Differentiation and function of cardiac valves in the adult Drosophila heart.

Drosophila, like all insects, has an open circulatory system for the distribution of haemolymph and its components. The circulation of the haemolymph is essentially driven by the pumping activity of the linear heart. The heart is constructed as a tube into which the haemolymph is sucked and pumped forward by rhythmic contractions running from the posterior to the anterior, where it leaves the heart tube. The heart harbours cardiac valves to regulate flow directionality, with a single heart valve differentiating during larval development to separate the heart tube into two chambers. During metamorphosis, the heart is partially restructured, with the linear heart tube with one terminal wide-lumen heart chamber being converted into a linear four-chambered heart tube with three valves. As in all metazoan circulatory systems, the cardiac valves play an essential role in regulating the direction of blood flow. We provide evidence that the valves in adult flies arise via transdifferentiation, converting lumen-forming contractile cardiomyocytes into differently structured valve cells. Interestingly, adult cardiac valves exhibit a similar morphology to their larval counterparts, but act differently upon heart beating. Applying calcium imaging in living specimens to analyse activity in valve cells, we show that adult cardiac valves operate owing to muscle contraction. However, valve cell shape dynamics are altered compared with larval valves, which led us to propose our current model of the opening and closing mechanisms in the fly heart.