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Vliv polyploidizace a hybridizace na fyziologii reprodukce u ryb
TICHOPÁD, Tomáš
Sexual reproduction is one of the most common traits of eukaryotes and one of the most debated topics in biology. It involves complicated gene regulatory networks coordinating meiotic division, recombination, and the production of haploid gametes. Although these pathways are highly conserved, they have been repeatedly modified in many ways throughout the evolution of life forms. Interspecific hybridization and polyploidization are well known to alter the reproductive potential of an individual, and both phenomena are commonly linked with the so-called asexuality, i.e., the production of (partly) clonal gametes. Asexuality has attracted considerable attention as an excellent model to study the evolutionary dis/advantages of sex. Because of numerous associations with polyploidy, asexuals have also been considered as potential missing links in the formation of polyploid species. Nevertheless, asexual eukaryotic lineages are not an easily definable group. They are distributed throughout the tree of life and employ a broad spectrum of independently arisen mechanisms for gamete production. These mechanisms can noticeably differ even between closely related taxa, ranging from entirely ameiotic processes to those involving distorted meiotic divisions, which have different evolutionary consequences for each asexual lineage. It is therefore vital to understand whether there are any general rules followed in the processes leading to the abandonment of sexual reproduction or induction of sterility. This thesis investigates the effect of hybridization and polyploidy on reproductive modifications in several fish taxa. The first investigation was focused on the regulation of unreduced gamete production in asexual allotriploid females of Cobitis elongatoides-taenia-taenia. Naturally, it is accomplished via premeiotic endoreduplication (PMER) and followed by the pairing of sister chromatids. It has been discovered that the regulation of PMER is a female's exclusive trait, but the PMER execution depends on germ cell hybrid origin. Therefore, the asexuality phenotype in Cobitis is linked to phenotypic sex differentiation. Secondly, the spermatozoa characteristics of F1 hybrids of common carp and gibel carp were examined to evaluate morphological malfunctions. Males showed classical hybrid dysgenesis as their reproductive potential was negatively affected by the hybridization event, probably due to the high divergence between species. Although most of the spermatozoa were aneuploid or polyploid, a small fraction of spermatozoa was capable of fertilization and gave rise to F2 hybrid offspring. Thirdly, germ cells of ancestral hybrid Poecilia formosa were examined to confirm the PMER pathway; however, it was discovered that this species only initiates first meiotic genes, but meiosis itself is skipped. Therefore, P. formosa represents a rare exception to apomixis in fish taxa. Finally, a successful and effective protocol for triploid zebrafish production was established. Although the primary goal of this protocol was the establishment of sterile recipients for transplantation purposes, it represents an excellent opportunity for research of autopolyploidization effect on sex differentiation as all produced triploid zebrafish were confirmed as males.
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