Activation of the Drosophila receptor tyrosine kinase Torso (Tor) only at the termini of the embryo is achieved by the localized expression of the maternal gene Torso-like (Tsl). Tor has a second function in the prothoracic gland as the receptor for prothoracicotropic hormone (PTTH) that initiates metamorphosis. Consistent with the function of Tor in this tissue, Tsl also localizes to the prothoracic gland and influences developmental timing. Despite these commonalities, in our studies of Tsl we unexpectedly found that tsl and tor have opposing effects on body size; tsl null mutants are smaller than normal, rather than larger as would be expected if the PTTH/Tor pathway was disrupted. We further found that whereas both genes regulate developmental timing, tsl does so independently of tor. Although tsl null mutants exhibit a similar length delay in time to pupariation to tor mutants, in tsl: tor double mutants this delay is strikingly enhanced. Thus, loss of tsl is additive rather than epistatic to loss of tor. We also find that phenotypes generated by ectopic PTTH expression are independent of tsl. Finally, we show that a modified form of tsl that can rescue developmental timing cannot rescue terminal patterning, indicating that Tsl can function via distinct mechanisms in different contexts. We conclude that Tsl is not just a specialized cue for Torso signaling but also acts independently of PTTH/Tor in the control of body size and the timing of developmental progression. These data highlight surprisingly diverse developmental functions for this sole Drosophila member of the perforin-like superfamily.MACPF | growth rate | ecdysis | heterochrony

Pore-forming members of the membrane attack complex/perforin-like (MACPF) protein superfamily perform wellcharacterized roles as mammalian immune effectors. For example, complement component 9 and perforin function to directly form pores in the membrane of Gram-negative pathogens or virally infected/transformed cells, respectively. In contrast, the only known MACPF protein in Drosophila melanogaster, Torso-like, plays crucial roles during development in embryo patterning and larval growth. Here, we report that in addition to these functions, Torso-like plays an important role in Drosophila immunity. However, in contrast to a hypothesized effector function in, for example, elimination of Gram-negative pathogens, we find that torso-like null mutants instead show increased susceptibility to certain Gram-positive pathogens such as Staphylococcus aureus and Enterococcus faecalis. We further show that this deficit is due to a severely reduced number of circulating immune cells and, as a consequence, an impaired ability to phagocytose bacterial particles. Together these data suggest that Torso-like plays an important role in controlling the development of the Drosophila cellular immune system. KEYWORDS genetics of immunity; membrane attack complex/perforin-like proteins; Torso-like; cellular immunity; Drosophila melanogaster; phagocytosis M EMBRANE attack complex/perforin-like (MACPF) proteins comprise a large, functionally diverse superfamily of molecules that include the mammalian immunity proteins complement component 9 (C9) and perforin. Both of these proteins function in immunity via forming pores in the membrane of target cells (Rosado et al. 2008;Law et al. 2010;Hadders et al. 2012). Structural studies have further revealed that MACPF proteins are related to an ancient family of bacterial pore-forming virulence factors, the cholesterol-dependent cytolysins (Rosado et al. 2007). These data suggest an ancestral role for pore-forming MACPF proteins in immune defense or attack. However, it is important to note that certain MACPF proteins perform roles outside immunity or pathogenicity. For example a group of MACPF proteins in mammals is important for brain development (Zheng et al. 1996;Adams et al. 2002;Kobayashi et al. 2014), and the subject of this study, Torso-like (Tsl), is crucial for embryonic and larval development in Drosophila (Stevens et al. 1990;Martin et al. 1994;Johnson et al. 2013). It is currently unclear whether pore formation is central to the function of these molecules.Many components of both cellular and humoral immunity share an ancient origin in metazoan evolution, and as such, studies in Drosophila have provided many crucial insights into the mechanisms of mammalian immunity. The Drosophila cellular immune response acts to eliminate pathogens and apoptotic cells using processes, including phagocytosis, encapsulation, and melanization. Plasmatocytes (hereon referred to as hemocytes) are the major hematopoietic cell type present in adult Drosophila (Lanot et al. 2001) and are responsib...