Monodelphis Research Articles

Locomotor kinetics and kinematics on inclines and declines in the gray short-tailed opossumMonodelphis domestica

Small terrestrial animals continually encounter sloped substrates when moving about their habitat; therefore, it is important to understand the mechanics and kinematics of locomotion on non-horizontal substrates as well as on level terrain. To this end, we trained gray short-tailed opossums (Monodelphis domestica) to move along level, 30 degrees inclined, and 30 degrees declined trackways instrumented with a force platform. Vertical, craniocaudal and mediolateral impulses, peak vertical forces, and required coefficient of friction (mu(req)) of individual limbs were calculated. Two high speed video cameras were used to simultaneously capture whole limb craniocaudal and mediolateral angles at limb touchdown, midstance and lift-off. Patterns on the level terrain were typical for non-primate quadrupeds: the forelimbs supported the majority of the body weight, forelimbs were net braking and hindlimbs net propulsive, and both limb pairs exerted small laterally directed impulses. M. domestica moved more slowly on sloped substrates in comparison to level locomotion, and exhibited a greater mu(req). On inclines, both limb pairs were more protracted at touchdown and more retracted at lift-off, fore- and hindlimbs had equal roles in body weight support, forelimbs exerted greater propulsive impulse than hindlimbs, and mu(req) was greater in the forelimbs than in hindlimbs. On declines, only the forelimbs were more protracted at touchdown; forelimbs supported the great majority of body weight while they generated nearly all of the braking impulse and, despite the disparity in fore- vs hindlimb function on the decline, mu(req) was not significantly different between limbs. These differences on the inclined and declined surfaces most likely result from (1) the location of the opossums' center of mass, which is closer to the forelimbs than to the hindlimbs, and (2) the greater functional range of the forelimbs versus the hindlimbs.

The cremasteric neuromuscular complex in male and female grey short-tailed opossums (Monodelphis domestica)

In marsupials, both sexes possess a cremaster muscle, that of the female being attached to the mammary gland and surrounding subcutaneous tissues. In the grey short-tailed opossum, the muscle in both sexes has a distinct proximal origin from the anterior superior iliac spine. The number of muscle fibres is higher in male than in female opossums on average, but the difference is not statistically significant. Retrograde tracing, achieved by injecting B-cholera toxin into the muscle, showed that labelled neurons occupied several spinal cord segments, the bulk being in L2–L4. There was no overall sex difference in the number of labelled neurons. It is concluded that cremaster muscles are present in both sexes of the grey short-tailed opossum Monodelphis domestica , contain approximately equal numbers of fibres, and differ only in their insertion target. Since the motor innervation also shows no dimorphisms in this species, it is concluded that the whole neuromuscular complex is homologous in the two sexes. This is in marked contrast to the situation in adult eutherian mammals where the muscle is possessed by males only, and where the cremasteric nucleus is highly dimorphic in terms of motor neuron numbers.

Placental Function in Two Distantly Related Marsupials

The biochemical composition of uterine and fetal fluids during pregnancy of the grey short-tailed opossum was compared with new and published data on the tammar wallaby. In the grey short-tailed opossum, there are three main phases of embryonic nourishment. During the first phase, the embryo obtains nutrients from uterine secretion transferred into the yolk sac. The amount of uterine secretion declines during the second phase up to the time of shell coat rupture. As a result, the protein concentration in yolk sac fluid also declines. During phase three, which begins with shell coat rupture, nutrients are predominantly available from the maternal blood. In the grey short-tailed opossum that lacks a vesicular, fluid-filled allantois, waste products such as urea are apparently stored in the yolk sac and from there pass into the maternal circulation across the invasive yolk sac placenta. In contrast, in the tammar wallaby, the main source of nutrients available to the late term fetus is glandular secretion that is complemented by substances from the maternal circulation via the chorio-vitelline placenta, and waste products are stored in the large, fluid-filled allantois.

