Thylamys elegans

Abstract: In this paper we studied the energetics and thermoregulation of the Chilean mouse-opossum Thylamys elegans (Dielphidae) a nocturnal small marsupial, endemic of southern South America. We studied their standard energetic and determined whether they exhibit shallow daily torpor or deep prolonged torpor as a function of ambient temperature and food availability. Thylamys elegans partially supports the hypothesis that Neotropical marsupials have somewhat a higher basal metabolic rate (BMR) and thermal conductance (Cm) than Australian ones. In fact, BMR was higher but Cm was lower than expected for their body mass. The higher mass-independent BMR of the Chilean mouse-opossum may be explained by its insectivorous food habits and its low Cm by its temperate habitats. Euthermic Chilean mouse-opossum showed daily fluctuations in body temperature being significantly higher during night time. In addition T. elegans entered in daily torpor and aroused spontaneously only was food was absent. That is, this species display a facultative type of daily torpor because propensity to enter in torpor was dependent of the combination of food absence and low ambient temperature. No torpor was observed when food was available. During torpor ambient temperature was slightly above ambient temperature between 0.3 to 0.5 °C. Torpor in this species as well as in marsupials in general, appears to be a flexible and an opportunistic response to unpredictable environmental conditions.

Abstract: We studied the retinal photoreceptors in the mouse opossum Thylamys elegans, a nocturnal South American marsupial. A variety of photoreceptor properties and color vision capabilities have been documented in Australian marsupials, and we were interested to establish what similarities and differences this American marsupial showed. Thylamys opsin gene sequencing revealed two cone opsins, a longwave-sensitive (LWS) opsin and a shortwave-sensitive (SWS1) opsin with deduced peak sensitivities at 560 nm and 360 nm (ultraviolet), respectively. Immunocytochemistry located these opsins to separate cone populations, a majority of LWS cones (density range 1,600-5,600/mm(2)) and a minority of SWS1 cones (density range 100-690/mm(2)). With rod densities of 440,000-590,000/mm(2), the cones constituted 0.4-1.2% of the photoreceptors. This is a suitable adaptation to nocturnal vision. Cone densities peaked in a horizontally elongated region ventral to the optic nerve head. In ventral-but not dorsal-retina, roughly 40% of the LWS opsin-expressing cones occurred as close pairs (double cones), and one member of each double cone contained a colorless oil droplet. The corneal electroretinogram (ERG) showed a high scotopic sensitivity with a rod peak sensitivity at 505 nm. At mesopic light levels, the spectral ERG revealed the contributions of a UV-sensitive SWS1 cone mechanism and an LWS cone mechanism with peak sensitivities at 365 nm and 555 nm, respectively, confirming the tuning predictions from the cone opsin sequences. The two spectral cone types provide the basis for dichromatic color vision, or trichromacy if the rods contribute to color processing at mesopic light levels.

Abstract: To investigate the phylogenetic relationships of living marsupials, morphometric and G-banded chromosome analyses were made in the Chilean species Dromiciops gliroides (Microbiotheria) and Thylamys elegans (Didelphimorphia). Chromosome arm lengths and patterns of G-bands were compared in at least eight bone marrow metaphase spreads in six and nine specimens, respectively. They were contrasted with those published for another 11 American and Australian genera. Three of six autosomal pairs (A1, A3, and C2) were uniquely shared by Dromiciops and some Australian species, being different in shape and G-banded patterns from those with similar total sizes in Thylamys and other South American didelphoid karyotypes. Such chromosomal correspondences suggest the past occurrence of at least three pericentric inversions. A table of character states constructed from chromosomal G-band comparisons is presented, showing that cytogenetic data agree with Szalay's (1982) hypothesis on the affinities of the South American Dromiciops with Australian marsupials.

Abstract: Six mouse opossum species are currently recognized for the genus Thylamys in South America, of which we hypothesized the evolutionary relationships for those located in the Andean Altiplano and canyons of the Region I of Chile, as well as for those located south of the Atacama Desert down to 37˚ S. To that goal we sequenced the cytochrome b mitochondrial gene and data were analyzed using different phylogenetic criteria such as parsimony, distance and likelihood available in the program PAUP 4.0. The different phylogenetic approaches agreed in recovering two well defined clades: one constituted by Thylamys pallidior that included specimens from the coast and the “pre-cordillera” of the Region I of Chile, and the other clade constituted by Thylamys elegans that included populations located south to the Atacama Desert. Therefore, we demonstrate that populations located in the canyons and andean areas of northern Chile belong to T. pallidior, while those located south to the Atacama desert are recognized as T. elegans, results that confirm the occurrence of two thylamyine species in Chile.