Our specimens had an uneven diploid number due to the presence of a multiple sex chromosome determination system (XX/XY1Y2) that arose via a Robertsonian rearrangement between the X chromosome and the largest acrocentric autosome, creating a Neo-X. The females had 2n = 16 with two Neo-X chromosomes, while the males had 2n = 17 with one Neo-X, one Y1 (true Y) and one Y2 (the homolog of the autosome translocated to the X). In Proechimys, translocations involving the sex chromosomes and autosomes are far less common than autosomal intrachromosomal rearrangements. Translocations involving sex chromosomes and autosomes may cause a new meiotic configuration, leading to low fertility among heterozygotes and creating post-zygotic barriers that can lead to chromosomal speciation [19, 20]. Many cytotaxonomic differences between species have arisen from variations in the sex chromosomes, coming mainly from constitutive heterochromatin addition/deletion, inversions and translocations . Members of genus Proechimys typically have a simple sex chromosome system ; thus, the simple system appears to be a symplesiomorphy, while the multiple sex chromosome system described herein is an autapomorphy.
We sought to assign our low-chromosome-number specimens to the appropriate species group of Proechimys (i.e., cuvieri, goeldii, guyannensis or longicaudatus). Based on morphological traits, we positively identified our specimens as belonging to the longicaudatus group. Notably, the specimens from Tanguro Farm represent an eastward extension of the previously recognized geographic distribution of the low-chromosome-number group of Proechimys (Figure 1).
Only two species are currently recognized in the longicaudatus group: P. brevicauda and P. longicaudatus. Two cytotypes have been described for longicaudatus; both had 2n = 28 [12, 13], and their karyotypes differed significantly from those described herein. The G-banding patterns were different from one another, and we found it difficult to compare the previous data with our present results. Future chromosome painting studies should allow us to define the precise homologies among these karyotypes. The C-banding patterns also did not allow for precise comparison of our results with the previous descriptions. In some of the Proechimys studied to date, the NORs have been found in the distal regions of the large arms of various pairs . Here, we found the NOR in the interstitial region of the large arm of chromosome pair 6.
We compared our data with the previously published karyotypes for Proechimys showing diploid numbers close to those of our samples, and found that our results were similar to those described in two prior papers [5, 12]. Our karyotypes were consistent with those described for P. gr. goeldii (2n = 15) , which had an identical Neo-X chromosome. In the previous paper, the authors recognized that their sample represented a female heterozygous for a fusion in the third autosome pair, explaining the uneven diploid number. Without the fusion, the 2n would be 16. The authors did not publish the G-banding for this specimen, but when we inverted their metaphase DAPI-banding image using Adobe Photoshop, the banding pattern was very similar to our G-banding results, confirming that the two karyotypes were quite similar. The previous authors studied only one female, however, and thus did not realize that their pair 1 was actually a Neo-X, as described herein. The chromosome they proposed as the X is homologous to pair 5 in the present work. Given the chromosomal similarities between our karyotypes and the 2n = 15 karyotype described in the previous paper , we propose that all Proechimys with this karyotype should be considered members of the longicaudatus group instead of the goeldii group. Paradoxically, when we analyzed the previously described specimen , which is deposited in the Museu de Zoologia da Universidade de São Paulo (São Paulo, Brazil), we concluded that it may indeed be morphologically associated with the goeldii group. In contrast, our specimens from Fazenda Tanguro were found to be morphologically associated with the longicaudatus group. Given the karyotypic similarities discussed above, we argue that the previously described specimen  is probably a composite of two different specimens and species, with the skin and skull belonging to a species of the goeldii group, while the karyotype belongs to a species of the longicaudatus group. This could be reasonably explained by a labeling mistake made in the field. If this is not the case, it would seem that either the previously described specimen represents a morphologically atypical individual of the longicaudatus group, or there are two karyotypically similar low-diploid-numbered species of Proechimys in the southern Amazon, one belonging to the longicaudatus group (our specimens) and another belonging to the goeldii group .
In the other previous work describing a low-diploid-numbered species of Proechimys, seven different karyotypes were described in 38 specimens of Proechimys sp. collected in the Para state, Brazil . These rodents were classified into three apparent species (designated sp1, sp2 and sp3). Of them, sp3 from the Jacaréacanga-Flexal locality (Figure 1; 6°16′48″S, 57°39′04″W) had 2n = 14-17 and a chromosome pair 1 that was identical to the Neo-X described herein and mis-identified in the previous paper .
Thus, our samples, Proechimys gr. goeldii with 2n = 15 , and Proechimys sp3  all have the same multiple sex chromosome determination system, similar diploid numbers, and consistent chromosomal morphologies and G-banding patterns, suggesting that they belong to the same taxon. However, their karyotypes differ with respect to other members of the longicaudatus group (2n = 28), indicating that the low-diploid-number specimens belong to a distinct species. It seems likely that any progeny resulting from a mating of individuals with 2n = 14-17 and 2n = 28-30 would have many meiotic problems and could be sterile hybrids with negative heterosis. Thus, the observed chromosomal differences could indicate reproductive isolation (King, 1983). Future analyses of additional specimens will be needed to confirm if the 2n = 14-17 specimens belong to one of the nominal taxa currently considered to be synonyms of P. brevicauda or P. longicaudatus (e.g., bolivianus, elassopus, gularis, leucomystax, ribeiroi, securus, and villacauda), or if they belong to a form that has not yet been formally described.