The Drosophila myogenic inhibitor Him gene is essential for adult muscle function and muscle stem cell maintenance

Vertebrate muscle fibres have a population of Muscle Stem Cells (MuSCs), or "satellite cells", vital to muscle growth, homeostasis and repair. In Drosophila, adult MuSCs with similar characteristics have only recently been described. This has opened up the Drosophila system for analysing how MuSCs operate in muscle maintenance, repair and ageing. Here we show that the Him gene is expressed in the adult muscle progenitors (AMPs), or myoblasts, that make the adult Drosophila thoracic flight and jump muscles. Notably, we also show that Him is expressed in the flight muscle MuSCs identifying Him as only the second genetic marker of these insect MuSCs. We then explored Him function. Him mutants have disrupted organisation of the thoracic jump muscle, resulting in reduced jumping ability. Him mutants also have a reduced pool of the myoblasts that will develop into the flight muscles. In the flight muscles themselves, Him mutants have an age-dependent decrease in the number of MuSCs, indicating that Him is required for maintenance of the adult muscle stem cell population. Moreover, this decrease in MuSCs coincides with a functional effect: there is an age-dependent decline in flight ability. Overall, Him is a novel marker of the Drosophila adult MuSC, and is required during ageing both to maintain MuSC number and flight ability.