Nectomys

Abstract: G- and C-banded chromosomes of Holochilus brasiliensis, Nectomys squamipes, Neaco- mys guianae, Sigmodon hispidus, and 11 species of Oryzomys are compared and cladistically ana- lyzed to document direction, type, and magnitude of chromosomal evolution in these species. Direction of chromosomal evolution was determined from the proposed primitive G-band se- quences for cricetid rodents. Numbers and types of chromosomal rearrangements identified in the largest 12 chromosomes of Oryzomys were 13 Robertsonian translocations (9 of which are polymorphic), 21 other translocations, 8 pericentric inversions, 1 paracentric inversion, and 12 rearrangements whose nature could not be identified from banding sequences. In 11 species of Oryzomys there are about 2.5 times as many euchromatic rearrangements as found in 18 species of Peromyscus. It is concluded that: (1) The early radiation of the peromyscine-neotomine-ory- zomyine lineages was accompanied by little chromosomal evolution. (2) Chromosomal evolu- tion in cricetine rodents does not appear to fit the predictions of the canalization model. (3) None of the currently accepted broadly-based models of chromosomal evolution appears ade- quate to explain the pattern of variation observed in the cricetines (especially the variation within and between Oryzomys and Peromyscus). (4) Nondifferentially stained chromosomes gen- erally will not be a useful predictor of phylogenetic relationships. (5) For some clades, G-banded sequences provide synapomorphies for the development of systematic hypotheses; however, several branching events cannot be resolved with the quality of G-bands available to us. Some of the phylogenetic hypotheses are: (1) that of the five genera of South American cricetids examined, Oryzomys and Holochilus appear to be the most closely related (based on morphology, Holochilus was thought to be more closely related to Sigmodon); (2) that Neacomys, Holochilus, and species of Oryzomys examined shared a common ancestry after diverging from the Nectomys lineage; and (3) chromosomal characters identified by us failed to document that Sigmodon is more closely related to the other South American genera (Oryzomys, Holochilus, Neacomys, and Nectomys) than it is to the North American cricetids (for example, Neotoma and Peromyscus). One example of lack of congruence documents convergence in chromosomal rearrangements. (Chro- mosomes; G-bands; C-bands; oryzomyines; cladistics; South America; canalization model.)

Abstract: Nectomys (Sigmodontinae, Oryzomyini) is currently composed by five species: Nectomys apicalis, N. magdalenae, N. palmipes, N. rattus, and N. squamipes. These rodents have strong morphologic adaptations to semiaquatic habit and inhabit forests along river and streams in almost the entire South American continent. Although Nectomys is not a very speciose genus, 23 nominal taxa are associated with this group. Among these names is Nectomys saturatus, described in 1897 and currently allocated as synonym of N. apicalis, the west-Amazon species of Nectomys. Based on the examination of type material of the genus Nectomys, we hypothesize that Nectomys saturatus is a valid species. To test this hypothesis, we examined morphological and morphometric traits of 570 specimens of genus Nectomys of the same age class, belonging to the five recognized species, including the N. saturatus type series. We analyzed the external dimensions through descriptive statistics and we assessed the variation in craniodental measurements by means of Discriminant Analysis. Our results indicate N. saturatus individuals are considerably larger than the samples of congeneric species in all external traits, and they were also discriminated from the other species in the multivariate space. Qualitatively, N. saturatus shows two exclusive traits, the presence of paralophule on M1 and almost parallel-sided interorbital region. In this sense, our set of evidences indicates that our hypothesis should be accepted and the name Nectomys saturatus represents a valid species.

Abstract: The evaluation of the role of rodents as natural hosts of Schistosoma mansoni was studied at the Pamparrao Valley, Sumidouro, RJ, with monthly captures and examination of the animals. Twenty-three Nectomys squamipes and 9 Akodom arviculoides with a shistosomal infection rate of 56.5% and 22.2% respectively eliminated a great majority of viable eggs. With a strain isolated from one of the naturally infected N. squamipes, we infected 75% of simpatric Biomphalaria glabrata and 100% of albino Mus musculus mice. The adult worms, isolated from N. squamipes after perfusion were located mainly in the liver (91.5%) and the mesenteric veins (8.5%). The male/female proportion was 2:1. The eggs were distributed on small intestine segments (proximal, medial and distal portions) and the large intestine without any significant differences in egg concentration of these segments. In A. arviculoides, the few eggs eliminated by the stools were viable and there was litlle egg retention on intestinal segments. Considering the ease to complete S. mansoni biological cycle in the Nectomys/Biomphalaria/Nectomys system under laboratory conditions, probably the same is likely to occur in natural conditions. In support to this hypotesis there are also the facts that human mansonic shistosomiasis has a very low prevalence in Sumidouro and endemicity among the rodents has not changed even after repetead treatments of the local patients. Based on our experiments, we conclude that N. squamipes has become a natural host of S. mansoni and possibly may participate in keeping the cycle of schistosomiasis transmission at Pamparrao Valley.