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

Date Published: April 16, 2019

Publisher: Public Library of Science

Author(s): Leony Dias de Oliveira, Willam Oliveira da Silva, Marlyson Jeremias Rodrigues da Costa, Iracilda Sampaio, Julio Cesar Pieczarka, Cleusa Yoshiko Nagamachi, Bi-Song Yue.


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/XY1Y2), 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.

Partial Text

Echimyidae is the most diverse family of Hystricognathi rodents in South America, with 22 genera and 88 currently recognized species [1]. Some members of this family are arboreal and sub-sampled, taking into account that the most employed methods favor the capture of specimens with terrestrial and/or scansorial habits [2]. There are also difficulties in the taxonomic identification, as a result of overlap of morphological characters among distinct genera [1]. The monophyly of the nine genera included in the Eumysopinae subfamily has been questioned based on morphological and molecular analyses, in which polytomies with the Dactylomyinae and Echimyinae subfamilies have been observed [1, 3, 4]. However, Lonchothrix and Mesomys have been consistently recognized as sister taxa [1, 5].

Our study provides the first cytogenetic data for Lonchothrix emiliae (2n = 64♀, 65♂/FN = 124) that can contribute to accurate taxonomic identification of this taxon, which is often confused with Mesomys. Based on our molecular and karyotypic data, we reinforce that these taxonomic entities are two distinct genera. In addition, the molecular data for Mesomys indicates that a substantial taxonomic revision of this group is needed to clarify its geographic boundaries, number of species, and evolutionary history. The Neo-X chromosome in each of L. emiliae and Proechimys may have resulted from independent events within family Echimyidae, in which case these species may be useful models for studies of sex chromosome evolution.