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

Abstract

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.. CC-BY-ND 4.0 International license It is made available under a (which was not peer-reviewed) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity.The copyright holder for this preprint . http://dx.doi.org/10.1101/397034 doi: bioRxiv preprint first posted online Aug. 21, 2018; 3 | INTRODUCTIONMammals and birds evolved independently from reptile-like ancestors as two vertebrate classes that are characterized by high metabolic rates and endothermy (Warren, 2008; Green., et al., 2014; Tattersall, 2016). When the hearts of mammals and reptiles are compared, it is evident that mammalian hearts are remodeled by incorporation of systemic and pulmonary vein myocardium to the atria (Jensen, Boukens, Wang, Moorman, Christoffels, 2014a; Carmona, Ariza, Cañete, Muñoz-Chápuli, 2018). The number of venous orifices to the left atrium can vary between 1 (dugongs) and 7 (armadillos), and the myocardial sleeve of the veins may be extensive (mouse) or all but gone (harbour porpoise) (Rowlatt, 1990; Mommersteeg et al.,2007). The number of orifices and the extent of myocardium even varies within the species (Nathan, Eliakim, 1966;Calkins et al., 2007, Rowlatt, 1990. In human, for example, the left atrium typically receives four pulmonary veins, but three and five veins are...