The Echimyidae family is considered the most taxonomically diverse among South American
hystricognathi rodents, comprising 25 genera and 93 species. The subfamily Eumysopinae is
represented by nine genera, among which we highlight the arboreal genera Mesomys, which has
four recognized species, and Lonchothrix described as monotypic (L. emiliae), both distributed
in the Amazon. Morphological, molecular and chromosomal studies in the genera Mesomys and
Lonchothrix have contributed to a better understanding of taxonomic design, phylogenetic
relationships and karyotypic patterns. Recent molecular investigations have shown an as yet
undescribed diversity, suggesting that these taxa are even more diverse than previously
assumed. Furthermore, the limits of geographic distribution in the Amazon for the species M.
hispidus and M. stimulax have been questioned by some authors. In this sense, the current study
sought to investigate the karyotypic diversity and the geographic limits of the Mesomys and
Lonchothrix genera, based on classical and molecular cytogenetic analysis and through
sequences of the mitochondrial genes Cytochrome b (Cytb) and Cytochrome Oxidase - Subunit
I (COI) from different locations in the Brazilian Amazon. The species M. stimulax and Mesomys
sp. n. presented 2n=60/NF=110, while M. hispidus presented 2n=60/NF=112 and Lonchothrix
emiliae presented 2n=66/NF=126, both unpublished karyotypes for the genera. Phylogenetic
analysis confirmed Mesomys and Lonchothrix as sister genera and showed a high intraspecific
rate in M. hispidus and Mesomys sp. n. from Itaituba, may be related to a new lineage in the
genus Mesomys.
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