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Entelegynae are the most specious group of spiders. Their karyotypes are characterized by the predominance of X1X20 sex chromosome system and exclusively acrocentric morphology of chromosomes. Mono-armed chromosomes can be the substrate for centric fusion, the rearrangement, in which two acrocentric chromosomes fuse at their centromeric regions and form single bi-armed chromosome. In Entelegynae, centric fusion events followed the "all or nothing" rule, where all acrocentric chromosomes fuse during one event or in a very rapid sequence and karyotype is fully saturated by bi-armed chromosomes. Although the cytogenetics of Entelegynae is relatively well studied, so far, "all or nothing" phenomenon has been observed only rarely, namely in 12 species from 5 families. In this thesis, new data about the occurrence of centric fusion in 12 species from 5 unrelated families are reported. In all cases the "all or nothing" rule was confirmed. Strong female meiotic drive is proposed as the main force responsible for changes in chromosomal morphology of Entelegynae. The persisting of bi-armed chromosomes is likely to be rare in Entelegynae. However, in the family Dictynidae, bi-armed chromosomes are fixed. For a better understanding of karyotype dynamics in this unique family, molecular cytogenetic approaches...

Chromosome aberrations play an important role in human pathologenesis as well as in evolution of many organisms. Robertsonian translocation or centric fusion are the most frequent chromosomal rearrangements in mammals. The most investigated model for their research is house mouse Mus musculus. The standard karyotype of a mouse consists from 40 acrocentric chromosomes, notably there are more than one hundred wild populations that are characterized by various combination of metacentrics, formed by centric fusion of different acrocentrics. Several models have been proposed for the formation of metacentrics, taking account of the sequential structure of their centromeres and short arms in mouse. Segregation distortion in favor of metacentrics is present in female and/or male meiosis, such as meiotic drive can positively affect the degree of fixation of metacentric chromosomes in certain population. Centric fusions have been also presented in evolution of other organisms. Cytogenetic data in spiders indicate the possibility of similar phenomena in the most diversified group of spiders, Entelegynae. Spiders are notable for their unusual sex chromosome systems. Centric fusions also play an important role in the evolution of sex chromosomes and the formation of neo-sex chromosome systems. So far, there is...

The aim of this study is a summary of existing knowledge about skewed sex ratio in birds. This controversial topic represents one of the most important questions of evolutionary ecology. We still miss consistence in finding ultimate causes just as proximate mechanisms although there was an expansion of experimental studies already in the last decade of the 20th century which was caused by an invention of the new technology in sex determination. On the other hand, there are many convincing proofs of skewed primary, secondary, tertiary and even quartenary sex ratio so the facultative manipulation or obligatory sex ratio bias is presumable. My Bachelor's thesis compares studies focused on the similar ultimate causes and tries to point out their deficiencies together with contributions to research of this topic. Observation of sex ratio includes many specifics which are mentioned in theoretical introduction. The second part offers potencial proximate mechanisms which could be responsible for sex ratio manipulation. Within the analysis of studies I found sex ratio manipulation of the first egg in clutch as common trend which could fit to selective ovum resorption as a proximate mechanism.