Ancient DNA of Guinea Pigs (Cavia spp.) Indicates a Probable New Center of Domestication and Pathways of Global Distribution.

Guinea pigs (Cavia spp.) have a long association with humans. From as early as 10,000 years ago they were a wild food source. Later, domesticated Cavia porcellus were dispersed well beyond their native range through pre-Columbian exchange networks and, more recently, widely across the globe. Here we present 46 complete mitogenomes of archaeological guinea pigs from sites in Peru, Bolivia, Colombia, the Caribbean, Belgium and the United States to elucidate their evolutionary history, origins and paths of dispersal. Our results indicate an independent centre of domestication of Cavia in the eastern Colombian Highlands. We identify a Peruvian origin for the initial introduction of domesticated guinea pigs (Cavia porcellus) beyond South America into the Caribbean. We also demonstrate that Peru was the probable source of the earliest known guinea pigs transported, as part of the exotic pet trade, to both Europe and the southeastern United States. Finally, we identify a modern reintroduction of guinea pigs to Puerto Rico, where local inhabitants use them for food. This research demonstrates that the natural and cultural history of guinea pigs is more complex than previously known and has implications for other studies regarding regional to global-scale studies of mammal domestication, translocation, and distribution.

Hearing differences in Hartley guinea pig stocks from two breeders.

Across the world, dozens of outbred Hartley guinea pig stocks are used for auditory experiments. The genetic makeup of these different stocks will differ due to differences in breeding protocols, history and genetic drift. In fact, outbred breeding protocols are not intended to produce genetically identical animals, neither across breeders, nor across time. For this reason, it is unclear how reproducible experimental results are likely to be using animals from different stocks. We evaluated the consistency of cochlear function using both clicks and tones in Hartley guinea pigs as a function of breeder (Kuiper and Charles River) and sex using archival Auditory Brain Stem Response (ABR) data and tissue from our own laboratory. Sound levels required to reach baseline threshold for click-induced ABRs were similar between male Charles River and male Kuiper guinea pig stocks. However, the median and average thresholds after exposure to high level noise were larger in the Kuiper population than in the Charles River population with corresponding threshold shifts higher in the Kuiper than in the Charles River animals. We evaluated the relationship between pure-tone thresholds and sex, age, breeder stock, left or right cochleas, weight and 5 test frequencies before and after noise exposure using a linear mixed statistical model. Across all frequencies, the effect of breeder on baseline threshold is statistically significant, with effect sizes most pronounced at the lower frequencies before exposure to noise. After noise exposure, the differences are minimal in the model, indicating that differences in threshold shift are chiefly due to differences in initial baseline hearing. However, a contingency calculation comparing response/no response at the highest speaker output at 32 kHz gave a statistically significant difference between the stocks: 28% of Kuiper cochleas responded to the highest output of the speaker as compared with 71.4% of Charles River cochleas, indicating that noise exposure induced a larger threshold shift in a greater proportion of Kuiper animals. Using our archival cochlear tissue from these studies, we confirmed the sex of each animal by PCR, then compared males and females of the Kuiper stock. Across all baseline frequencies, the effect of sex on threshold is statistically significant, with no noticeable difference after exposure. The effect sizes for baseline thresholds are most pronounced at lower frequencies. These data demonstrate that Hartley guinea pig stocks from different breeders are not uniform in their auditory characteristics, and that due to these differences, results and conclusions can differ among laboratories. Moreover, within a single stock, males and females can provide different data, confirming that male and female animals must be individually evaluated in any auditory protocol.

Temperature Preference in IAF Hairless and Hartley Guinea Pigs (Cavia porcellus).

The Hairless strain of guinea pigs (Cavia porcellus) is the result of a spontaneous recessive mutation first identified at the Institute Armand Frappier (IAF) in 1978. Despite the longstanding availability of this strain, little is known about its thermoregulatory behavior. The aim of this study was to determine temperature preference in Hartley and Hairless guinea pigs by observing each strain in a ring-shaped apparatus containing a nonlinear temperature gradient. Temperatures were maintained by separately controlled heating mats lining the apparatus. Set point temperatures ranged from 24 to 38 °C. Guinea pigs (Hartley female, Hairless female, and Hairless male guinea pigs; n = 8 each group) were placed either singly or in pairs at 1 of the 8 randomized starting points within the apparatus. Subjects were observed for 30 min and coded for location within the temperature gradient by both frequency and duration. When placed singly in the apparatus, all 3 groups spent more time in the 30 °C zones. However, when placed as pairs with a cagemate, Hartley female guinea pigs spent more time in the cooler range of temperatures from 24 to 30 °C, whereas Hairless guinea pigs preferred a range of 30 to 38 °C. These results confirm a temperature preference of 30 ± 2 °C for both Hartley and Hairless guinea pigs when singly housed. However, data from the paired housing condition suggest that context plays an important role in thermoregulatory behavior.

