O paradoxo da variação cariotípica em pequenas populações de peixes neotropicais: o modelo de Rineloricaria lanceolata (SILURIFORMES: LORICARIIDAE)

Abstract

The genus Rineloricaria is the most species-rich within the Loricariidae family, with 71 currently accepted species. These Neotropical fish are distinguished by a remarkable characteristic: karyotypic diversity. This is exemplified by the presence of structural and numerical chromosomal polymorphisms, which are predominantly associated with Robertsonian-type rearrangements. However, cytogenetic data is only available for a limited number of species within this taxon, and there is a paucity of research exploring other aspects of this phenomenon. These aspects include the maintenance of these polymorphisms throughout the life history of the species and the potential evolutionary consequences of these polymorphisms for the maintenance of populations. This dissertation aimed to study the polymorphic population of the species Rineloricaria lanceolata from Córrego da Onça, Mato Grosso do Sul, using bioinformatics and cytogenetic methods to investigate and propose hypotheses about the mechanisms of emergence and maintenance of polymorphisms in these fish. The cytogenetic characterisation of the sample revealed notable differences between the cytotypes present in the individuals of the population, including the observation of a unprecedented cytotype in the species. Meiotic preparations were created from this cytotype, which belonged to a male individual with 2n=47 chromosomes. This enabled the behaviour of chromosome segregation during meiosis to be analysed with help of ribosomal and satellite probes. The 5S rDNA probe was instrumental in identifying the chromosomes involved in the Robertsonian rearrangement that gave rise to the 2n=47 cytotype from its ancestral 2n=48 form. These chromosomes form a trivalent pairing during meiosis, resulting in the production of unbalanced gametes in equal frequencies. The satellitome, or library of repetitive genome sequences, was constructed from the total DNA of an individual, with 36 satDNA families being characterised. Although the Robertsonian rearrangements observed here are structural events that diminish adaptive value, the maintenance of polymorphisms may offer an adaptative advantage according to the drift robustness hypothesis here suggested. This process should be repeated in other polymorphic populations of the genus, not as a recurring event, but with some underlying molecular mechanism for chromosomal instability that has yet to be elucidated.