THALAMIC DEVELOPMENT OF THE GRAY SHORT-TAILED OPOSSUM (MONODELPHIS DOMESTICA)

Few studies have examined thalamic development in the embryonic mammalian brain. For a better understanding of the development of the thalamus, the time sequence of the location of neurons in thalamic nuclei was examined in a developing marsupial embryo and fetus. To accomplish this, uptake of tritiated thymidine was examined in the gray short-tailed opossum (Monodelphis domestica) from 3 days before birth through 16 days after birth. Total neurogenesis was optimal 4 days after birth. Plots of the labeled cells revealed that thalamic and cortical development are almost concurrent, similar to the time course of eutherian mammals. A consistent time gradient of proliferation in dorsal thalamus was not found. Rather, sectors of the dorsal thalamus showed significant differences regardless of day of development. The dorsal sector showed more neurogenesis than the ventral sector, caudal more than rostral, and lateral more than the medial sector. An analysis of individual nuclei of dorsal thalamus showed no significant differences in amount of neurogenesis with day of injection.

Modo-UG, a marsupial nonclassical MHC class I locus

Modo-UG is a class I gene located in the MHC of the marsupial Monodelphis domestica, the gray, short-tailed opossum. Modo-UG is expressed as three alternatively spliced mRNA forms, all of which encode a transmembrane form with a short cytoplasmic tail that lacks phosphorylation sites typically found in classical class I molecules. The three alternative mRNAs would encode a full-length form, an isoform lacking the alpha2 domain, and one lacking both alpha2 and alpha3 domains. Genotyping both captive-bred and wild M. domestica from different geographic regions revealed no variation in the residues that make up Modo-UG's peptide-binding groove. Modo-UG's low polymorphism is contrasting to that of a nearby class I locus, Modo-UA1, which has a highly polymorphic peptide-binding region. Absence of functional polymorphism in Modo-UG is therefore not a general feature of opossum class I genes but the result of negative selection. Modo-UG is the first MHC linked marsupial class I to be described that appears to clearly have nonclassical features.

Evolution of the “OR37” Subfamily of Olfactory Receptors: A Cross-Species Comparison

Genes encoding the olfactory receptors of the "OR37" subfamily of the mouse are characterized by special features including a clustered expression pattern, assembly in two distinct gene clusters, and highly conserved putative promoter motifs. Mining the rat and dog databases revealed that these two species possess highly conserved clusters of OR37 genes at two syntenic genomic loci. In a prototherian mammal, the platypus (Ornithorhynchus anatinus), none of the characteristic OR37 genes were found. Examination of a metatherian mammal, the gray short-tailed opossum (Monodelphis domestica) revealed seven canonical OR37 genes, all phylogenetically related to cluster II genes and also organized similar to cluster II of eutherian species. In addition, their 5' upstream regions comprised sequence motifs related to the putative regulatory sequences of cluster II genes. Typical cluster I OR37 genes were identified only in the eutherian mammals examined, including the evolutionary ancient anteater, wherein OR37 genes related to both clusters were present. Together, these results reveal novel information concerning the phylogenetic origin and important evolutionary steps of the mammalian-specific OR37 olfactory receptor family.

Erratum: Development, inheritance, and linkage-group assignment of 60 novel microsatellite markers for the gray, short-tailed opossum Monodelphis domestica

Erratum: Development, inheritance, and linkage-group assignment of 60 novel microsatellite markers for the gray, short-tailed opossum <i>Monodelphis domestica</i>

Linkage mapping and physical localization of the major histocompatibility complex region of the marsupial Monodelphis domestica

We used genetic linkage mapping and fluorescence in situhybridization (FISH) to conduct the first analysis of genic organization and chromosome localization of the major histocompatibility complex (MHC) of a marsupial, the gray, short-tailed opossum Monodelphis domestica. Family based linkage analyses of two M. domestica MHC Class I genes (UA1, UG) and three MHC Class II genes (DAB, DMA, and DMB) revealed that these genes were tightly linked and positioned in the central region of linkage group 3 (LG3). This cluster of MHC genes was physically mapped to the centromeric region of chromosome 2q by FISH using a BAC clone containing the UA1 gene. An interesting finding from the linkage analyses is that sex-specific recombination rates were virtually identical within the MHC region. This stands in stark contrast to the genome-wide situation, wherein males exhibit approximately twice as much recombination as females, and could have evolutionary implications for maintaining equality between males and females in the ability to generate haplotype diversity in this region. These analyses also showed that three non-MHC genes that flank the MHC region on human chromosome 6, myelin oligodendrocyte glycoprotein (MOG), bone morphogenetic protein 6 (BMP6), and prolactin (PRL), are split among two separate linkage groups (chromosomes) in M. domestica. Comparative analysis with eight other vertebrate species suggests strong conservation of the BMP6–PRL synteny among birds and mammals, although the BMP6–PRL–MHC–ME1 synteny is not conserved.