Enriched open field facilitates exercise and social interaction in 2 strains of guinea pigs (Cavia porcellus).

Current housing guidelines for laboratory rodents include recommendations for enrichment. Working with guinea pigs, we have developed an open-field enrichment paradigm that provides several aspects of this species' natural environment. These naturalistic aspects include access to increased space for exploration, access to western timothy (Phleum pratense L.) hay, and grouping as a herd to facilitate social interaction. To determine the immediate effect on behavior from access to the enriched environment, female guinea pigs from 2 strains, IAF Hairless and NIH Hartley, were observed in both standard home cages and an open-field enriched environment. Subjects were housed with cagemates in pairs for the home-cage observation and were grouped as a herd when in the open-field arena. Behaviors were videorecorded for 1 h and then scored. Salivary cortisol levels were measured both prior to and immediately after behavioral observations. Analyses revealed higher levels of activity and social interaction in the open-field arena compared with the home cage, with no significant change in salivary cortisol levels. These results suggest that exposure to the open-field environment provide increased opportunities for exercise and social enrichment. Although additional studies are needed to determine long-term effects on experimental outcomes, the open-field configuration holds promise as a laboratory enrichment paradigm for guinea pigs.

Dinâmica da inversão do saco vitelino em preás (Galea spixii Wagler, 1831) / Dynamics of yolk sac inversion in galea (Galea spixii Wagler, 1831)

O objetivo deste trabalho foi estudar o período de inversão do saco vitelino bem como a dinâmica resultante deste processo na gestação inicial em preás, utilizando-se microscopia de luz, microscopia eletrônica de varredura e de transmissão. No décimo segundo dia de gestação observou-se o desenvolvimento dos endodermas parietal e visceral delimitando a cavidade do saco vitelino. O endoderma parietal foi evidenciado revestindo a superfície fetal da placenta corioalantoidea bem como contornando o espaço delimitado pela decídua capsular. Estes endodermas apresentaram formato prismático e encontraram-se separados do trofoblasto por uma desenvolvida membrana de Reichert. Já o endoderma visceral continha vasos vitelínicos e possuía vilosidades apenas em determinadas áreas. No décimo quarto dia de gestação verificou-se a inversão do saco vitelino, caracterizada pela degeneração do endoderma parietal e trofoblasto mural, associado ao desaparecimento gradual da membrana de Reichert. Como consequência deste fenômeno, o endoderma visceral passou a constituir uma interface com o epitélio uterino. Após a inversão, o endoderma parietal que permaneceu íntegro foi aquele que se apoiava na superfície da placenta corioalantóidea, apresentando células em formato colunar alto e característica de epitélio pseudoestratificado. O endoderma visceral apresentou numerosas vilosidades apicais principalmente em regiões próximas a placenta corioalantóidea. Com o contínuo desenvolvimento do embrião e placenta corioalantóidea, observou-se o surgimento de importante área de aposição entre os endodermas visceral e parietal. A inversão do saco vitelino representou uma disposição anatômica favorável ao desenvolvimento embrionário, além de ser uma característica evolutiva nesta espécie de roedor.

The aim of this study was to study the time of yolk sac inversion as well as the dynamics resulting from this process in galea throughout pregnancy. For this, conventional histological techniques, scanning electron microscopy and transmission electron microscopy were used. Parietal and visceral endoderm delimiting the yolk sac cavity was observed at 12 days of pregnancy. The parietal endoderm was coating the fetal surface of the chorioallantoic placenta as well as delimiting the decidua capsularis area. This endoderm had prismatic format and were apart from the trophoblast by an enlarged Reichert's membrane. The visceral endoderm had vitelline vessels and there were villi only in certain areas. At 14 days of pregnancy the yolk sac inversion was characterized by the degeneration of parietal endoderm and mural trophoblast, and also the gradual disappearance of the Reichert's membrane. So it made the visceral endoderm establish an interface with the uterine epithelium. After the inversion, the parietal endoderm which remained intact was the one that rested on the chorioallantoic placenta surface. It presented cells with high columnar format and pseudostratified epithelium featured. The visceral endoderm presented many apical villi, especially in areas close to the chorioallantoic placenta. The continued development of the embryo and chorioallantoic placenta evidenced the emergence of an important apposition area between visceral and parietal endoderm. The yolk sac inversion represented an anatomical arrangement in favor of the embryo development as well as an evolutionary trait in this rodent species.

