorted radial metaphases of uncultured cells (spermatogonia and oogonia) not subjected to spindle inhibitors. Colchicinized metaphases from uncultured (spleen and intestine) and cultured tissues (blood) were also analyzed. We report two antagonic non-random chromosome arrangements in untreated premeiotic cells: the parallel configuration with homologue chromosomes associated side by side in the metaphase plate and the antiparallel configuration having homologue chromosomes with antipolar distribution in the metaphase ring. The antiparallel aspect also appeared in colchicinized cells. The spatial chromosome arrangement in both configurations is groupal size-dependent and maintained through meiosis. We also describe, in untreated gonia cells, endomitosis followed by reductional mitosis which restores the diploid number. In B. jararaca males we observed that some gonad regions present changes in the meiotic mechanism. In this case, endoreduplicated cells segregate the diplochromosomes to opposite poles forming directly endoreduplicated second metaphases of meiosis with the suppression of first meiosis. By a successive division, these cells form nuclei with one set of chromosomes. Chromosome doubling in oogonia is known in hybrid species and in parthenogenetic salamanders and lizards. This species also presented chromosome rearrangements leading to aneuploidies in mitosis and meiosis. It is suggested that somatic pairing, endomitosis, meiotic alterations, and chromosomal aberrations can be correlated processes. Similar aspects of nuclei configurations, endomitosis and reductional mitosis were found in other Viperidae and Colubridae species.

Chromosome order in HeLa cells changes during mitosis and early G1, but is stably maintained during subsequent interphase stages

Whether chromosomes maintain their nuclear positions during interphase and from one cell cycle to the next has been controversially discussed. To address this question, we performed long-term live-cell studies using a HeLa cell line with GFP-tagged chromatin. Positional changes of the intensity gravity centers of fluorescently labeled chromosome territories (CTs) on the order of several μm were observed in early G1, suggesting a role of CT mobility in establishing interphase nuclear architecture. Thereafter, the positions were highly constrained within a range of ∼1 μm until the end of G2. To analyze possible changes of chromosome arrangements from one cell cycle to the next, nuclei were photobleached in G2 maintaining a contiguous zone of unbleached chromatin at one nuclear pole. This zone was stably preserved until the onset of prophase, whereas the contiguity of unbleached chromosome segments was lost to a variable extent, when the metaphase plate was formed. Accordingly, chromatin patterns observed in daughter nuclei differed significantly from the mother cell nucleus. We conclude that CT arrangements were stably maintained from mid G1 to late G2/early prophase, whereas major changes of CT neighborhoods occurred from one cell cycle to the next. The variability of CT neighborhoods during clonal growth was further confirmed by chromosome painting experiments.

Dynamics of gametic disequilibria between loci linked to chromosome inversions: the recombination-redistributing effect of inversions

SummaryThe total gametic disequilibrium between two loci linked to polymorphic inversions can be partitioned into two types of components: within and between chromosome arrangements. The within components depend on the gametic disequilibrium within each chromosome arrangement. The between components depend on the locus-inversion disequilibria. This partitioning has practical applications and is indispensable for studying the dynamics of these systems because inversions greatly reduce recombination in the heterokaryotypes while allowing free, and sometimes different, recombination in each of the homokaryotypes. We provide equations for the per generation change of the various disequilibria for systems with two and three chromosome arrangements, and the general recursive equations predicting the disequilibria after any number of generations for the case of two arrangements. Simulation studies were carried out using different values of the recombination parameters and all possible initial conditions. The results show a complex convergence to linkage equilibrium in inversion systems. The various disequilibria can have local maxima and minima while approaching equilibrium and, moreover, their dynamics cannot be described, in general, using a single parameter, i.e. an effective recombination rate. We conclude that the effects of inversions on gametic disequilibria must be carefully considered when dealing with disequilibriain inversion systems. The formulae provided in this paper can be used for such purpose.

Changes in relative fitness with temperature among second chromosome arrangements in Drosophila melanogaster.

Development time and body weight of In(2L)t, R (a putative short inversion on the left arm of the second chromosome) and ST (standard) karyotypes of Drosophila melanogaster were measured at different temperatures. Frequency changes were followed in populations polymorphic for In(2L)t and ST and kept under different environmental conditions. These experiments were carried out in order to explain the worldwide latitudinal clines for In(2L)t and other inversions. To avoid interactions with the Adh and alpha Gpdh loci, which also have latitudinal clines, all karyotypes were homozygous AdhS alpha GpdhF. In(2L)t homokaryotypes had a longer development time and a lower weight than the other karyotypes at all temperatures. R/ST heterokaryotypes had the shortest development time and ST/ST had the smallest weight decrease with increasing temperature. The differences among the In(2L)t and ST karyotypes in development time were further analyzed in an experiment where the age at which 50% of the larvae were able to become adults, without further food ingestion, was determined. In polymorphic populations at 20 degrees and 25 degrees a significant decline of In(2L)t frequencies was observed. At 29.5 degrees and 33 degrees there was no change in In(2L)t frequencies but a significant excess of heterokaryotypes occurred. On ethanol-supplemented food the most drastic decline in In(2L)t frequency was observed. Populations transferred at 2- and 3-week intervals at 25 degrees exhibited large differences in final In(2L)t frequencies. The frequency changes could in part be attributed to the differences in development time and to previously observed differences in high temperature resistance. The experiments prove that the karyotypes are under selection. The results are discussed in relation to the geographic distribution of In(2L)t.