Karyotypes of the polyploid parthenogenetic species Saga pedo from four localities in France and the Republic of Macedonia were constructed and compared. All these karyotypes consist of 70 chromosomes, which is more than twice that in other species of the genus. The chromosomes differ from each other, making the matching of homologues difficult. Karyotypes of French specimens are similar, except for differences in the heterochromatin. Compared to that of the Macedonian specimens those from French specimens differ by the shortening of a single chromosome. The difficulty experienced in identifying tetrads and even pairs of chromosomes indicates that either many chromosome rearrangements have occurred since the polyploidisation event(s) or that the addition of quite different genomes is the cause. On the other hand, that the karyotypes are similar indicates a common origin of both the Macedonian and French populations. Thus, most chromosome changes preceded the separation from their common ancestor. Both the DNA content and chromosome analyses suggest that the S. pedo karyotype is pentaploid and not tetraploid as previously proposed. This odd ploidy number rules out the hypothesis that it could only have originated by endoreduplication. It is more likely that it originated by the association of five copies of the 14,X haploid karyotype, which exists in the gametes of the closely related species, S. campbelli and S. rammei (male / female 2n = 27, X / 28, XX). Fertilization of a parthenogenetic 56, XXXX female by a 14, X spermatozoa could have resulted in the last increase in ploidy.
It is known that chromosomes occupy non-random positions in the cell nucleus. However, it is not clear to what extent their nuclear positions, together with their neighborhood, are conserved in daughter cells. To address specific aspects of this problem, we used the model of the chromosomes carrying ribosomal genes that are organized in clusters termed Nucleolus Organizer Regions (NORs). We compared the association of chosen NOR-bearing chromosomes (NOR-chromosomes) with nucleoli, as well as the numbers of nucleoli, in the pairs of daughter cells, and established how frequently the daughter cells had equal numbers of the homologs of certain NOR-chromosomes associated with individual nucleoli. The daughter cells typically had different numbers of nucleoli. At the same time, using immuno-FISH with probes for chromosomes 14 and 15 in HeLa cells, we found that the cell pairs with identical combinations appeared significantly more frequently than predicted by the random model. Thus, although the total number of chromosomes associated with nucleoli is variable, our data indicate that the position of the NOR-bearing chromosomes in relation to nucleoli is partly conserved through mitosis., M. Kalmárová, E. Smirnov, L. Kováčik, A. Popov, I. Raška., and Obsahuje bibliografii a bibliografické odkazy