Molecular cloning and expression of the IL-10 gene from guinea pigs.

The Guinea pig (Cavia porcellus) is one of the most relevant small animals for modeling human tuberculosis (TB) in terms of susceptibility to low dose aerosol infection, the organization of granulomas, extrapulmonary dissemination and vaccine-induced protection. It is also considered to be a gold standard for a number of other infectious and non-infectious diseases; however, this animal model has a major disadvantage due to the lack of readily available immunological reagents. In the present study, we successfully cloned a cDNA for the critical Th2 cytokine, interleukin-10 (IL-10), from inbred Strain 2 guinea pigs using the DNA sequence information provided by the genome project. The complete open reading frame (ORF) consists of 537 base pairs which encodes a protein of 179 amino acids. This cDNA sequence exhibited 87% homology with human IL-10. Surprisingly, it showed only 84% homology with the previously published IL-10 sequence from the C4-deficient (C4D) guinea pig, leading us to clone IL-10 cDNA from the Hartley strain of guinea pig. The IL-10 gene from the Hartley strain showed 100% homology with the IL-10 sequence of Strain 2 guinea pigs. In order to validate the only published IL-10 sequence existing in Genbank reported from C4D guinea pigs, genomic DNA was isolated from tissues of C4D guinea pigs. Amplification with various sets of primers showed that the IL-10 sequence reported from C4D guinea pigs contained numerous errors. Hence the IL-10 sequence that is being reported by us replaces the earlier sequence making our IL-10 sequence to be the first one accurate from guinea pig. Recombinant guinea pig IL-10 proteins were subsequently expressed in both prokaryotic and eukaryotic cells, purified and were confirmed by N-terminal sequencing. Polyclonal anti-IL-10 antibodies were generated in rabbits using the recombinant IL-10 protein expressed in this study. Taken together, our results indicate that the DNA sequence information provided by the genome project is useful to directly clone much needed cDNAs necessary to study TB in the guinea pig. The newly cloned guinea pig IL-10 cDNA and recombinant proteins will serve as valuable resources for immunological studies in the guinea pig model of TB and other diseases.

Morfologia do epidídimo de cobaio (Cavia porcellus) em diferentes fases do desenvolvimento pós-natal / Morphological differentiation of the epididymis of guinea pig (Cavia porcellus) at different ages

A evolução morfológica do ducto epididimário de cobaio (Cavia porcellus, L.) foi avaliada nas faixas etárias de 10, 20, 30, 45,60, 70, 90 e 100 dias após o nascimento, mostrando-se complexa, o que é decorrente da própria diferenciação pós-natal do epitélio epididimário. Assim sendo, o segmento inicial epididimário mostrou um aumento de altura epitelial correspondente aos valores médios de alturas das células principais do epitélio tubular, crescente após 45 dias de idade pós-natal. As alturas epiteliais obtidas em nível do segmento médio foram maiores em idades juvenis (de 10 a 45 dias) do que na idade pré-puberal (60 dias), tendo diminuído entre os 70 e 100 dias de idade. No segmento terminal epididimário, verificou-se um decréscimo marcante da altura epitelial tubular a partir da idade puberal (70 dias) até a idade adulta (a partir de 90 dias). Além disso, este segmento apresentou ondulações epiteliais que cessaram aos 70 dias de idade, momento em que a luz tubular se preenche com espermatozoides e esfoliações celulares. Os tipos celulares do epitélio tubular epididimário, células principais, basais e apicais, foram observados em todas as idades. Em epidídimos de animais muito jovens, observou-se a predominância de células colunares indiferenciadas. A partir de 20 dias de idade, houve prevalência natural das células principais sobre os demais tipos celulares, como é peculiar ao epitélio epididimário de mamíferos, de modo geral.

