First cytogenetic information for Lonchothrix emiliae and taxonomic implications for the genus taxa Lonchothrix + Mesomys (Rodentia, Echimyidae, Eumysopinae)

Abstract

The taxonomic identification of Lonchothrix emiliae (Rodentia, Echimyidae, Eumysopinae) is problematic because of the overlap of morphological characters with its sister clade represented by species in the genus Mesomys which, like L . emiliae , is distributed throughout the Amazonian biome. Cytogenetic studies reported the karyotype of L . emiliae as 2n = 60/FN = 116, but this karyotype and samples were later designated as M . hispidus . To evaluate the karyotype diversity of Lonchothrix and Mesomys , and to provide data useful as karyological diagnostic characters, in the present study we made a comparative analysis of specimens of L . emiliae and M . stimulax collected from two Brazilian Amazonian localities, using C-banding, G-banding, FISH using rDNA 45S and telomeric probes, and Cytochrome-b (Cytb) sequences. The results indicate that L . emiliae has 2n = 64♀, 65♂/FN = 124 and a multiple sexual system (XX/XY 1 Y 2 ), while M . stimulax has 2n = 60/FN = 116. The Neo-X system found in L . emiliae also occurs in two Proechimys species, but cytogenetic analysis indicated an independent origin for these systems. The rDNA 45S analysis showed interstitial signals at one autosomal pair for each species, while an ITS found in L . emiliae was not coincident with the NOR. The molecular analysis confirmed Lonchothrix and Mesomys are sister genera, and the high level of intraspecific genetic divergence (7.1%) in M . stimulax suggests that it may be a species complex.

New karyotype for Mesomys stimulax (Rodentia, Echimyidae) from the Brazilian Amazon: A case for species complex?

“…The specimen of Mesomys stimulax studied herein has 2n = 60 and FN = 110, with 26 pairs of biarmed chromosomes, three small acrocentric pairs, and a medium-size submetacentric X chromosome (Figure 3a; Figure S2). Constitutive Heterochromatin (CH, Figure 3b Mesomys hispidus (Dias de Oliveira et al, 2019;Orlando et al, 2003;Patton et al, 2000) shares the same 2n and FN of the previously reported specimens of M. stimulax (Patton et al, 2000) and probably shares the same chromosomal differences with the karyotype of M. stimulax herein described. In our sample, more CH was present in the centromeric and pericentromeric regions of almost all pairs, with some large blocks observed in pairs 21, 23, and 26 (Figure 3b) while in M. hispidus there are no such large blocks (Dias de Oliveira et al, 2019).…”

“…Constitutive Heterochromatin (CH, Figure 3b Mesomys hispidus (Dias de Oliveira et al, 2019;Orlando et al, 2003;Patton et al, 2000) shares the same 2n and FN of the previously reported specimens of M. stimulax (Patton et al, 2000) and probably shares the same chromosomal differences with the karyotype of M. stimulax herein described. In our sample, more CH was present in the centromeric and pericentromeric regions of almost all pairs, with some large blocks observed in pairs 21, 23, and 26 (Figure 3b) while in M. hispidus there are no such large blocks (Dias de Oliveira et al, 2019). The difference in FN may be due to pericentric inversions or centromeric repositioning (Rocchi et al, 2012), while the variation in amount of CH is likely to reflect the addition/ deletion of repetitive sequences.…”

“…This same karyotype (2n = 60 and FN = 116) was assigned to specimens of M. stimulax collected from both banks of the lower Xingu River (Patton et al, 2000;Emmons, personal communication) and from the left bank of the lower Tapajós River (Dias de Oliveira et al, 2019). Only the M. stimulax karyotype (2n = 60; FN = 116) has been analyzed with chromosome banding and molecular cytogenetics (Dias de Oliveira et al, 2019). More detailed cytogenetic studies of these species are needed to improve our understanding of the real karyotypic diversity in this genus and shed light on the mechanisms involved in its diversification.…”

“…The karyotype 2n = 60 and FN = 116 was described for individuals of M. hispidus collected from sites at the Samuel Dam in Madeira River (Leal-Mesquita, 1991), the Juruá River south of the Solimões River, the upper Urucu River, Jaú River north of the Solimões River, Brazil, and Tambopata, Peru (Patton et al, 2000;Emmons, personal communication). This same karyotype (2n = 60 and FN = 116) was assigned to specimens of M. stimulax collected from both banks of the lower Xingu River (Patton et al, 2000;Emmons, personal communication) and from the left bank of the lower Tapajós River (Dias de Oliveira et al, 2019). Only the M. stimulax karyotype (2n = 60; FN = 116) has been analyzed with chromosome banding and molecular cytogenetics (Dias de Oliveira et al, 2019).…”

Paralonchothrix gen. nov., the first record of Echimyini (Rodentia, Octodontoidea) in the late Miocene of Southern South America

“…Lonchothrix emiliae lives in lowland mature and secondary (capoeira) rainforests in a limited area of eastern Brazilian Amazonia (Patton 2015b;Fabre et al 2016). Mesomys has a wider geographic range, occupying lowland rainforest throughout the Amazon Basin and the Guianan region, and even extending up to an elevation of 2000 m in the upper montane forests along the eastern slope of the Andes (Patton et al 2000;Voss et al 2001;Orlando et al 2003;Upham et al 2013;Dias de Oliveira et al 2019). Preserved morphology in Paralonchothrix does not allow reliable inference on its habits.…”