Percival-Smith A, Hayden D J
Department of Zoology, University of Western Ontario, London, Ontario N6A 5B7, Canada.
Genetics. 1998 Sep;150(1):189-98. doi: 10.1093/genetics/150.1.189.
Sex Combs Reduced (SCR) activity is proposed to be required cell nonautonomously for determination of tarsus identity, and Extradenticle (EXD) activity is required cell autonomously for determination of arista identity. Using the ability of Proboscipedia to inhibit the SCR activity required for determination of tarsus identity, we found that loss-of-EXD activity is epistatic to loss-of-SCR activity in tarsus vs. arista determination. This suggests that in the sequence leading to arista determination SCR activity is OFF while EXD activity is ON, and in the sequence leading to tarsus determination SCR activity is ON, which turns EXD activity OFF. Immunolocalization of EXD in early third-instar larval imaginal discs reveals that EXD is localized in the nuclei of antennal imaginal disc cells and localized in the cytoplasm of distal imaginal leg disc cells. We propose that EXD localized to the nucleus suppresses tarsus determination and activates arista determination. We further propose that in the mesodermal adepithelial cells of the leg imaginal discs, SCR is required for the synthesis of a tarsus-inducer that when secreted acts on the ectoderm cells inhibiting nuclear accumulation of EXD, such that tarsus determination is no longer suppressed and arista determination is no longer activated.
性梳减少（SCR）活性被认为在确定跗节特征时非细胞自主地发挥作用，而额外齿（EXD）活性在确定触角芒特征时细胞自主地发挥作用。利用触须基因抑制确定跗节特征所需的SCR活性的能力，我们发现在跗节与触角芒的确定过程中，EXD活性缺失相对于SCR活性缺失是上位性的。这表明在导致触角芒确定的序列中，SCR活性关闭而EXD活性开启，而在导致跗节确定的序列中，SCR活性开启，这会关闭EXD活性。在三龄幼虫早期成虫盘阶段对EXD进行免疫定位显示，EXD定位于触角成虫盘细胞的细胞核中，并定位于成虫腿盘远端细胞的细胞质中。我们提出定位于细胞核的EXD抑制跗节的确定并激活触角芒的确定。我们进一步提出，在腿成虫盘的中胚层上皮下细胞中，SCR是合成跗节诱导物所必需的，该诱导物分泌后作用于外胚层细胞，抑制EXD的核积累，从而使跗节的确定不再受到抑制，触角芒的确定不再被激活。
