Figure 5
Example of candidate gene transcript abundance associated with horn expression. (a) Quantitative real-time PCR was conducted on the O. nigriventris InR transcript, using 28S as a control for overall levels of RNA (Pfaffl, 2004; Johnson et al, 2005). At the beginning of horn growth (early Period II) there is no evidence for differential expression of the InR gene between large males which grow horns, and small males and females who lack horns. However, during the period of maximal horn growth (late Period II) there is a highly significant (P<0.0001) difference in expression of the InR gene that is associated with horn growth: horn cells from small males and females had significantly higher levels of InR transcript than similar cells from large males. (b) The insulin signaling pathway, illustrating one explanation for this result: as signaling through the insulin pathway is increased overall in large males, the expression of InR decreases in horn discs due to kinase-dependent inactivation of its transcriptional activator FOXO by PKB/Akt (red bar; Kramer et al, 2003; Puig and Tijan, 2005). (c) In horn discs of small males and females pathway activity is truncated at some point downstream from the insulin receptor (tissue ‘reprogramming’ resulting from the mechanism of horn dimorphism). Reduced signaling through this pathway keeps FOXO in an activated state, causing an upregulation of InR transcription (Puig and Tijan, 2005) and shutting off cell proliferation through the transcriptional inhibitor 4E-BP (red arrows; Junger et al, 2003). Data from D Emlen, L Corley, I Dworkin and Q Szafran, in preparation.
