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Functional and evolutionary aspects of the supernumerary chromosomes in fish based on the Astyanax model (Teleostei, Caraciformes), revealed by genomic and structural analysis


B chromosomes are enigmatic structural elements in the genome of some organisms. Interestingly, these chromosomes do not recombine with complement A chromosomes and thus followed a different path during evolution, carrying mainly redundant genetic information already present on standard complement chromosomes, in addition to regions of repetitive sequences. In fish populations of the Astyanax genus, several types of B chromosomes are found in relation to size and morphology, with the metacentric variant being the most recurrent in several species. Due to its morphological similarity and the sharing of repetitive DNA sequences, it was proposed that this variant would have a common origin in the carrier species in this group. However, the conclusions obtained from the analysis of repetitive DNA sequences are fragile, since this type of sequence has a fast evolution and a high dispersion rate in chromosomes. Regarding the effects of B chromosomes in Astyanax, it has been shown that these elements are associated with greater expression of the dmrt1 gene in maturing male gonads. Despite the existence of several cytogenetic and structural studies already carried out on these elements in Astyanax, little is known about the functional activity of some genes present and identified in these elements, usually involved in controlling the cell cycle, as well as about their rate of transmission to offspring, which could give indications about its function and role played in the evolutionary stage of populations. In order to test the hypothesis of common origin of B chromosomes in Astyanax, the presence of genes encoding proteins in these elements will be investigated in some species of this genus. In addition, the expression of genes found in individuals with (+B) and non-carriers (0B) and the possible expression of these genes from sequences present exclusively on the B chromosome will be investigated, to verify patterns of activity in this type of chromosome. It will be checked whether the possible expression of these genes translates into functional proteins, and analyzes will be carried out to identify differences in the overall protein profile in individuals +B and 0B. Also, the analysis of the transmission rate of this genomic element in a population of A. paranae chosen as a model will make it possible to identify cellular mechanisms that act in the processes that guarantee the maintenance of these elements in the populations. In this sense, it is considered that the use of last generation molecular genetic methodology serving as a basis for genomic and functional studies constitutes an adequate tool to unveil aspects of the origin, presence, structure, maintenance, functioning, inheritance and evolution of these elements in Astyanax and, therefore, in fish and other organisms. This new approach certainly represents a major advance in research in this area. On the other hand, information about the origin and maintenance of these elements could provide knowledge about the development of anomalous cellular processes resulting from uncontrolled cell cycle. (AU)

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