title: Proechimys (Rodentia, Echimyidae): characterization and taxonomic considerations of a form with a very low diploid number and a multiple sex chromosome system.
abstract: BACKGROUND
  Proechimys is the most diverse genus in family Echimyidae, comprising 25 species (two of which are polytypic) and 39 taxa. Despite the numerous forms of this rodent and their abundance in nature, there are many taxonomic problems due to phenotypic similarities within the genus and high intraspecific variation. Extensive karyotypic variation has been noted, however, with diploid numbers (2n) ranging from 14 to 62 chromosomes. Some heteromorphism can be found, and 57 different karyotypes have been described to date.
  
  
  RESULTS
  In the present work, we describe a cytotype with a very low 2n. Specimens of Proechimys cf. longicaudatus were collected from two different places in northern Mato Grosso state, Brazil (12°54″S, 52°22″W and 9°51'17″S, 58°14'53″W). The females and males had 16 and 17 chromosomes, respectively; all chromosomes were acrocentric, with the exception of the X chromosome, which was bi-armed. The sex chromosome system was found to be XY1Y2, originating from a Robertsonian rearrangement involving the X and a large acrocentric autosome. Females had two Neo-X chromosomes, and males had one Neo-X and two Y chromosomes. NOR staining was found in the interstitial region of one autosomal pair.
  
  
  CONCLUSIONS
  Comparison of this karyotype with those described in the literature revealed that Proechimys with similar karyotypes had previously been collected from nearby localities. We therefore suggest that this Proechimys belongs to a different taxon, and is either a new species or one that requires reassessment.
authors.affiliation: Laboratório de Citogenética, Instituto de Ciências Biológicas, Universidade Federal do Pará, Campus do Guamá, Av, Bernardo Sayão, sn, Guamá, Belém - Pará, 66075-900, Brazil.
authors.forename: Paulo J S
authors.initials: PJ
authors.lastname: Amaral
authors.forename: Cleusa Y
authors.initials: CY
authors.lastname: Nagamachi
authors.forename: Renata C R
authors.initials: RC
authors.lastname: Noronha
authors.forename: Marlyson J R
authors.initials: MJ
authors.lastname: Costa
authors.forename: Adenilson L
authors.initials: AL
authors.lastname: Pereira
authors.forename: Rogerio V
authors.initials: RV
authors.lastname: Rossi
authors.forename: Ana C
authors.initials: AC
authors.lastname: Mendes-Oliveira
authors.forename: Julio C
authors.initials: JC
authors.lastname: Pieczarka
mesh_terms: D000818:Animals; D001938:Brazil; D033481:Chromosomes, Mammalian; D005260:Female; D059785:Karyotype; D008297:Male; D012377:Rodentia / Q000145:classification* / Q000235:genetics* / Q000502:physiology; D019849:Sex Determination Processes; D014960:X Chromosome; D014998:Y Chromosome