Dev Genes Evol (2009) 219:427–435 DOI 10.1007/s00427-009-0301-0 SHORT COMMUNICATION A conserved function of the zinc finger transcription factor Sp8/9 in allometric appendage growth in the milkweed bug Oncopeltus fasciatus Nina D. Schaeper & Nikola-Michael Prpic & Ernst A. Wimmer Received: 9 June 2009 / Accepted: 30 August 2009 / Published online: 16 September 2009 # The Author(s) 2009. This article is published with open access at Springerlink.com Abstract The genes encoding the closely related zinc finger transcription factors Buttonhead (Btd) and D-Sp1 are expressed in the developing limb primordia of Drosophila melanogaster and are required for normal growth of the legs. The D-Sp1 homolog of the red flour beetle Tribolium castaneum, Sp8 (appropriately termed Sp8/9), is also required for the proper growth of the leg segments. Here we report on the isolation and functional study of the Sp8/9 gene from the milkweed bug Oncopeltus fasciatus. We show that Sp8/9 is expressed in the developing appendages throughout development and that the downregulation of Sp8/9 via RNAi leads to antennae, rostrum, and legs with shortened and fused segments. This supports a conserved role of Sp8/9 in allometric leg segment growth. However, all leg segments including the claws are present and the expression of the leg genes Distal-less, dachshund, and homothorax are proportionally normal, thus providing no evidence for a role of Sp8/9 in appendage specification. Keywords Sp transcription factors . Leg development . Allometric organ growth . Appendage evolution . Insect development Communicated by S. Roth N. D. Schaeper : N.-M. Prpic : E. A. Wimmer (*) Georg-August-Universität, Johann-Friedrich-Blumenbach-Institut für Zoologie und Anthropologie, Abteilung Entwicklungsbiologie, GZMB, Ernst-Caspari-Haus, Justus-von-Liebig-Weg 11, 37077 Göttingen, Germany e-mail: ewimmer@gwdg.de Introduction The members of the Sp gene family encode evolutionarily conserved proteins, which are characterized by the presence of three zinc finger motifs (for nucleic acid binding) as well as an additional short conserved motif called Btd box (a transactivation domain), and are involved in a variety of developmental processes in both insects and vertebrates (reviewed in Zhao and Meng 2005). In Drosophila, two different Sp factor encoding genes, buttonhead (btd) and D-Sp1, have been shown to be expressed during leg development (Wimmer et al. 1996; Schöck et al. 1999; Estella et al. 2003). The btd gene is required for the specification and formation of the entire leg disc primordia (Estella et al. 2003; McKay et al. 2009). The gene is acting upstream of the well- characterized leg specification gene Distal-less (Dll) and seems to be capable of activating the entire leg developmental cascade when expressed ectopically (Estella et al. 2003). The role of the D-Sp1 gene during leg development is less clear, but seems to be partially redundant with btd (Schöck et al. 1999; Estella et al. 2003). The homolog of D-Sp1 in the red flour beetle Tribolium castaneum, Sp8, has been shown to be involved in the control of allometric growth of the leg segments, but the evidence for a role of Sp8 in the specification of the entire leg primordium is ambiguous (Beermann et al. 2004). Orthologous genes to D-Sp1 and Tc Sp8 have also been identified in the mouse, termed Sp8 and Sp9 (Bell et al. 2003; Treichel et al. 2003; Kawakami et al. 2004). Please note that the murine Sp8 gene has also been called inaccurately mBtd (Treichel et al. 2003), because it was named at a time when its orthology to either Drosophila btd or Drosophila D-Sp1 was unclear. Intriguingly, both murine genes are also involved in limb outgrowth. Here we present the isolation of an Sp factor encoding gene from the milkweed bug O. fasciatus. Although our