Project description:The six-layered neocortex is exclusively present in mammals and mediates sensory-motor and higher-order functions. Key differences in this structure and its connections exist between the main mammalian groups: eutherians and marsupials, however, the molecular changes that underlie these known morphological differences remain unknown. This question is particularly difficult to address because small and transient changes in gene expression during development may be crucial to brain formation, which would not be detectable in adult transcriptomic analyses. To address this question of the developmental origin of changes in the evolution of the mammalian neocortex, we performed transcriptomic analysis on the marsupial fat-tailed dunnart (Sminthopsis crassicaudata) at postnatal ages P12 and P20 corresponding to the generation of infragranular (layers 5/6) and supragranular (layers 2/3) neurons, respectively. We assembled a de novo transcriptome of the neocortex of fat-tailed dunnarts using RNA-seq data from all samples, then differential gene expression analysis performed across the two ages. Additional cross-species analysis was performed against existing mouse neocortical datasets in the NCBI Sequence Read Archive at equivalent developmental ages embryonic (E) day 12.5 (SRR1509162, SRR1509163, SRR1509164) and E16 (SRR5755669, SRR5755670, SRR5755671, SRR5755672). We identified 12,632 protein-coding transcripts orthologous to mouse RNA reference sequences (Refseq) in the dunnart neocortical transciptome. The results also revealed divergences in gene sets known to be enriched in different neuronal populations, revealing a more advanced stage of maturation in the marsupial neocortex at the period of infragranular birth compared to the eutherian mouse.
Project description:Marsupials exhibit unique biological features that provide fascinating insights into many aspects of mammalian development. These include their distinctive mode of reproduction, altricial stage at birth, and the associated heterochrony that is required for their crawl to the pouch and teat attachment. Marsupials are also an invaluable resource for mammalian comparative biology, forming a distinct lineage from the extant placental and egg-laying monotreme mammals. Despite their unique biology, marsupial resources are lagging behind those available for placentals. The fat-tailed dunnart (Sminthopsis crassicaudata) is a laboratory based marsupial model, with simple and robust husbandry requirements and a short reproductive cycle making it amenable to experimental manipulations. Here we present a detailed staging series for the fat-tailed dunnart, focusing on their accelerated development of the forelimbs and jaws. This study provides the first skeletal developmental series on S. crassicaudata and provides a fundamental resource for future studies exploring mammalian diversification, development and evolution.
Project description:Melanopsin confers photosensitivity to a subset of retinal ganglion cells and is responsible for many non-image-forming tasks, like the detection of light for circadian entrainment. Recently, two melanopsin genes, Opn4m and Opn4x, were described in non-mammalian vertebrates. However, only one form, Opn4m, has been described in the mammals, although studies to date have been limited to the placentals and have not included the marsupials. We report here the isolation and characterization of an Opn4 gene from an Australian marsupial, the fat-tailed dunnart (Sminthopsis crassicaudata), and present evidence which suggests that the Opn4x gene was lost before the placental/marsupial split. In situ hybridization shows that the expression of Opn4 in the dunnart eye is restricted to a subset of ganglion cells, a pattern previously reported for rodents and primates. These Opn4-positive cells are randomly distributed across the dunnart retina. We also undertook a comparative analysis with the South American marsupial, the grey short-tailed opossum (Monodelphis domestica), and two placental mammals, mouse and human. This approach reveals that the two marsupials show a higher sequence identity than that seen between rodents and primates, despite separating at approximately the same point in time, some 65-85 Myr ago.
