The systematics of the genus Didelphis (Marsupialia: Didelphidae) in North and Middle America

Gardner, Alfred L.

doi:10.5962/bhl.title.156476

Cited by 79 publications

(93 citation statements)

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“…These features make the ganglion cell layer a suitable place for the search of correlations between developmental patterns and its mature organization. Finally, opossums, and particularly Didelphis marsupialis, are presumed to retain many features of mammalian ancestors (Romer 1936;Gardner 1973;Ebbesson 1984). Thus, information on retinogenesis in these taxa may be of much value for comparative purposes.…”

Section: Introductionmentioning

confidence: 99%

Genesis of neurons of the retinal ganglion cell layer in the opossum

et al. 1992

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In this study, we have examined the genesis of neurons of the retinal ganglion cell layer of the opossum Didelphis marsupialis by [3H]-thymidine autoradiography. Our results suggest that most neurons surviving to adulthood are generated in postnatal life from day 1 to day 23. Cells are generated according to a coarse gradient from the retinal geometric center to the periphery. Regional analysis of soma size distributions in different cohorts suggest that this gradient is actually formed by two partially-overlapping, concentric waves of cell proliferation. Most medium and large ganglion cells are formed during the early wave, whereas most displaced amacrine cells and small ganglion cells are formed during the late wave. Our results confirm the appropriateness of the opossum as a model for studies of development of the mammalian visual system.

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Section: Introductionmentioning

confidence: 99%

Genesis of neurons of the retinal ganglion cell layer in the opossum

et al. 1992

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“…These results put the observations of the regression plots and the boxplots of centroid size and Procrustes distance in a statistical context, since the observation of those plots insinuates that males could be peramorphic relative to females. According to Gardner (1973) and Bergallo and Cerqueira (1994), during pregnancy and lactation, didelphid females stop growing since energetic resources are allocated in reproduction, while males continue to grow. In species that live for more than one cycle (e.g., M. domestica; see Bergallo and Cerqueira 1994), females continue to grow after the reproductive season, but they never reach male size.…”

Section: Discussionmentioning

confidence: 99%

“…Individuals were classified into five ageclasses (2-6) according to tooth eruption and wear, based on the model proposed by Gardner (1973) for Didelphis: 2-M2, m2 and m3 in position; 3-M3 in position and permanent premolars erupting; 4-permanent premolars and m4 in position; 5-all teeth erupted but with little wear; 6-all teeth moderately to excessively worn. Due to the one-year life cycle of M. dimidiata, dental ages become a good proxy for age, since litters are born in a 2-month period (December and January for Buenos Aires Province, Argentina) and adult males die a year later, while females live a couple of months more (Reig 1964;Pine et al 1985;Baladrón et al 2012).…”

Section: Materials and Landmarksmentioning

confidence: 99%

Peramorphic males and extreme sexual dimorphism in Monodelphis dimidiata (Didelphidae)

Chemisquy

2015

Zoomorphology

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The southern short-tailed opossum, Monodelphis dimidiata, is a species known not only for its semelparous life cycle, but also for the extreme sexual dimorphism of adults, where males are not only larger, but also have distinctive morphological characters in their skull. Using geometric morphometrics and a suite of statistical tests, I analyzed the postweaning ontogenetic development of this species in order to evaluate the ageclass where sexual dimorphism becomes significant and the amount of change exhibited by both sexes. My results showed that M. dimidiata partly follows the ontogenetic pattern described for didelphids by previous authors. The character that escapes the general pattern is rostral length, which becomes shorter instead of lengthening throughout the development. This change could be related to an increment in the bite force in the anterior part of the dentition. The amount of sexual dimorphism found for this species is larger than the reported previously for other American marsupials, and I also found a higher rate of growth in males at the attaining of sexual maturity. Based on my results and the information available for other didelphids, I can suggest that M. dimidiata males undergo through a process of hypermorphosis, resulting in a peramorphic condition. It is possible that the extreme sexual dimorphism present in this species is related to reproductive success, specially taking into account their semelparous life cycle.

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“…It is probable that the partial segregation of optic terminals is the primitive state, since studies of karyotypes, geographical distribution and behavior of the Didelphids suggest that marsupialis was the original stock and that speciation of virginiana occurred in marsupialis isolates (Gardner 1973). It is interesting to note these proposed primitive features (segregated retino-geniculate terminals; beta pathway) in the light of evidence suggesting that early marsupials adopted an arboreal way of life, and that only later marsupials included terrestrial forms (Marshall 1979).…”

Section: Phylogenetic Considerationsmentioning

confidence: 99%

Fiber order in the opossum's optic tract

1992

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The distribution of axons by size in the optic tract of the South American opossum, Didelphis marsupialis was studied. Thin and semi-thin sections were examined, and measurements of axonal diameter were made on electron micrographs taken from various locations across the optic tract of normal opossums. In order to determine the contributions of the different axon diameter classes to the crossed and uncrossed retinofugal pathways, measurements were also made from the tracts of opossums in which one eye had been enucleated 5 weeks previously. Within the opossum's optic tract, the axons are partially segregated by their size: the deepest parts of the tract contain only fine and medium-sized axons, whereas coarse axons are also present superficially. In the middle of the tract, all three size classes are present. At increasingly superficial positions, there is a steady reduction in the proportion of medium-diameter axons, and an increase in the number of the finest axons. Medium and coarse axons contribute to both the crossed and uncrossed pathways, and the uncrossed component is displaced superficially relative to the crossed component. The fine axons in the deeper parts of the tract arise from both retinae, while those in the superficial parts of the tract, near the pial surface, are virtually all crossed. The opossum's optic tract thus displays the segregation of axons by size found in placental mammals, and follows a pattern reminiscent of that found in carnivores. Such a common organizational plan, particularly the similarities between the didelphids and carnivores, is suggestive of an early acquisition of parallel visual pathways in mammalian phylogeny. Since the fiber order in the optic tract of eutherians is a chronological map of axonal arrival during development, these results suggest that a conserved developmental mechanism has led to a common organizational plan.

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The systematics of the genus Didelphis (Marsupialia: Didelphidae) in North and Middle America

Cited by 79 publications

References 0 publications

Genesis of neurons of the retinal ganglion cell layer in the opossum

Genesis of neurons of the retinal ganglion cell layer in the opossum

Peramorphic males and extreme sexual dimorphism in Monodelphis dimidiata (Didelphidae)

Fiber order in the opossum's optic tract

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