Biology subjects

Christoffels, V.

Publications and source records attributed to Christoffels, V..

2 recordsLinked to original sources

bioRxiv2019

Heart development in the lizards (Varanidae) with the greatest extent of ventricular septation

Among lizards, only monitor lizards (Varanidae) have a functionally divided cardiac ventricle. This enables them to sustain higher systemic blood pressures and higher metabolic rates than other reptiles of similar size. The division results from the concerted action of three partial septa, which may have homology to the full ventricular septum of mammals and archosaurs. Homology, however has only been inferred from anatomical comparisons of hearts of adult monitors whereas gene expression during heart development has not been studied. We show in developing monitors that the partial septa that separate the left and right ventricle, the muscular ridge and bulbuslamelle, express the evolutionary conserved transcription factors Tbx5, Irx1 and Irx2, orthologues of which mark the full ventricular septum. Compaction of embryonic trabeculae contributes to the formation of these septa. The septa are positioned, however, to the right of the atrioventricular junction and they do not partake in the separation of incoming atrial blood streams. Instead, the vertical septum within the left ventricle separates the atrial blood streams. It expresses Tbx3 and Tbx5, which orchestrate the formation of the electrical conduction axis of the full ventricular septum. These patterns of expression are more pronounced in monitors than in other lizards, and are associated with a deep electrical activation near the vertical septum, contrasting the primitive base-to-apex activation of other lizards. We conclude that current concepts of ventricular septum formation apply well to the monitor septa and that there is evolutionary conservation of ventricular septum formation among amniote vertebrates.

evolutionary biology↗

bioRxiv2018

Little variation in the morphology of the atria across 13 orders of birds

Mammals and birds acquired high performance hearts and endothermy during their independent evolution from amniotes with many reptile characters. A literature review shows that the variation in atrial morphology is greater in mammals than in ectothermic reptiles. We therefore hypothesized that the transition from ectothermy to endothermy associated with greater variation in cardiac structure. We tested the hypothesis in birds, by assessing the variation in 15 characters in hearts from 13 orders of birds. Hearts were assessed by gross morphology and histology, and we focused on the atria as they have multiple features that lend themselves to quantification. We found bird hearts to have multiple features in common with ectothermic reptiles (synapomorphies), for instance the presence of three sinus horns. Convergent features were shared with crocodylians and mammals, such as the cranial offset of the left atrioventricular junction. Other convergent features like the compact organization of the atrial walls were shared with mammals only. Sinus myocardium expressing Isl1 was node-like (Mallard), thickened (chicken), or anatomically indistinct from surrounding myocardium (Lesser redpoll). Some features were distinctively avian (apomorphies), including the presence of a left atrial antechamber, and the ventral merger of the left and right atrium, which was found in parrots and passerine birds. Most features, however, exhibited little variation. For instance, there were always three systemic veins and two pulmonary veins, whereas among mammals there are 2-3 and 1-7, respectively. Our findings suggest that the transition to high cardiac performance does not necessarily lead to greater variation in cardiac structure.

evolutionary biology↗