Lamka M L, Boulet A M, Sakonju S
Howard Hughes Medical Institute, Department of Human Genetics, University of Utah, Salt Lake City 84112.
Development. 1992 Dec;116(4):841-54. doi: 10.1242/dev.116.4.841.
The Abdominal-B (Abd-B) gene, a member of the bithorax complex (BX-C), specifies the identities of parasegments (PS) 10-14 in Drosophila. Abd-B codes for two structurally related homeodomain proteins, ABD-B m and ABD-B r, that are expressed in PS10-13 and PS14-15, respectively. Although ABD-B m and r proteins have distinct developmental functions, ectopic expression of either protein during embryogenesis induces the development of filzkörper and associated spiracular hairs, structures normally located in PS13, at ectopic sites in the larval thorax and abdomen. These results suggest that other parasegmental differences contribute to the phenotype specified by ABD-B r activity in PS14. Both ABD-B m and r repress the expression of other homeotic genes, such as Ubx and abd-A, in PS10-14. However, the importance of these and other cross-regulatory interactions among homeotic genes has been questioned. Since ectopic UBX protein apparently failed to transform abdominal segments, González-Reyes et al. (González-Reyes, A., Urquía, N., Gehring, W.J., Struhl, G. and Morata, G. (1990). Nature 344, 78-80) proposed a functional hierarchy in which ABD-A and ABD-B activities override UBX activity. We tested this model by expressing UBX and ABD-B m proteins ectopically in wild-type and BX-C-deficient embryos. Ectopic ABD-B m does not prevent transformations induced by ectopic UBX. Instead, ectopic UBX and ABD-B m proteins compete for the specification of segmental identities in a dose-dependent fashion. Our results support a quantitative competition among the homeotic proteins rather than the existence of a strict functional hierarchy. Therefore, we suggest that cross-regulatory interactions are not irrelevant but are important for repressing the expression of competing homeotic proteins. To explain the apparent failure of ectopic UBX to transform the abdominal segments, we expressed UBX at different times during embryonic development. Our results show that ectopic UBX affects abdominal cuticular identities if expressed during early stages of embryogenesis. In later embryonic stages, abdominal segments become resistant to transformation by ectopic UBX while thoracic segments remain susceptible. Head segments also show a similar stage-dependent susceptibility to transformation by ectopic UBX in early embryogenesis but become resistant in later stages. These results suggest that abdominal and head identities are determined earlier than are thoracic identities.
腹节B（Abd - B）基因是双胸复合体（BX - C）的成员之一，它决定了果蝇中副节（PS）10 - 14的特征。Abd - B编码两种结构相关的同源结构域蛋白，即ABD - B m和ABD - B r，它们分别在PS10 - 13和PS14 - 15中表达。尽管ABD - B m和r蛋白具有不同的发育功能，但在胚胎发育过程中，这两种蛋白的异位表达都会在幼虫胸部和腹部的异位位点诱导filzkörper和相关气门毛的发育，这些结构通常位于PS13。这些结果表明，其他副节差异也对PS14中由ABD - B r活性所决定的表型有影响。ABD - B m和r都能抑制PS10 - 14中其他同源异型基因的表达，如Ubx和abd - A。然而，同源异型基因之间这些及其他交叉调节相互作用的重要性受到了质疑。由于异位表达的UBX蛋白显然未能转化腹部体节，冈萨雷斯 - 雷耶斯等人（冈萨雷斯 - 雷耶斯，A.，乌尔基亚，N.，盖林，W.J.，斯特鲁尔，G.和莫拉塔，G.（1990年）。《自然》344卷，78 - 80页）提出了一种功能层次结构，其中ABD - A和ABD - B的活性优先于UBX的活性。我们通过在野生型和BX - C缺陷型胚胎中异位表达UBX和ABD - B m蛋白来测试这个模型。异位表达的ABD - B m并不能阻止异位表达的UBX所诱导的转化。相反，异位表达的UBX和ABD - B m蛋白以剂量依赖的方式竞争体节特征的决定。我们的结果支持同源异型蛋白之间存在定量竞争，而不是存在严格的功能层次结构。因此，我们认为交叉调节相互作用并非无关紧要，而是对于抑制竞争性同源异型蛋白的表达很重要。为了解释异位表达的UBX明显未能转化腹部体节的现象，我们在胚胎发育的不同时间表达UBX。我们的结果表明，如果在胚胎发育早期表达，异位表达的UBX会影响腹部表皮特征。在胚胎发育后期，腹部体节对异位表达的UBX诱导的转化产生抗性，而胸部体节仍然敏感。头部体节在胚胎发育早期对异位表达的UBX诱导的转化也表现出类似的阶段依赖性敏感性，但在后期变得抗性。这些结果表明，腹部和头部特征的决定比胸部特征的决定更早。