The morphological evolution of the epididymal duct of guinea pigs (Cavia porcellus, L.) was studied on 10, 20, 30, 45, 60, 70, 90 and 100 days of age, being complex, which is due to the proper differentiation postnatal in the epididymal epithelium. Thus, it was observed that the initial segment of the epididymis reveals an increase of epithelial height corresponding to the average height of the main tubular epithelium cells, generally progressive after 45 days of age. The epithelial height in the middle segment were higher in younger stages (10 to 45 days) than in the prepubertal age (60 days), and have a decrease among 70 to 100 days of age. The ductular terminal segment starts a gradual decrease of the epithelial heights from puberal age (70 days), until adult age (90 to 100 days). In addition, this segment showed epithelial waviness that disappeared after 70 days of age, when the lumen fills it with sperm and cellular exfoliation. The cell types of the epididymal tubular epithelium, principal cells, basal cells and apical cells, were observed in all ages. In the epididymis of very young animals, there was a predominance of undifferentiated columnar cells. After 20 days of age, there was natural prevalence of the principal cells on other cell types, what is a feature similar to other mammals.

Guinea pig ID-like families of SINEs.

Previous studies have indicated a paucity of SINEs within the genomes of the guinea pig and nutria, representatives of the Hystricognathi suborder of rodents. More recent work has shown that the guinea pig genome contains a large number of B1 elements, expanding to various levels among different rodents. In this work we utilized A-B PCR and screened GenBank with sequences from isolated clones to identify potentially uncharacterized SINEs within the guinea pig genome, and identified numerous sequences with a high degree of similarity (>92%) specific to the guinea pig. The presence of A-tails and flanking direct repeats associated with these sequences supported the identification of a full-length SINE, with a consensus sequence notably distinct from other rodent SINEs. Although most similar to the ID SINE, it clearly was not derived from the known ID master gene (BC1), hence we refer to this element as guinea pig ID-like (GPIDL). Using the consensus to screen the guinea pig genomic database (Assembly CavPor2) with Ensembl BlastView, we estimated at least 100,000 copies, which contrasts markedly to just over 100 copies of ID elements. Additionally we provided evidence of recent integrations of GPIDL as two of seven analyzed conserved GPIDL-containing loci demonstrated presence/absence variants in Cavia porcellus and C. aperea. Using intra-IDL PCR and sequence analyses we also provide evidence that GPIDL is derived from a hystricognath-specific SINE family. These results demonstrate that this SINE family continues to contribute to the dynamics of genomes of hystricognath rodents.

Sexual dimorphism in the craniofacial growth of the guinea pig (Cavia porcellus).

Variation between the sexes during ontogeny is frequently overlooked in discussions of the phylogenetic patterns of adult sexual dimorphism. Different growth trajectories can produce identical degrees and direction of adult dimorphism and the possibility exists that similarities in adults may be the result of differing growth patterns, suggesting independent evolutionary pathways among species to the seemingly identical adult morphology. We quantified the sexual dimorphism in craniofacial skeletal growth of Cavia porcellus, the guinea pig, using longitudinally collected radiographs. Guinea pigs have male-biased sexual dimorphism in size and in growth parameters, despite literature reports to the contrary. These results, analyzed with equivalent data for five species of rodents, and two outgroups representing similarly sized mammals, a rabbit and a marsupial, indicate that some aspects of sexual differences in growth follow phylogenetic lines, while others are a function of whether the species has male- or female-biased dimorphism.

Guinea pig serum erythropoietin (EPO) selectively stimulates guinea pig erythroid progenitors: human or mouse erythroid progenitors do not form erythroid burst-forming unit colonies in response to guinea pig serum EPO.

Erythropoietin (EPO) is the primary regulator of mammalian erythropoiesis, providing a proliferative and differentiative signal to the early EPO-responsive erythroid progenitors, burst-forming unit-erythroid (BFU-E) and colony-forming unit-erythroid, as well as to later EPO-responsive erythroid progenitors. EPO is secreted by the kidney in response to hypoxia and anemia. There is an extensive biological crossreactivity between human EPO and the EPOs of other mammals. Necas et al. have reported that this crossreactivity may not include the guinea pig (Cavia porcelllus). Because the specificity of the guinea pig's erythropoietic responses may be of biological significance, we compared guinea pig hypoxic serum with mouse (m) and human (h) recombinant (r) EPOs for their ability to induce erythroid progenitor proliferation and differentiation in semisolid cultures. Guinea pig bone marrow mononuclear cells (BMMCs) formed BFU-E colonies in response to guinea pig hypoxic serum, rhEPO, or rmEPO in a dose-dependent fashion. Neither human nor mouse BMMCs responded to guinea pig hypoxic serum; however, guinea pig hypoxic serum exerted no inhibitory effect on human or mouse in vitro erythroid differentiation in the presence of rhEPO or rmEPO. The intensity of the EPO band on Western blotting analysis of guinea pig hypoxic serum was significantly greater than in nonhypoxic serum. This suggests that guinea pig erythropoiesis is mediated by EPO and stimulated by hypoxia in a fashion similar to that observed in human and mouse erythropoiesis. Furthermore, guinea pig EPO did not stimulate human or mouse erythroid differentiation in vitro, whereas guinea pig erythroid progenitors could be stimulated by human or mouse EPO, suggesting structural differences in guinea pig EPO and EPO receptor (EPOR) compared with human or mouse EPO and EPOR. These differences probably evolved after the guinea pig's ancestors diverged from myomorph rodents. Further characterization of the guinea pig EPO and EPOR should facilitate our understanding of the interaction between EPO and EPOR.

