Basic cytogenetics and physical mapping of ribosomal genes in four Astyanax species (Characiformes, Characidae) collected in Middle Paraná River, Iguassu National Park: considerations on taxonomy and systematics of the genus

Abstract

Karyotypes and chromosomal characteristics of both minor and major rDNAs in four fish species known popularly as “lambaris”, namely Astyanax abramis (Jenyns, 1842), Astyanax asuncionensis Géry, 1972, Astyanax correntinus (Holmberg, 1891) and Astyanax sp. collected from downstream of the Iguassu Falls (Middle Paraná River basin), preservation area of the Iguassu National Park, were analyzed by conventional and molecular protocols. Astyanax abramis had diploid chromosome number 2n=50 (4m+30sm+8st+8a) and single AgNORs (pair 22), Astyanax asuncionensis had 2n=50 (8m+24sm+6st+12a) and single AgNORs (pair 20), Astyanax sp. had 2n=50 (4m+26sm+8st+12a) and single AgNORs (pair 25), and Astyanax correntinus had 2n=36 (12m+16sm+2st+6a) and multiple AgNORs (pairs 12, 15, 16, 17). FISH with 18S rDNA showed a single site for Astyanax abramis, Astyanax asuncionensis and Astyanax sp. and multiple for Astyanax correntinus (14 sites). FISH with 5S rDNA showed single 5S-bearing loci chromosome pair only for Astyanax asuncionensis and multiple for Astyanax abramis (four sites), Astyanax correntinus (five sites) and Astyanax sp. (four sites). Distinct distribution patterns of heterochromatin were observed for karyotypes of all species, with the exception of the first acrocentric chromosome pair characterized by centromeric, interstitial-proximal and telomeric blocks of heterochromatin on the long arm, which may represent homeology between karyotypes of Astyanax abramis and Astyanax asuncionensis. Our study showed species-specific characteristics which can serve in diagnosis and differentiation between Astyanax abramis and Astyanax asuncionensis, considered cryptic species, as well as strengthening the occurrence of a species of Astyanax not yet described taxonomically. In addition, the data obtained from first cytogenetic studies in Astyanax correntinus suggest a high similarity with Astyanax schubarti Britski, 1964, suggesting that these species may belong to the same morphological group and that can be phylogenetically related.

Trends in Karyotype Evolution in Astyanax (Teleostei, Characiformes, Characidae): Insights From Molecular Data

“…In fact, this species shows the 5S rDNA sites located in the proximal region of two pairs of metacentric chromosomes ( Almeida-Toledo et al, 2002 ). Interestingly, A. currentinus , a species recently described ( Mirande et al, 2015 ), presents 2n = 36 chromosomes and the same distribution pattern of A. schubarti 5S rDNA, with an additional odd site ( Paiz et al, 2015 ). The authors suggest that these species may belong to the same morphological group and are phylogenetically related.…”

“…Clade 3 is characterized by a synapomorphy: only one pair of chromosomes carrying a rDNA 5S site on its pericentromeric region ( Kavalco et al, 2011 ) and in the literature ( Almeida-Toledo et al, 2002 ; Fernandes and Martins-Santos, 2006 ; Peres et al, 2008 , 2012 ; Hashimoto et al, 2011 ; Paiz et al, 2015 ). One exception is A. abramis , which presents in addition to the site of the chromosomal marker pair, a pair of extra-acrocentric chromosomes with proximal marking ( Paiz et al, 2015 ; Piscor et al, 2015 ). This differentiated pattern may be an autapomorphy of A. abramis , whose molecular distinctiveness within the group was also evident in our analysis ( Figure 5 ).…”