American soles (family Achiridae) have been characterized by remarkable chromosomal variation even though several species lack basic cytogenetic information. This trend indicates that chromosomal traits can be useful to taxonomy once the morphological identification of some taxa in this family (e.g., Achirus species) is controversial. In this work, we expand the cytogenetic data in Achiridae by providing the first karyotypic analysis of Achirus achirus. An unusual multiple sex chromosome system (XXXX/XXY) was detected, once males presented 2n = 33 with three unpaired chromosomes (XXY) while females presented 2n = 34 with two acrocentric pairs (XX and XX) found in hemizygosis in males. The 18S rDNA clusters were observed interspersed with GC-rich sites in a single pair. However, the 5S rRNA genes were dispersed through the genome of A. achirus in a sex-specific manner (10 clusters in males and 12 in females), as a result of the presence of ribosomal cistrons in X and X chromosomes. This pattern allowed us to infer that Y chromosome has evolved by partial deletion followed by fusion of proto-X and proto-X homologous chromosomes. The high rate of genomic evolution in Achiridae could have favored their reproductive isolation and speciation even in sympatric conditions.
ABSTRACT. Loricariidae (Siluriformes, Hypostominae) is one of the most diverse catfish families. In spite of the wide distribution of loricariids in South America, cytogenetic reports are available for only a few species, mostly from southern and southeastern Brazil. We made the first chromosomal analysis of Hypostomus aff. unae from the Contas River basin in northeastern Brazil. Four populations isolated by short distances but from distinct landscapes were studied based on conventional staining, C-banding, argyrophilic nucleolar organizer regions (Ag-NOR), CMA 3 / DAPI fluorochrome staining, and fluorescent in situ hybridization with 18S rDNA probes. Although sharing the same diploid number (2n = 76) and NOR locations, each population presented exclusive karyotype formulae and specific patterns of heterochromatic and AT-rich regions. The derived karyotypes of H. aff. unae (2n >54; high number of acrocentrics bearing AT-rich interstitial heterochromatin) indicated a divergent karyoevolution, mostly driven by centric fissions, pericentric inversions and particular heterochromatin dispersion models. This finding of distinct evolutionary units in H. aff. unae will be useful for understanding the natural history of loricariids from relatively unexplored coastal basins in South America.
The wrasses (family Labridae) represent a suitable model to understand chromosomal evolution and to test the efficacy of cytotaxonomy since they display a remarkable karyotypic variation, rarely reported in marine Perciformes, as well as a high number of species and complex systematics. Therefore, we provided new chromosomal data in 5 labrids from South Atlantic (Doratonotus megalepis, Halichoeres dimidiatus, Halichoeres penrosei, Thalassoma noronhanum, and Xyrichtys novacula) and carried out a detailed comparative analysis of karyotypic data in Labridae using multivariate approaches. Basal diploid values (2n = 48) were observed in most of species studied in the present work but D. megalepis (2n = 46), along with distinct karyotype formulae. Single 18S rDNA sites interspersed with GC-rich heterochromatin were also commonly reported except for both Halichoeres species (2 18S rDNA-bearing pairs), following a species-specific pattern. These data show the high rates of chromosomal evolution in wrasses, ranging from microstructural rearrangements to centric fusions. A revision of chromosomal data in Labridae based on multivariate analysis of 74 taxa allowed inferring karyoevolutionary trends within tribes and genera of wrasses. The dendrogram obtained was in agreement with recent systematic hypotheses. In spite of the independent occurrence of some chromosomal rearrangements, karyoevolutionary trends could be identified within tribes of Labridae. Moreover, the karyotypic features are also suitable as cytotaxonomic markers of wrasses.
Few chromosomal reports are available for the endemic fish fauna from coastal basins in northeastern Brazil, and regional biodiversity remains partially or completely unknown. This is particularly true for Loricariidae, the most diverse family of armored catfishes. In the present work, allopatric populations of Hypostomus cf. wuchereri (Siluriformes: Loricariidae) from two basins in Bahia (northeastern Brazil) were cytogenetically analyzed. Both populations shared 2n = 76 chromosomes, a karyotype formula of 10m+18sm+48st/a (FN = 104) and single terminal GC-rich NORs on the second metacentric pair. Nevertheless, microstructural differences were detected by C-banding, fluorochrome staining and chromosomal digestion with restriction enzymes (Alu I, Bam HI, Hae III, and Dde I). The population from Una River (Recôncavo Sul basin) showed conspicuous heterochromatin blocks and a remarkable heterogeneity of base composition (presence of interspersed AT/GC-rich and exclusively AT- or GC-rich sites), while the population from Mutum river (Contas River basin) presented interstitial AT-rich C-bands and terminal GC/AT-rich heterochromatin. Each enzyme yielded a specific band profile per population which allowed us characterizing up to five heterochromatin families in each population. Based on the present data, we infer that these populations have been evolving independently, as favored by their geographic isolation, probably representing cryptic species. Poucos dados cromossômicos estão disponíveis para a fauna de peixes endêmicos das bacias costeiras do nordeste do Brasil, e a biodiversidade regional continua a ser parcial ou completamente desconhecida. Isto é particularmente verdadeiro para Loricariidae, a mais diversa família de cascudos. No presente trabalho, populações alopátricas de Hypostomus cf. wuchereri (Siluriformes: Loricariidae) de duas bacias hidrográficas da Bahia (nordeste do Brasil) foram citogeneticamente analisadas. Ambas as populações compartilham 2n = 76 cromossomos, uma fórmula cariotípica de 10m+18sm+48st/a (FN = 104) e um único sinal terminal de RONs GC-ricas no segundo par metacêntrico. No entanto, diferenças microestruturais foram detectados pelo bandeamento C, coloração com fluorocromos e digestão cromossômica com enzimas de restrição (Alu I, BamH I, Hae III e Dde I). A população do rio Una (bacia do Recôncavo Sul) apresentou blocos de heterocromatina conspícuos e grande heterogeneidade de composição de base (presença de sítios AT/GC-ricos intercalados e exclusivamente AT ou GC-ricos), enquanto a população do rio Mutum (bacia do Rio de Contas) apresentou bandas C intersticiais AT-ricas e heterocromatina terminal GC/AT-ricas. Cada enzima gerou um perfil específico de bandas por população que nos permitiu caracterizar até cinco famílias de heterocromatina em cada população. Baseado nos presentes dados, podemos inferir que essas populações têm evoluido de forma independente, favorecidas pelo seu isolamento geográfico, provavelmente representando espécies crípticas
Lignobrycon myersi is a threatened freshwater fish species and endemic of a few coastal rivers in northeastern Brazil. Even though the Brazilian laws prohibit the fisheries of threatened species, L. myersi is occasionally found in street markets, being highly appreciated by local population. In order to provide a reliable DNA barcode dataset for L. myersi, we compared mitochondrial sequences of cytochrome c oxidase subunit I (COI) from fresh, frozen, and salt-preserved specimens. Phylogenetically related species (Triportheus spp.) and other fish species (Astyanax fasciatus) commonly mixed with L. myersi in street markets were also included to test the efficiency of molecular identification. In spite of the differences in conservation processes and advanced deterioration of some commercial samples, high-quality COI sequences were obtained and effective in discriminating L. myersi specimens. In addition, while populations from Contas and Almada River basins seem to comprise a single evolutionary lineage, the specimens from Cachoeira River were genetically differentiated, indicating population structuring. Therefore, DNA barcoding has proved to be useful to trace the illegal trading of L. myersi and to manage threatened populations, which should focus on conservation of distinct genetic stocks and mitigation on human impacts along their range.
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