1. Department of Cell & Developmental Biology, University of Pennsylvania Medical Center, 1215 BRB II/III, 421 Curie Blvd, Philadelphia, Pennsylvania 19104-6058, USA
2. These authors contributed equally to this work.

Correspondence to: Stephen DiNardo Correspondence and requests for materials should be addressed to S.D. (e-mail: Email: sdinardo@mail.med.upenn.edu).

Abstract

Stem cells divide both to produce new stem cells and to generate daughter cells that can differentiate¹. The underlying mechanisms are not well understood, but conceptually are of two kinds². Intrinsic mechanisms may control the unequal partitioning of determinants leading to asymmetric cell divisions that yield one stem cell and one differentiated daughter cell. Alternatively, extrinsic mechanisms, involving stromal cell signals, could cause daughter cells that remain in their proper niche to stay stem cells, whereas daughter cells that leave this niche differentiate. Here we use Drosophila spermatogenesis as a model stem cell system³ to show that there are excess stem cells and gonialblasts in testes that are deficient for Raf activity. In addition, the germline stem cell population remains active for a longer fraction of lifespan than in wild type. Finally, raf is required in somatic cells that surround germ cells. We conclude that a cell-extrinsic mechanism regulates germline stem cell behaviour.