The guinea-pig is not a rodent.

In 1991 Graur et al. raised the question of whether the guinea-pig, Cavia porcellus, is a rodent. They suggested that the guinea-pig and myomorph rodents diverged before the separation between myomorph rodents and a lineage leading to primates and artiodactyls. Several findings have since been reported, both for and against this phylogeny, thereby highlighting the issue of the validity of molecular analysis in mammalian phylogeny. Here we present findings based on the sequence of the complete mitochondrial genome of the guinea-pig, which strongly contradict rodent monophyly. The conclusions are based on cumulative evidence provided by orthologically inherited genes and the use of three different analytical methods, none of which joins the guinea-pig with myomorph rodents. In addition to the phylogenetic conclusions, we also draw attention to several factors that are important for the validity of phylogenetic analysis based on molecular data.

Phylogenetic place of guinea pigs: no support of the rodent-polyphyly hypothesis from maximum-likelihood analyses of multiple protein sequences.

Graur et al.'s (1991) hypothesis that the guinea pig-like rodents have an evolutionary origin within mammals that is separate from that of other rodents (the rodent-polyphyly hypothesis) was reexamined by the maximum-likelihood method for protein phylogeny, as well as by the maximum-parsimony and neighbor-joining methods. The overall evidence does not support Graur et al.'s hypothesis, which radically contradicts the traditional view of rodent monophyly. This work demonstrates that we must be careful in choosing a proper method for phylogenetic inference and that an argument based on a small data set (with respect to the length of the sequence and especially the number of species) may be unstable.

Neural BC1 RNA as an evolutionary marker: guinea pig remains a rodent.

The traditional morphologically grounded placement of South American guinea pig-like rodents (Caviomorpha) within one of the two rodent suborders, Hystricognathi, has been disputed by recent analysis of protein and nucleic acid sequence data. The Caviomorpha and possibly all Hystricognathi would be considered a separate order, distinct from the other rodent suborder, Sciurognathi, and thus of the order Rodentia, and would be placed closer phylogenetically to other mammals [Graur, D., Hide, W. A. & Li, W.-H. (1991) Nature (London) 351, 649-652]. To address the discrepancy between morphological comparisons and sequence analyses, we have applied an alternative form of molecular analysis. We demonstrate that BC1 RNA, a neural-specific small cytoplasmic RNA that is the product of a retropositionally generated gene (a gene derived by reverse transcription of RNA followed by insertion of the DNA copy into the genome), is present in Sciurognathi and guinea pig but not in other mammalian orders including Lagomorpha, Artiodactyla, and Primates. The species-confined, tissue-specific expression of a retroposed sequence therefore supports the morphological evidence for monophyly of Rodentia inclusive of guinea pig and demonstrates the usefulness of such molecular genetic markers. Furthermore, the conservation and tissue-specific expression of the BC1 RNA gene in the two divergent rodent suborders suggests that this macromolecule has been exapted into a functional role (i.e., coopted into a variant or novel function) in the rodent nervous system.

Taxonomical classification of the guinea pig based on its Cu/Zn superoxide dismutase sequence.

Cu-Zn superoxide dismutase was purified from guinea pig (Cavia porcellus) liver up to electrophoretic homogeneity and its amino acid sequence was elucidated by automated Edman degradation of proteolytic fragments and mass spectrometry. The protein was classified as a typical mammalian cytosolic Cu-Zn superoxide dismutase by molecular mass, specific activity, amino acid sequence and N-terminal acetylation. A dendrogram constructed from previously known vertebrate cytosolic Cu-Zn superoxide dismutase sequences reflects the commonly accepted taxonomy and phylogenetic relationships of the species, whereas the guinea pig sequence is similarly remote form muriform rodents, lagomorphs, equiforms and primates. The data appear incompatible with the assumption that the Caviomorpha with the representative Cavia porcellus form a common phylogenetic clade with the muriform rodents but rather have to be considered a distinct order of mammals. The degree of similarity of the sequences further suggests that the mammalian clade diverged into rodents, primates, lagomorphs and caviomorphs at about the same time.

