Integrating morphological, molecular and cytogenetic data for F2 sea turtle hybrids diagnosis revealed balanced chromosomal sets

Hybridization could be considered part of the evolutionary history of many species. The hybridization among sea turtle species on the Brazilian coast is atypical and occurs where nesting areas and reproductive seasons overlap. Integrated analysis of morphology and genetics is still scarce, and there is no evidence of the parental chromosome set distribution in sea turtle interspecific hybrids. In this study, chromosome markers previously established for pure sea turtle species were combined with morphological and molecular analyses aiming to recognize genetic composition and chromosome sets in possible interspecific hybrids initially identified by mixed morphology. The data showed that one hybrid could be an F2 individual among Caretta caretta × Eretmochelys imbricata × Chelonia mydas, and another is resulting from backcross between C. caretta × Lepidochelys olivacea. Native alleles of different parental lineages were reported in the hybrids, and, despite this, it was verified that the hybrid chromosome sets were still balanced. Thus, how sea turtle hybridism can affect genetic features in the long term is a concern, as the implications of the crossing-over in hybrid chromosomal sets and the effects on genetic function are still unpredictable.

Heterochromatin and microsatellites detection in karyotypes of four sea turtle species: Interspecific chromosomal differences

The wide variation in size and content of eukaryotic genomes is mainly attributed to the accumulation of repetitive DNA sequences, like microsatellites, which are tandemly repeated DNA sequences. Sea turtles share a diploid number (2n) of 56, however recent molecular cytogenetic data have shown that karyotype conservatism is not a rule in the group. In this study, the heterochromatin distribution and the chromosomal location of microsatellites (CA)n, (GA)n, (CAG)n, (GATA)n, (GAA)n, (CGC)n and (GACA)n in Chelonia mydas, Caretta caretta, Eretmochelys imbricata and Lepidochelys olivacea were comparatively investigated. The obtained data showed that just the (CA)n, (GA)n, (CAG)n and (GATA)n microsatellites were located on sea turtle chromosomes, preferentially in heterochromatic regions of the microchromosomes (mc). Variations in the location of heterochromatin and microsatellites sites, especially in some pericentromeric regions of macrochromosomes, corroborate to proposal of centromere repositioning occurrence in Cheloniidae species. Furthermore, the results obtained with the location of microsatellites corroborate with the temperature sex determination mechanism proposal and the absence of heteromorphic sex chromosomes in sea turtles. The findings are useful for understanding part of the karyotypic diversification observed in sea turtles, especially those that explain the diversification of Carettini from Chelonini species.

The karyotypes and evolution of ZZ/ZW sex chromosomes in the genus Characidium (Characiformes, Crenuchidae)

Available data on cytotaxonomy of the genus Characidium Reinhardt, 1867, which contains the greatest number of species in the Characidiinae (Crenuchidae), with 64 species widely distributed throughout the Neotropical region, were summarized and reviewed. Most Characidium species have uniform diploid chromosome number (2n) = 50 and karyotype with 32 metacentric (m) and 18 submetacentric (sm) chromosomes. The maintenance of the 2n and karyotypic formula in Characidium implies that their genomes did not experience large chromosomal rearrangements during species diversification. In contrast, the internal chromosomal organization shows a dynamic differentiation among their genomes. Available data indicated the role of repeated DNA sequences in the chromosomal constitution of the Characidium species, particularly, in sex chromosome differentiation. Karyotypes of the most Characidium species exhibit a heteromorphic ZZ/ZW sex chromosome system. The W chromosome is characterized by high rates of repetitive DNA accumulation, including satellite, microsatellite, and transposable elements (TEs), with a varied degree of diversification among species. In the current review, the main Characidium cytogenetic data are presented, highlighting the major features of its karyotype and sex chromosome evolution. Despite the conserved karyotypic macrostructure with prevalent 2n = 50 chromosomes in Characidium, herein we grouped the main cytogenetic information which led to chromosomal diversification in this Neotropical fish group.

Dispersion of transposable elements and multigene families: Microstructural variation in Characidium (Characiformes: Crenuchidae) genomes

Eukaryotic genomes consist of several repetitive DNAs, including dispersed DNA sequences that move between chromosome sites, tandem repeats of DNA sequences, and multigene families. In this study, repeated sequences isolated from the genome of Characidium gomesi were analyzed and mapped to chromosomes in Characidium zebra and specimens from two populations of C. gomesi. The sequences were transposable elements (TEs) named retroelement of Xiphophorus (Rex); multigene families of U2 small nuclear RNA (U2 snRNA); and histones H1, H3, and H4. Sequence analyses revealed that U2 snRNA contains a major portion corresponding to the Tx1-type non-LTR retrotransposon Keno, the preferential insertion sites of which are U2 snRNA sequences. All histone sequences were found to be associated with TEs. In situ localization revealed that these DNA sequences are dispersed throughout the autosomes of the species, but they are not involved in differentiation of the specific region of the W sex chromosome in C. gomesi. We discuss mechanisms of TE invasion into multigene families that lead to microstructural variation in Characidium genomes.