Reconstructing an Ancestral Mammalian Immune Supercomplex from a Marsupial Major Histocompatibility Complex

The first sequenced marsupial genome promises to reveal unparalleled insights into mammalian evolution. We have used theMonodelphis domestica (gray short-tailed opossum) sequence to construct the first map of a marsupial major histocompatibility complex (MHC). The MHC is the most gene-dense region of the mammalian genome and is critical to immunity and reproductive success. The marsupial MHC bridges the phylogenetic gap between the complex MHC of eutherian mammals and the minimal essential MHC of birds. Here we show that the opossum MHC is gene dense and complex, as in humans, but shares more organizational features with non-mammals. The Class I genes have amplified within the Class II region, resulting in a unique Class I/II region. We present a model of the organization of the MHC in ancestral mammals and its elaboration during mammalian evolution. The opossum genome, together with other extant genomes, reveals the existence of an ancestral “immune supercomplex” that contained genes of both types of natural killer receptors together with antigen processing genes and MHC genes.

Status and applications of genomic resources for the gray, short-tailed opossum, Monodelphis domestica, an American marsupial model for comparative biology

Owing to its small size, favourable reproductive characteristics, and simple husbandry, the gray, short-tailed opossum, Monodelphis domestica, has become the most widely distributed and intensively utilised laboratory-bred research marsupial in the world today. This article provides an overview of the current state and future projections of genomic resources for this species and discusses the potential impact of this growing resource base on active research areas that use M. domestica as a model system. The resources discussed include: fully arrayed, bacterial artificial chromosome (BAC) libraries; an expanding linkage map; developing full-genome BAC-contig and chromosomal fluorescence in situ hybridisation maps; public websites providing access to the M. domestica whole-genome-shotgun sequence trace database and the whole-genome sequence assembly; and a new project underway to create an expressed-sequence database and microchip expression arrays for functional genomics applications. Major research areas discussed span a variety of genetic, evolutionary, physiologic, reproductive, developmental, and behavioural topics, including: comparative immunogenetics; genomic imprinting; reproductive biology; neurobiology; photobiology and carcinogenesis; genetics of lipoprotein metabolism; developmental and behavioural endocrinology; sexual differentiation and development; embryonic and fetal development; meiotic recombination; genome evolution; molecular evolution and phylogenetics; and more.

In silico identification of opossum cytokine genes suggests the complexity of the marsupial immune system rivals that of eutherian mammals

BackgroundCytokines are small proteins that regulate immunity in vertebrate species. Marsupial and eutherian mammals last shared a common ancestor more than 180 million years ago, so it is not surprising that attempts to isolate many key marsupial cytokines using traditional laboratory techniques have been unsuccessful. This paucity of molecular data has led some authors to suggest that the marsupial immune system is 'primitive' and not on par with the sophisticated immune system of eutherian (placental) mammals.ResultsThe sequencing of the first marsupial genome has allowed us to identify highly divergent immune genes. We used gene prediction methods that incorporate the identification of gene location using BLAST, SYNTENY + BLAST and HMMER to identify 23 key marsupial immune genes, including IFN-γ, IL-2, IL-4, IL-6, IL-12 and IL-13, in the genome of the grey short-tailed opossum (Monodelphis domestica). Many of these genes were not predicted in the publicly available automated annotations.ConclusionThe power of this approach was demonstrated by the identification of orthologous cytokines between marsupials and eutherians that share only 30% identity at the amino acid level. Furthermore, the presence of key immunological genes suggests that marsupials do indeed possess a sophisticated immune system, whose function may parallel that of eutherian mammals.

