Allopatric chromosomal variation in Nematocharax venustus Weitzman, Menezes & Britski, 1986 (Actinopterygii: Characiformes) based on mapping of repetitive sequences

Abstract

Characiformes is the most cytogenetically studied group of freshwater Actinopterygii, but karyotypical data of several taxa remain unknown. This is the case of Nematocharax, regarded as a monotypic genus and characterized by marked sexual dimorphism. Therefore, we provide the first cytogenetic report of allopatric populations of Nematocharax venustus based on distinct methods of chromosomal banding and fluorescence in situ hybridization (FISH) with repetitive DNA probes (18S and 5S rDNA). The karyotype macrostructure was conserved in all specimens and populations, independently on sex, since they shared a diploid number (2n) of 50 chromosomes divided into 8m+26sm+14st+2a. The heterochromatin was mainly distributed at pericentromeric regions and base-specific fluorochrome staining revealed a single pair bearing GC-rich sites, coincident with nucleolar organizer regions (NORs). On the other hand, interpopulation variation in both number and position of repetitive sequences was observed, particularly in relation to 5S rDNA. Apparently, the short life cycles and restricted dispersal of small characins, such as N. venustus, might have favored the divergence of repetitive DNA among populations, indicating that this species might encompass populations with distinct evolutionary histories, which has important implications for conservation measures.Characiformes é o grupo de Actinopterygii de água doce mais estudado citogeneticamente, porém dados cariotípicos de vários taxa permanecem desconhecidos. Este é o caso de Nematocharax, considerado um gênero monotípico e caracterizado pelo acentuado dimorfismo sexual. Em vista disso, nós fornecemos a primeira descrição citogenética de populações alopátricas de Nematocharax venustus, baseada em métodos distintos de bandamento cromossômico e hibridação fluorescente in situ (FISH) com sondas de DNA repetitivo (DNAr 18S e 5S). A macroestrutura cariotípica mostrou-se conservada em todos os espécimes e populações, independentemente do sexo, uma vez que compartilharam um número diploide (2n) de 50 cromossomos dividido em 8m+26sm+14st+2a. A heterocromatina distribuiu-se principalmente nas regiões pericentroméricas e a coloração com fluorocromos base-específicos revelou um único par portador de sítios GC-ricos, coincidentes com as regiões organizadoras de nucléolo (RONs). Por outro lado, foi observada uma variação interpopulacional no número e na posição das sequências repetitivas, especialmente em relação ao DNAr 5S. Aparentemente, ciclos de vida curtos e dispersão restrita dos pequenos caracídeos, tal como N. venustus, podem ter favorecido a divergência do DNA repetitivo entre as populações, indicando que essa espécie pode englobar populações com distintas histórias evolutivas, o que tem implicações importantes para medidas de conservação.

Integrative approach reveals a new species of Nematocharax (Teleostei: Characidae)

“…conspicuous roundish dark caudal blotch on the terminal portion of the caudal peduncle). Besides morphological and morphometric characters, N. varii can also be distinguished from N. venustus by cytogenetic traits (Barreto et al ., ) and molecular divergence (Barreto et al ., ; present study). (See Discussion for more details regarding the molecular differences between both species).…”

“…Accordingly, the upper Contas River population also had the largest genetic distance when compared with the others based on nuclear genes (Tables S7 to S11). Thus, the present results along with previous reports based on DNA barcode, geometric morphometrics and cytogenetics (Barreto et al ., , 2016b) reinforce the status of the Nematocharax lineage from the upper Contas River as a new species for the genus, described below as Nematocharax varii .…”

“…Differentiation of the evolutionary lineage from the upper Contas River (presently referred as N. varii ) is presumably related to genetic drift, which can especially effect small and isolated populations such as those in headwaters from the upper Contas River, as also mentioned by Barreto et al . ().…”

“…Combinations of genetic and morphological data have consistently supported the population of Nematocharax from the upper Contas River, on the Diamantina Plateau, as a distinct species, herein named as N. varii . Additional support for N. varii as a unique taxon is provided by previous reports based on cytogenetics, geometric morphometrics and DNA barcode (Barreto et al ., , 2016b). On the other hand, within the distribution range of N. venustus , five divergent mitochondrial lineages were observed (Figures and ), but with shared haplotypes in all nuclear loci (Supporting Information Figure S2) and lacking conspicuous morphological differences.…”

“…However, N. costai is now considered to be a junior synonym of N. venustus because morphological differences between the two species were inconsistent (Menezes et al ., ). More recently, cytogenetic markers, DNA barcoding and geometric morphometrics have revealed distinct lineages in Nematocharax (Barreto et al ., , ), with the population from the upper Contas River representing the most divergent lineage. Chromosome mapping of repetitive sequences has demonstrated that samples from this locality have a unique C‐banding pattern and a distinctive distribution of 18S rDNA cistrons and GC‐rich sites (Barreto et al ., ).…”