Diversity and Karyotypic Evolution in the Genus <b><i>Neacomys </i></b>(Rodentia, Sigmodontinae)

Abstract

Neacomys (Sigmodontinae) comprises 8 species mainly found in the Amazonian region. We describe 5 new karyotypes from Brazilian Amazonia: 2 cytotypes for N. paracou (2n = 56/FNa = 62-66), 1 for N. dubosti (2n = 64/FNa = 68), and 2 for Neacomys sp. (2n = 58/FNa = 64-70), with differences in the 18S rDNA. Telomeric probes did not show ITS. We provide a phylogeny using Cytb, and the analysis suggests that 2n = 56 with a high FNa is ancestral for the genus, as found in N. paracou, being retained by the ancestral forms of the other species, with an increase in 2n occurring independently in N. spinosus and N. dubosti. Alternatively, an increase in 2n may have occurred in the ancestral taxon of the other species, followed by independent 2n-reduction events in Neacomys sp. and in the ancestral species of N. tenuipes, N. guianae, N. musseri, and N. minutus. Finally, a drastic reduction event in the diploid number occurred in the ancestral species of N. musseri and N. minutus which exhibit the lowest 2n of the genus. The karyotypic variations found in both intra- and interspecific samples, associated with the molecular phylogeny, suggest a chromosomal evolution with amplification/deletion of constitutive heterochromatin and rearrangements including fusions, fissions, and pericentric inversions.

Advances in cytogenetics of Brazilian rodents: cytotaxonomy, chromosome evolution and new karyotypic data

“…According to the authors, both cytotypes differed in the number of biarmed pairs due to amplification/deletion of constitutive heterochromatin in the short arms of pairs 24, 26, and 27 (from Marajó Island) and pericentric inversion involving pairs 28 (metacentric) and 24 (acrocentric) from Marajó Island and Marabá, respectively. These karyotypes could not be assigned to any species described so far, and molecular phylogeny of these samples corroborates the cytogenetic data that it might be a new species (Silva et al 2015). …”

“…Our molecular phylogeny suggests that this specimen with 2n = 58, FN = 66, from Vila Rica may be an undescribed species that belongs to the same one reported by Silva et al (2015) with 2n = 58, FN = 64, but with a new fundamental number, probably due to pericentric inversions (Fig. 2).…”

“…Additionally, a sample from Igarapé-Açu (MTR12842), Rio Abacaxis (Amazonas, Brazil) was recovered as the sister group of these two clades. Although the phylogenetic reconstruction lacks N. tenuipes Thomas, 1900 (because the unique sequence available in GenBank has only 177pb), it is unlikely that samples with 2n = 58 belong to N. tenuipes once this species is distributed in Colombia and Venezuela and did not nest in the clade of N. tenuipes of the molecular phylogeny presented by Silva et al (2015). In addition, our phylogenetic reconstruction recovered Neacomys as monophyletic with high support values (1PP/ 99ML).…”

“…For instance, recently, Silva et al (2015) described two new cytotypes for Neacomys: 2n = 58, FN = 64, from samples collected in Marabá, and 2n = 58, FN = 70, from samples collected in Chaves, Marajó Island, localities from Pará State. According to the authors, both cytotypes differed in the number of biarmed pairs due to amplification/deletion of constitutive heterochromatin in the short arms of pairs 24, 26, and 27 (from Marajó Island) and pericentric inversion involving pairs 28 (metacentric) and 24 (acrocentric) from Marajó Island and Marabá, respectively.…”

“…Herein, we describe the same diploid (2n = 58), but with a different fundamental number (66) to Neacomys collected in Vila Rica, Mato Grosso State (approximately 700 km from those samples described by Silva et al 2015). The karyotype comprises 23 acrocentric pairs decreasing in size (pair 1 is the largest of the complement), and five small biarmed pairs.…”