Citogenética comparativa em tartarugas marinhas da família Cheloniidae (Reptilia: Testudines): diversificação cariotípica microestrutural

As tartarugas marinhas representam um componente primitivo e singular da diversidade biológica. Neste momento evolutivo são reconhecidas sete espécies de tartarugas marinhas no mundo, sendo que cinco delas ocorrem no litoral brasileiro, as quais são: Dermochelys coriacea, Caretta caretta, Chelonia mydas, Eretmochelys imbricata e Lepidochelys olivacea, sendo as três últimas alvo desse estudo. Em tartarugas marinhas a identificação de sequências repetitivas e métodos de citogenética molecular são inexistentes, as análises cromossômicas destas espécies está limitada a descrição do número diploide, fórmulas cariotípicas e bandamentos convencionais, destacando a importância desse estudo como pioneiro para as espécies. Neste estudo foram avaliados os mecanismos cromossômicos responsáveis pela variação interespecífica em C. mydas, E. imbricata e L. olivacea amostradas em cativeiro e na natureza para o conhecimento da organização, evolução e diversidade cariotípica. Foram caracterizadas citogeneticamente C. mydas, E. imbricata e L. olivacea utilizando técnicas baseadas na coloração convencional por Giemsa e hibridação in situ fluorescente (FISH) com sonda de DNA ribossomal (DNAr) 18S e telomérica (TTAGGG)n. As três espécies estudadas apresentam 2n=56 cromossomos e variações na fórmula cariotípica quanto a morfologia cromossômica e número de microcromossomos quando comparado ao cariótipo considerado ancestral para a subordem. A FISH permitiu a identificação de um par de microcromossomos detentor do sítio de DNAr nas três espécies. O sítio de DNAr 18S está distribuído de forma desigual entre o par demonstrando que mecanismos de crossing-over desigual atuaram na origem do heteromorfismo de tamanho deste cístron. A sonda (TTAGGG)n permitiu detectar um sítio telomérico intersticial (ITS – interstitial telomeric sites) no par microcromossomo 14 na espécie C. mydas, o qual é colocalizado ao sítio de DNAr 18S. A presença deste ITS sem a alteração do 2n sugere sua inserção durante o reparo de quebras da dupla fita no sítio do DNAr 18S pela via de ação da telomerase. Diante deste contexto, este estudo de citogenética molecular evidencia variações microestruturais nos cromossomos das tartarugas marinhas e demonstram que os cariótipos das tartarugas marinhas da família Cheloniidae não são extremamente conservados como previamente descritos e, que variações microestruturais nos cromossomos atuaram na diversificação cariotípica do grupo.Apoio: CNPq, Fundação Araucária, Projeto Rebimar, Projeto de Monitoramento de Praias da Bacia de Santos, Projeto TAMAR ICMBio

Identification and chromosome mapping of repetitive elements in the Astyanax scabripinnis (Teleostei: Characidae) species complex

Most part of the eukaryotic genome is composed of repeated sequences or multiple copies of DNA, which were considered as "junk DNA", and may be associated to the heterochromatin. In this study, three populations of Astyanax aff. scabripinnis from Brazilian rivers of Guaratinguetá and Pindamonhangaba (São Paulo) and a population from Maringá (Paraná) were analyzed concerning the localization of the nucleolar organizer regions (Ag-NORs), the As51 satellite DNA, the 18S ribosomal DNA (rDNA), and the 5S rDNA. Repeated sequences were also isolated and identified by the Cot - 1 method, which indicated similarity (90%) with the LINE UnaL2 retrotransposon. The fluorescence in situ hybridization (FISH) showed the retrotransposon dispersed and more concentrated markers in centromeric and telomeric chromosomal regions. These sequences were co-localized and interspaced with 18S and 5S rDNA and As51, confirmed by fiber-FISH essay. The B chromosome found in these populations pointed to a conspicuous hybridization with LINE probe, which is also co-located in As51 sequences. The NORs were active at unique sites of a homologous pair in the three populations. There were no evidences that transposable elements and repetitive DNA had influence in the transcriptional regulation of ribosomal genes in our analyses.