Instituto Gulbenkian de Ciência, Rua da Quinta Grande 6, Oeiras, Portugal.
Evodevo. 2013 Mar 1;4(1):7. doi: 10.1186/2041-9139-4-7.
Morphological innovation is an elusive and fascinating concept in evolutionary biology. A novel structure may open up an array of possibilities for adaptation, and thus is fundamental to the evolution of complex multicellular life. We use the respiratory appendages on the dorsal-anterior side of the Drosophila eggshell as a model system for morphological novelty. To study the co-option of genetic pathways in the evolution of this novelty we have compared oogenesis and eggshell patterning in Drosophila melanogaster with Ceratitis capitata, a dipteran whose eggs do not bear dorsal appendages.
During the final stages of oogenesis, the appendages are formed by specific groups of cells in the follicular epithelium of the egg chamber. These cells are defined via signaling activity of the Dpp and EGFr pathways, and we find that both pathways are active in C. capitata oogenesis. The transcription factor gene mirror is expressed downstream of EGFr activation in a dorsolateral domain in the D. melanogaster egg chamber, but could not be detected during C. capitata oogenesis. In D. melanogaster, mirror regulates the expression of two important genes: broad, which defines the appendage primordia, and pipe, involved in embryonic dorsoventral polarity. In C. capitata, broad remains expressed ubiquitously throughout the follicular epithelium, and is not restricted to the appendage primordia. Interestingly pipe expression did not differ between the two species.
Our analysis identifies both broad and mirror as important nodes that have been redeployed in the Drosophila egg chamber patterning network in the evolution of a morphologically novel feature. Further, our results show how pre-existing signals can provide an epithelium with a spatial coordinate system, which can be co-opted for novel patterns.
形态创新是进化生物学中一个难以捉摸而又引人入胜的概念。一个新的结构可能为适应开辟一系列的可能性，因此对复杂多细胞生命的进化至关重要。我们使用果蝇卵壳背前部的呼吸附属物作为形态新颖性的模型系统。为了研究遗传途径在这种新颖性进化中的共适应，我们比较了果蝇和黑腹果蝇的卵子发生和卵壳模式形成，后者的卵子没有背部附属物。
在卵子发生的最后阶段，这些附属物是由卵室滤泡上皮中的特定细胞群形成的。这些细胞通过 Dpp 和 EGFr 途径的信号活性来定义，我们发现这两个途径在 C. capitata 的卵子发生中都很活跃。转录因子基因 mirror 在 D. melanogaster 卵室的背侧和外侧区域，在 EGFr 激活的下游表达，但在 C. capitata 的卵子发生中无法检测到。在 D. melanogaster 中，mirror 调节两个重要基因的表达：broad，定义附属物原基，和 pipe，参与胚胎背腹极性。在 C. capitata 中，broad 仍然在整个滤泡上皮中广泛表达，而不限于附属物原基。有趣的是，pipe 的表达在两个物种之间没有差异。
我们的分析确定了 broad 和 mirror 作为重要的节点，它们在果蝇卵室模式形成网络的进化中被重新部署，以形成一种形态新颖的特征。此外，我们的结果表明，预先存在的信号如何为上皮组织提供一个空间坐标系，这个坐标系可以被重新用于新的模式。
