Yorkie and Scalloped regulate availability of Serrate signaling cells required for crystal cell differentiation in the larval lymph gland. Gabriel B. Ferguson, Julian Martinez-Agosto. University of California, Los Angeles, Department Of Human Genetics. Los Angeles, CA.

   Cellular microenvironments established by the spatial and temporal expression of specific signaling molecules are critical for both the maintenance and lineage specific differentiation of stem and progenitor cells. The Drosophila Lymph Gland is an ideal system in which signaling can be studied within the highly relevant and complex process of hematopoiesis. Lymph Gland homeostasis is largely regulated by a cluster of cells known as the Poster Signaling Center or PSC. These cells secrete the signaling factors Hedgehog and PVF1 which are required to maintain the hematopoietic progenitors in an undifferentiated state. The PSC also expresses the Notch ligand Serrate which has been shown to be a critical component in the differentiation of the Crystal Cell lineage of mature hemocytes in Drosophila. While specification of Crystal Cells is known to be dependent on Serrate activity, activation of Notch signaling, and its downstream target the transcription factor Lozenge, we provide evidence that the Serrate expressing cells in the PSC are not required for this process. Here, we report that the Hippo Pathway effectors Yorkie and Scalloped are required for the expression of Serrate in interior signaling cells of Drosophila Lymph Gland. These Serrate expressing cells are distinct from the niche cells and are required for the specification of the Crystal Cell Lineage of hemocytes in Drosophila. Furthermore, we found that Yorkie expression is activated by Notch signaling specifically in Crystal Cells to promote cell survival via the anti- apoptosis factor Diap1. These findings demonstrate a role for Hippo pathway components in normal hematopoiesis and establish internal signaling cells as an alternative niche for blood progenitor cells.