The molecular taxonomy and evolution of the guinea pig.

On the basis of 18 protein sequences totaling 2,413 aligned amino acid sites, it is suggested that the guinea pigs and the myomorphs (rat-like rodents) are not monophyletic. Rather, the evolutionary lineage leading to the guinea pig seems to have branched off prior to the divergence among myomorphs, lagomorphs, primates, chiropterans, artiodactyls, and carnivores. It is suggested therefore that the Caviomorpha (guinea pig-like rodents) and possibly the Hystricomorpha (porcupine-like rodents) should be elevated in taxonomic rank and conferred an ordinal status distinct from the Rodentia. This suggestion calls for a reevaluation of the morphological evolution of guinea pigs and further molecular studies on the possibility of paraphyly of the order Rodentia. If the monophyly of rodents holds, it must be concluded that the pattern of molecular evolution in many guinea pig genes has been extremely unusual and that the causes for this pattern should be sought. It is also suggested that claims of large differences in the rate of molecular evolution between guinea pigs and myomorphs may have been exaggerated in many cases as a result of an erroneous phylogenetic position for the guinea pig. The average rate of amino acid replacement in the guinea pig seems to be comparable to that in the rat and the mouse. However, the data indicate that myomorph and caviomorph genes evolve, on average, about two times faster than their human counterparts. Finally, our analysis provides evidence against the hypothesis that the gundi (an African rodent) represents the most ancient rodent lineage.

The biochemical phylogeny of guinea-pigs and gundis, and the paraphyly of the order rodentia.

1. Molecular data indicate that caviomorphs (guinea-pig-like rodents) and myomorphs (rat-like rodents) are not monophyletic. 2. Rather, the evolutionary lineage leading to the guinea-pig may have branched off prior to the divergence among myomorphs, lagomorphs, primates, chiropterans, artiodactyls, and carnivores. 3. Thus, the guinea-pig lineage probably represents an ancient eutherian lineage, and should be conferred an independent ordinal status. 4. The gundis (Ctenodactylidae) also seem to have branched off before the divergence among myomorphs, primates, and artiodactyls, but after the divergence of the guinea-pig. 5. Therefore, the order Rodentia as defined at the present time is in all probability a paraphyletic group devoid of taxonomic validity. 6. Previous claims pertaining to large differences in the rate of molecular evolution between guinea-pigs and myomorphs may have been exaggerated in many cases as a result of the erroneous phylogenetic position attributed to the guinea-pig. 7. The average rate of amino acid replacement in the guinea-pig is comparable to that in the rat and the mouse. 8. Protein-coding genes of myomorphs and caviomorphs evole, on average, about two times faster than their counterparts in gundis and humans.

Molecular phylogeny.

The avalanche of molecular sequence data from a wide variety of organisms and genes makes the construction and testing of evolutionary trees a widespread and demanding activity. We present the most recent advances in the interpretation of molecular data, as well as recent phylogenetic results affecting both molecular evolutionary biology and other areas of biological research.

Is the guinea-pig a rodent?

The guinea-pig (Cavia porcellus), traditionally classified as a New World hystricomorph rodent, often shows anomalous morphological and molecular features in comparison with other eutherian mammals. For example, its insulin differs from that of other mammals in anabolic and growth-promoting activities and in its capability to form hexamers. Indeed, the literature about the molecular evolution of guinea-pigs abounds in references to 'convergent evolution', 'extremely rapid rates of substitution', and 'unique evolutionary mechanisms'. These claims are based on the assumption that the guinea-pig is a rodent. Our phylogenetic analyses of amino-acid sequence data, however, imply that the guinea-pig diverged before the separation of the primates and the artiodactyls from the myomorph rodents (rats and mice). If true, then the myomorphs and the caviomorphs do not constitute a natural clade, and the Caviomorpha (or the Histricomorpha) should be elevated in taxonomical rank and regarded as a separate mammalian order distinct from the Rodentia. If, as suggested by recent data, the myomorphs branched off before the divergence among the carnivores, lagomorphs, artiodactyls and primates, then the new order would represent an early divergence in eutherian radiation.