Development, inheritance, and linkage-group assignment of 60 novel microsatellite markers for the gray, short-tailed opossum Monodelphis domestica

Short-tandem-repeat (SSR) or microsatellite polymorphisms are some of the most extensively employed genetic markers in contemporary linkage mapping studies. To date, only a limited number of microsatellites have been isolated in the gray, short-tailed opossum Monodelphis domestica, a South American marsupial widely used for comparative biological and biomedical research. To increase the number of potentially useful mapping markers, we screened 2 microsatellite-enriched genomic libraries containing alternatively (CA)n or (GA)n repeats. A total of 184 clones were sequenced, from which 60 polymorphic microsatellite markers were successfully optimized. The efficiency of this enrichment protocol for M. domestica microsatellite isolation is discussed, and suggestions to improve the outcome are made. All 60 loci showed high allelic diversity, with allele numbers ranging from 2 to 10 in a subset of 33 unrelated animals. Normal Mendelian inheritance was confirmed for all loci by analyzing allelic segregation in 5 two-generation families. One microsatellite appeared to be X linked, and null alleles were found in 5 others. Two-point linkage analyses were implemented using the data on the 5 families, leading to the assignment of 59 of these loci to the existing linkage groups. The 60 novel microsatellites developed in this study will contribute significantly to the M. domestica linkage map, and further QTL mapping studies.

Organization of the Olfactory and Respiratory Skeleton in the Nose of the Gray Short-Tailed Opossum Monodelphis domestica

The internal nasal skeleton in Monodelphis domestica, the gray short-tailed opossum, primarily supports olfactory and respiratory epithelia, the vomeronasal organ, and the nasal gland. This scaffold is built by the median mesethmoid, and the paired vomer and ethmoid bones. The mesethmoid ossifies within the nasal septum cartilage. The bilateral ethmoid segregates respiratory and olfactory regions, and its geometry offers insight into the functional, developmental, and genomic organization of the nose. It forms through partial coalescence of separate elements known as turbinals, which in Monodelphis comprise the maxilloturbinal, nasoturbinal, five endoturbinals, and two ectoturbinals. Geometry of the ethmoid increases respiratory mucosal surface area by a factor of six and olfactory mucosal surface by nearly an order of magnitude. Respiratory epithelium warms and humidifies inspired air, recovers moisture as air is exhaled, and may help mediate brain temperature. In contrast, the olfactory skeleton functions as a series of small funnels that support growth of new olfactory neurons throughout life. Olfactory mucosa lines the mouth of each funnel, forming blind olfactory recesses known as the ethmoid cells, and neuronal axons are funneled from the epithelium through tiny olfactory foramina in the cribriform plate, into close proximity with target glomeruli in the olfactory bulb of the brain where each axon makes its first synapse. The skeleton may thus mediate topological correspondence between odorant receptor areas in the nose with particular glomeruli in the olfactory bulb, enabling growth throughout life of new olfactory neurons and proper targeting by their axons. The geometric arrangement of odorant receptors suggests that a measure of volatility may be a component in the peripheral olfactory code, and that corresponding glomeruli may function in temporal signal processing. Supporting visualizations for this study are available online at www.DigiMorph.org.

Efferent connections of the main olfactory bulb in the opossum ( Monodelphis domestica): A characterization of the olfactory entorhinal cortex in a marsupial

Olfactory projections have been investigated for decades, but few reports using modern, sensitive neural tracers are available. In marsupials, only lesion-degeneration studies exist and they are restricted to the genera Didelphis and Trichosurus. Some of the territories described as olfactory-recipient such as the upper portion of the rhinal fissure and the vomeronasal amygdala are, however, controversial. Also, the characterization of the olfactory portion of the entorhinal cortex is far from clear in acallosal mammals. The present report investigates, using biotinylated dextran-amine, the olfactory connections in the short-tailed opossum ( Monodelphis domestica) and characterizes the olfactory portion of the entorhinal cortex in non-placental mammals. The data indicate that olfactory projections do not reach the upper portion of the rhinal fissure, but partially end in the vomeronasal amygdala, i.e., the medial and posteromedial cortical amygdaloid nuclei; thus, although olfactory and vomeronasal system have largely segregated outputs, areas of overlap should be restudied. The olfactory portion of the entorhinal cortex is much larger than previously described, extending up to the occipital pole of the cerebral hemisphere. Collectively, these data contribute to our understanding of the organization of the hippocampal formation in marsupials.

Acoustic startle response in the opossum &lt;i&gt;Monodelphis domestica&lt;/i&gt; in comparison with the Wistar albino rat

The acoustic startle response (ASR) was studied in 20 gray short-tailed opossums (Monodelphis domestica) and its characteristics were compared with those obtained from responses of 20 Wistar albino rats. The animals were exposed to 10 ms, 110 dB white noise acoustic pulses in the Coulbourn apparatus. Amplitude of ASR was normalized to the weight of animals and then analyzed. The results show that probability of a freezing response is much higher in the opossums that generally respond with lower startle amplitudes in comparison to rats. These differences may reflect different emotional characteristics of the two species, different reactions to fear in opossum and/or different ecological specializations of the two species.

