Patel N H, Hayward D C, Lall S, Pirkl N R, DiPietro D, Ball E E
Department of Organismal Biology and Anatomy and Howard Hughes Medical Institute, University of Chicago, 5841 S. Maryland Ave., MC1028, Chicago, IL 60637, USA.
Development. 2001 Sep;128(18):3459-72. doi: 10.1242/dev.128.18.3459.
While the expression patterns of segment polarity genes such as engrailed have been shown to be similar in Drosophila melanogaster and Schistocerca americana (grasshopper), the expression patterns of pair-rule genes such as even-skipped are not conserved between these species. This might suggest that the factors upstream of pair-rule gene expression are not conserved across insect species. We find that, despite this, many aspects of the expression of the Drosophila gap gene hunchback are shared with its orthologs in the grasshoppers S. americana and L. migratoria. We have analyzed both mRNA and protein expression during development, and find that the grasshopper hunchback orthologs appear to have a conserved role in early axial patterning of the germ anlagen and in the specification of gnathal and thoracic primordia. In addition, distinct stepped expression levels of hunchback in the gnathal/thoracic domains suggest that grasshopper hunchback may act in a concentration-dependent fashion (as in Drosophila), although morphogenetic activity is not set up by diffusion to form a smooth gradient. Axial patterning functions appear to be performed entirely by zygotic hunchback, a fundamental difference from Drosophila in which maternal and zygotic hunchback play redundant roles. In grasshoppers, maternal hunchback activity is provided uniformly to the embryo as protein and, we suggest, serves a distinct role in distinguishing embryonic from extra-embryonic cells along the anteroposterior axis from the outset of development - a distinction made in Drosophila along the dorsoventral axis later in development. Later hunchback expression in the abdominal segments is conserved, as are patterns in the nervous system, and in both Drosophila and grasshopper, hunchback is expressed in a subset of extra-embryonic cells. Thus, while the expected domains of hunchback expression are conserved in Schistocerca, we have found surprising and fundamental differences in axial patterning, and have identified a previously unreported domain of expression in Drosophila that suggests conservation of a function in extra-embryonic patterning.
虽然诸如engrailed等体节极性基因的表达模式在黑腹果蝇和美洲沙漠蝗（蚱蜢）中已显示出相似性，但诸如偶数缺失等成对规则基因的表达模式在这些物种之间并不保守。这可能表明成对规则基因表达上游的因子在昆虫物种间并不保守。我们发现，尽管如此，果蝇间隙基因驼背蛋白的表达在许多方面与其在美洲沙漠蝗和东亚飞蝗中的直系同源物是共有的。我们分析了发育过程中的mRNA和蛋白质表达，发现蚱蜢驼背蛋白的直系同源物似乎在胚盘的早期轴向模式形成以及颚和胸原基的特化中具有保守作用。此外，驼背蛋白在颚/胸区域中不同的阶梯状表达水平表明，蚱蜢驼背蛋白可能以浓度依赖的方式起作用（如在果蝇中），尽管形态发生活性不是通过扩散形成平滑梯度来建立的。轴向模式形成功能似乎完全由合子驼背蛋白执行，这与果蝇有根本区别，在果蝇中母体和合子驼背蛋白发挥冗余作用。在蚱蜢中，母体驼背蛋白活性作为蛋白质均匀地提供给胚胎，并且我们认为，从发育开始就沿前后轴在区分胚胎细胞和胚外细胞方面发挥独特作用——果蝇是在发育后期沿背腹轴进行这种区分。腹部节段中驼背蛋白的后期表达是保守的，神经系统中的模式也是如此，并且在果蝇和蚱蜢中，驼背蛋白都在一部分胚外细胞中表达。因此，虽然驼背蛋白表达的预期区域在沙漠蝗中是保守的，但我们发现了轴向模式形成中令人惊讶的根本差异，并在果蝇中鉴定出一个以前未报道的表达区域，这表明在胚外模式形成中存在功能保守。
