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RNA directed DNA methylation in Arabidopsis thaliana
Motylová, Šárka ; Fischer, Lukáš (advisor) ; Moravec, Tomáš (referee)
The differential transcriptional activity of the genome is provided by epigenetic modifications, which include DNA methylation, alteration of histone N-terminal amino acids and changes in histone variants. RNA interference is a regulatory process, in which transcriptional or post-transcriptional silencing of exogenous or endogenous sequences is mediated by the action of small RNAs derived from these sequences. The 24-nucleotide siRNAs, forming a fraction of small RNAs, direct de novo DNA methylation and participate in the maintenance of DNA methylation (RNA-directed DNA methylation; RdDM), which facilitates transcriptional silencing of heterochromatin and transposable elements representing a large part of plant genomes. The presence of two RNA polymerases involved in this pathway is characteristic for flowering plants, which were discovered for the first time in the genome of Arabidopsis thaliana, which has also become the main plant model for the study of RdDM. Polymerase IV transcribes siRNA precursors; siRNAs are subsequently associated with AGO4 proteins and guide methylation enzymes to the target sequences via complementarity with polymerase V transcripts.
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Evolution of selected karyotype characters in tetrapulmonate arachnids
Jílková, Klára ; Král, Jiří (advisor) ; Marec, František (referee)
The class Arachnida is not thoroughly explored from the cytogenetic point of view. Previous studies suggest a high diversity of karyotypes and sex determination in arachnids. This study deals with the evolution of sex chomosomes, nucleolar organizer regions (NOR), and telomeric repeats in the tetrapulmonate clade of arachnids, particularly in groups of ancient origin. Sex chromosomes were detected in two orders. Detection of NORs in a large set of species supports the hypothesis that the ancestral karyotype of arachnids contained NOR on one pair of autosomes only. The number of NORs has increased during the evolution of some groups of Pedipalpi. The NORs are located in terminal or subterminal chromosomal regions in most tetrapulmonates. The occurrence of the "insect" telomeric motif was confirmed in majority of tetrapulmonates. Interstitital telomeric repeats were not detected with the exception of one species. Keywords: arachnids, meiosis, sex chromosomes, telomeres, nucleolar organizer, heterochromatin
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Evolution of karyotype and sex determination in iguanas (Squamata: Pleurodonta)
Altmanová, Marie ; Kratochvíl, Lukáš (advisor) ; Zrzavá, Magda (referee)
Squamate reptiles in general possess an unusual variability in karyotype and sex determining mechanisms. However, in these two aspects, iguanas (Pleurodonts) are considered as a relatively conservative group of lizards. So far only genotypic sex determination with male heterogamety has been detected in this lineage. However, the sex chromosomes have not been revealed in many species by classical cytogenetics, probably due to their homomorphy. Significant variability in karyotype was observed only in the species-rich genera Anolis, Sceloporus and Liolaemus. The aim of this thesis was to investigate the genome of available species from the main iguanid lineages using classical and molecular cytogenetic methods. As well as finding the karyotype characteristics, which may represent synapomorphies of main iguanid lineages, the other aim was to try to identify sex chromosomes. This study confirmed previously published karyotypes of 13 species and established new descriptions of karyotype for eight species. The chromosomes of all studied specimens were examined by methods of classical and molecular cytogenetics, 21 species covering eight iguanid families were analysed in this thesis. The majority of studied species shared the apparently ancestral karyotype of the group, with diploid chromosome number...
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Yeast gene silencing.
Tarabová, Eva ; Kuthan, Martin (advisor) ; Schierová, Michaela (referee)
Each cell contains a complete copy of the entire genetic equipment of the organism. However not all genes are expresed, cells are differentiated in higher eukaryots and only certain proteins are transcribed in each cell. This is possible thanks to a gene silencing, that is stable throughout the whole cell cycle and epigeneticaly inherited from one generation to another. Gene silencing serves also in the maintainance of the chromosomal integrity, it is connected with the right progression of the cell division. It even enables mating type switching and ensures right cells' identity in yeasts. The basis is compact and a higher-ordered structure of chromatin called heterochromatin. The mechanism is common to many various organisms, although the proteins, which ensure silencing, are different.
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Karyotype analysis of selected representatives of two pedipalpid orders, Amblypygi and Uropygi
Sember, Alexandr ; Král, Jiří (advisor) ; Ráb, Petr (referee)
Karyotype analysis of selected species from arachnid orders Amblypygi and Uropygi Whip spiders (Amblypygi) and whip scorpions (Uropygi) represent relict arachnid orders which has been found already at Upper Carboniferous strata. Although cytogenetic data from amblypygids and uropygids might be important to reconstruct karyotype evolution of arachnids, cytogenetics of these orders is almost unknown. Presented study is aimed in analysis of karyotype and meiosis in 16 species of Amblypygi and 4 species of Uropygi. Both groups are characterized by considerable range of diploid chromosome numbers (2n = 24 - 86 in Amblypygi and 36 - 66 in Uropygi). Analysed species does not exhibit morfologically differentiated sex chromosomes. Differentiation of sex chromosomes on molecular level was revealed in amblypygid Paraphrynus mexicanus by comparative genome hybridization. Obtained data indicate XY/XX sex chromosome system in this species. Comparison of karyotype data indicates reduction of chromosome numbers during evolution of both orders. In Amblypygi, this reduction was accompanied by increase of number of biarmed chromosomes. This trend is not apparent in Uropygi. Karyotypes of most analysed amblypygids and uropygids are also characterized by low amount of heterochromatin. Most studied species exhibit two pairs...
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Rapid turnover of the W chromosome in geographical populations of wild silkmoths, Samia cynthia ssp.
ŠÍCHOVÁ, Jindra
Geographical subspecies of wild silkmoths, Samia cynthia ssp. (Lepidoptera: Saturniidae) exhibit a unique polymorphism in chromosome numbers, resulting from variations in the sex chromosome systems. Three different sex chromosome constitutions were identified: Z0/ZZ in S. c. ricini (2n=27/28), neo-Wneo-Z/neo-Zneo-Z in S. c. walkeri (2n=26/26), and neo-WZ1Z2/Z1Z1Z2Z2 in S. cynthia subsp. indet. (2n=25/26). It has been proposed, that the common ancestor of S. cynthia subspecies had a classical WZ/ZZ constitution with diploid chromosome number of 2n=28/28 and the neo-sex chromosomes in S. c. walkeri and S. cynthia subsp. indet. arose by repeated sex chromosome-autosome fusions. Our sampling effort enabled us to examine five populations of S. cynthia with an ancestral-like karyotype with the aim to verify a hypothesis about sex chromosome evolution in this species complex. Obtained results suggest that the curious WZ system of S. cynthia pryeri may represent an ancestral state of the Samia species complex. However, they do not exclude an alternative hypothesis of its derived origine.
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