Karyotypic and morphological divergence between two cryptic species of Eigenmannia in the Amazon basin with a new occurrence of XX/XY sex chromosomes (Gymnotiformes: Sternopygidae)

Abstract

Eigenmannia species are widely distributed in the Neotropics, with eight valid species currently recognized. Populations of Eigenmannia from three locations in the eastern Amazon were investigated using cytogenetic and morphological techniques, revealing two taxa designated here as Eigenmannia sp. "A" and Eigenmannia sp. "B". The species differ in three morphometric characters, two meristic characters, and one osteological character. Eigenmannia sp. "A" presents 2n = 34 (22 m/sm+12 st/a) and Eigenmannia sp. "B" presents 2n = 38 (14 m/sm+24st/a) and simple differentiated sex chromosomes of the type XX/XY. In both species the Constitutive Heterochromatin (CH) rich in A-T bases is distributed in the centromeric region of all chromosomes. Eigenmannia sp. "B" also presents CH blocks in the interstitial region of chromosome pairs 8, 9 and X which are positively stained with CMA3, indicating G-C rich regions. The NOR is located on the short arm of chromosome pair 17 of Eigenmannia sp. "A" and on the short arm of pair 14 of Eigenmannia sp. "B". FISH with rDNA probes hybridized to different-sized regions between homologs, suggesting heteromorphism. The differentiation of the X chromosome in Eigenmannia sp. "B" could be the result of amplification of repetitive DNA sequences.

Analysis of multiple chromosomal rearrangements in the genome of Willisornis vidua using BAC-FISH and chromosome painting on a supposed conserved karyotype

“…A useful tool for characterizing bird species, as well as for understanding their evolutionary history and genome organization is through karyotype analysis [ 18 ]. Karyotypic studies can be employed for the detection of rearrangements involved in speciation events in evolutionary comparative studies and could be helpful in defining cryptic species without obviously genetic divergence, but with chromosomal differences [ 18 – 20 ]. Several evolutionary studies involving karyotypic characters have been conducted (reviewed in Griffin et al [ 22 ] and Kretschmer et al [ 18 ]), but most are focussed on poultry due to the ease of obtaining samples, or on birds that are of significant commercial value, such as the Psittacidae, targets of biopiracy [ 23 ].…”

Highest Diploid Number Among Gymnotiformes: First Cytogenetic Insights into Rhabdolichops (Sternopygidae)

“…12,13 The karyotypic formulae of the studied Rhabdolichops species resemble those found in most species of Eigenmannia, especially, those with higher diploid numbers. 7,13,14 However, they are quite different from the published karyotypes of Sternopygus species, which only have biarmed chromosomes. 12,13 The high diploid number found in Rhabdolichops suggests that chromosome fission may have contributed to the chromosomal evolution of this genus.…”

“…The species of genera Gymnotus and Eigenmannia have been the most widely studied to date (for review see Refs. 6,7 ). The Gymnotiformes already studied show great karyotypic diversity, with diploid numbers ranging from 2n = 22/24 in Apteronotus albifrons 8 to 2n = 54 in Gymnotus inaequilabiatus, 9 Gymnotus carapo, 9 Gymnotus mamiraua, 10 and Gymnotus paraguensis.…”

“…11 In Sternopygidae, cytogenetic results have been published only for Sternopygus macrurus, which has a conserved 2n = 46 karyotype, 12,13 and several chromosomally diversified species of Eigenmannia, which reportedly have 2n = 28, 32, 32/31, and 38. 7,14,15 Sex-chromosome systems have been found in some species of Eigenmannia; they range from female heterogametic systems (ZZ/ZW) to single or multiple male heterogametic systems (XX/XY or X 1 X 1 X 2 X 2 /X 1 X 2 Y). 7,12,[15][16][17] The major ribosomal genes (the 18S or 45S rDNAs) and that encoding the 5S rDNA have been mapped in some Eigenmannia species, with the latter exhibiting multiple centromeric locations in most of the tested species.…”

“…7,14,15 Sex-chromosome systems have been found in some species of Eigenmannia; they range from female heterogametic systems (ZZ/ZW) to single or multiple male heterogametic systems (XX/XY or X 1 X 1 X 2 X 2 /X 1 X 2 Y). 7,12,[15][16][17] The major ribosomal genes (the 18S or 45S rDNAs) and that encoding the 5S rDNA have been mapped in some Eigenmannia species, with the latter exhibiting multiple centromeric locations in most of the tested species. 14 Interstitial telomere sequences (ITSs) were found in a species of Eigenmannia with a low diploid number.…”