Department of Organismal Biology and Anatomy, University of Chicago, Chicago, IL 60637, USA.
Published: June 1999
AI Article Synopsis
Article Abstract
The ascendancy of dinosaurs on land near the close of the Triassic now appears to have been as accidental and opportunistic as their demise and replacement by therian mammals at the end of the Cretaceous. The dinosaurian radiation, launched by 1-meter-long bipeds, was slower in tempo and more restricted in adaptive scope than that of therian mammals. A notable exception was the evolution of birds from small-bodied predatory dinosaurs, which involved a dramatic decrease in body size. Recurring phylogenetic trends among dinosaurs include, to the contrary, increase in body size. There is no evidence for co-evolution between predators and prey or between herbivores and flowering plants. As the major land masses drifted apart, dinosaurian biogeography was molded more by regional extinction and intercontinental dispersal than by the breakup sequence of Pangaea.
Embryo implantation requires tightly coordinated signaling between the blastocyst and the endometrium, and is crucial for the establishment of a uteroplacental unit that persists until term in eutherian mammals. In contrast, marsupials, with a unique life cycle and short gestation, make only brief fetal-maternal contact and lack implantation. To better understand the evolutionary link between eutherian implantation and its ancestral equivalent in marsupials, we compare single-cell transcriptomes from the receptive and non-receptive endometrium of the mouse and guinea pig with that of the opossum, a marsupial.
The evolutionary transition from early synapsids to therian mammals involved profound reorganization in locomotor anatomy and function, centered around a shift from "sprawled" to "erect" limb postures. When and how this functional shift was accomplished has remained difficult to decipher from the fossil record alone. Through biomechanical modeling of hindlimb force-generating performance in eight exemplar fossil synapsids, we demonstrate that the erect locomotor regime typifying modern therians did not evolve until just before crown Theria.
Therian female mammals compensate for the dosage of X-linked gene expression by inactivating one of the X-chromosomes. X-inactivation is facilitated by the master regulator Xist long non-coding RNA, which coats the inactive-X and facilitates heterochromatinization through recruiting different chromatin modifiers and changing the X-chromosome 3D conformation. However, many mechanistic aspects behind the X-inactivation process remain poorly understood.
The study talks about a special way some genes work differently depending on whether they come from the mom or dad, called genomic imprinting, which is found in certain mammals like koalas and other marsupials.
Researchers found a specific area in the gene PRKACB that showed this imprinting by checking how a special chemical called methylation was added to it.
They discovered that in two types of marsupials, the gene from the mom was modified differently than from the dad, showing that PRKACB is a new gene that behaves this way in marsupials, which might be important for its function in these animals.
The epigenetic phenomenon of genomic imprinting is puzzling. While epigenetic modifications in general are widely known in most species, genomic imprinting in the animal kingdom is restricted to autosomes of therian mammals, mainly eutherians, and to a lesser extent in marsupials. Imprinting causes monoallelic gene expression.
