Haemocytes, the immune system cells of Drosophila, are produced during larval development by the fly's lymph gland, a structure that is established by mid-embryogenesis. Now, for the first time, Svetlana Minakhina and Ruth Steward identify Drosophila haematopoietic stem cells (HSCs) in the lymph glands of embryos and young larvae (see p. ). They use lineage analysis to show that single lymph gland cells give rise to large, persistent clones (indicative of the presence of stem cells that can both self-renew and produce pluripotent daughter cells) or to smaller, transient clones (indicative of non-stem cells). Interestingly, even single cells marked very early in lymph gland development can form both types of clones, challenging the long-held belief that all the cells in embryonic primordia form stem cells. Finally, the researchers show that the conserved zinc finger protein Zfrp8/PDCD2 is essential for the maintenance of the HSCs but not of their pluripotent daughter cells. This factor, they suggest, might play a similar role in vertebrate haematopoiesis.