TOOTH ERUPTION IN MONODELPHIS DOMESTICA AND ITS SIGNIFICANCE FOR PHYLOGENY AND NATURAL HISTORY

The gray short-tailed opossum (Monodelphis domestica) began tooth eruption at 32 days with the deciduous premolars (dp3 and dP3). All but 5 teeth completed eruption by 56 days, with only p3, m4, P3, M3, and M4 unerupted at that age. Eruption was complete by 134 days with eruption of P3 and M4. We found no significant differences between sexes in tooth eruption timing, but significant differences occurred among litters at some tooth positions. Timing and sequence of tooth eruption differed somewhat as seen in live young versus that seen in a series of skulls of known age. Live juveniles can be placed into a series of 6 age classes based on emergence of teeth through the gingiva, whereas juvenile skeletal material is best placed into age classes based solely on eruption of upper molars. Other systems of age classes used in didelphid marsupials that are based on sequence of eruption of P3 and M4 are not generally applicable because of variation in this sequence. In didelphids delayed eruption of Il may be functionally related to weaning, whereas weaning and 1st reproduction are not obviously correlated with age class based on molar eruption.

Food Preferences of Captive Gray Short-tailed Opossums (Monodelphis domestica)

The gray short-tailed opossum has been a subject in behavioral and biomedical studies for the last quarter century, but researchers know little about its preferred diet. The authors describe a study designed to determine food preferences of this species for the purpose of identifying suitable rewards to be used in behavioral studies.

The role of the vomeronasal system in food preferences of the gray short-tailed opossum, Monodelphis domestica

Although feeding deficits have been reported in snakes and lizards following vomeronasal system disruption, no deficit has been previously reported in a mammal. We tested gray short-tailed opossums with items from four different food categories prior to occluding access to the vomeronasal organ. Preoperatively, opossums preferred meat to fruit or vegetables. Following occlusion of the nasopalatine canal, but not after control treatment, opossums failed to demonstrate food preferences.

Skin and gland but not urine odours elicit conspicuous investigation by female grey short-tailed opossums, Monodelphis domestica

Chemical communication plays a major role in mammalian behaviour. One mammal with well-developed chemical senses is the grey short-tailed opossum, a small, South American marsupial that has become common in laboratory research. Male and female opossums scent-mark with different body parts, but apparently not with urine, and females enter oestrus following their exposure to male odours. To elucidate the effects of body odours on the behaviour of female opossums, we investigated their ability to discriminate between conspecific odours originating from different body parts. We simultaneously presented female opossums with two cotton balls smeared either with skin and glandular secretions or urine of conspecifics, or with distilled water or blank controls, and we analysed the time females spent investigating each stimulus. Odours of the male suprasternal gland elicited intensive investigation by the test females. Male flank and submandible odours, and female submandible odours, were also attractive to the females. Investigation of male and female urine was comparable to investigation of water and blank controls. We suggest that in this species, which inhabits semiarid areas, urine marking is avoided to reduce dehydration, and consequently urine has lost its biological significance as a chemosignal. On the other hand, glandular and skin secretions, which are more stable than aqueous deposits in dry conditions, allow the opossums to communicate with each other effectively and also to retain body fluids.

Modification of Odor Investigation and Discrimination in Female Opossums (Monodelphis domestica) Following the Ablation of the Accessory Olfactory Bulbs.

To determine whether the vomeronasal system of the Brazilian short-tailed opossum (Monodelphis domestica) is important to the response to conspecific chemical signals, the authors tested female opossums with conspecific odors, before and after ablation of their accessory olfactory bulbs (AOBs). Anesthesia and sham treatments did not modify females' discrimination of conspecific odors when tested against water, between male and female odors, or between different odors from the same male donors. Odor investigation was partially diminished following partial ablation of the AOB, and complete ablation of the AOBs further impaired the ability of females to discriminate between certain odors. These findings provide the first evidence for the importance of the vomeronasal system in the detection of chemosignals of known origin in opossums.