Estudo de marcadores cormossômicos em tartarugas marinhas da família Cheloniidade (Reptilia: Testunides): ênfase na diversidade cariotípica

Abstract

Sea turtles represent a primitive and particular element of the biological diversity, belonging to the oldest lineage of the living reptiles and being an important part of the marine ecosystem. In this evolutive moment, it is recognized seven species of sea turtles in the whole world, whereas five of them exist in the brazilian coast: Dermochelys coriacea, the unique representant of the Dermochelydae family, and the four other species belonging to the Cheloniidae family: Caretta caretta, Chelonia mydas, Eretmochelys imbricata e Lepidochelys olivacea, the last three are the main point of this study. Over the last 20 years, molecular genetic has performed an important role concerning the sea turtles conservation, however it has been focused on molecular markers only to observe patterns of migration and genetic diversity of the individuals. Citogenetic studies with turtles are scarce and they aim only the fresh water turtles, with comercial interest. All sea turtles species of the brazilian coast are under threat of extinction, besides being considered flagship species to the biodiversity conservation. However, there is no citogenetic studies to C. mydas, E. imbricata and L. olivacea of the brazilian coast. Thus, it is necessary to perform classical and molecular citogenetic studies to these species, focusing on the karyotypic evolution of the group, highlighting the importance of this study as pioneer to the sea turtle species. Therefore, the main goal here is the evaluation of the chromosomal mechanisms responsible to the interspecific variation and mechanisms that probable generated interspecific variation in C. mydas, E. imbricata and L. olivacea karyotypes, sampled in captivity and in the nature, in order to know the organization, evolution and diversity of karyotype. Techniques based on conventional Giemsa staining, G-band and fluorescence in situ hybridization (FISH) with 18S rDNA and (TTAGGG)n probes were performed. The three species studied showed 2n = 56 chromosomes and some variations in the karyotypic formula about the chromosomal morphology and number of microchromosomes. The 18S rDNA probe allowed identification of a single pair of microchromosomes with the rDNA site in the three species, which are distributed unequally between the chromosomes of the pair, showing that unequal crossing-over mechanisms were probably acting in the origin of the heteromorphism size of that cistron. The (TTAGGG)n probe highlighted one interstitial telomeric site (ITS) in the microchromosome pair 14 of C. mydas species of different location, which co-located with the 18S rDNA site. The presence of this ITS, without alteration of the basal 2n, suggests that they were inserted during the repair of double DNA strands breakage in the 18S rDNA site, with telomerase action during the course of evolution. In view of what has been exposed here, this molecular cytogenetic study shows chromosome microstructure variation in sea turtles and helps in understanding the karyotypic diversification of the group.