University of Chicago, Dept. of Organismal Biology and Anatomy, CLSC 921B, 920 E. 58th Street, Chicago, IL 60637, USA.
Dev Biol. 2010 May 1;341(1):282-90. doi: 10.1016/j.ydbio.2010.01.040. Epub 2010 Feb 6.
The amnioserosa is an extraembryonic epithelium that evolved in higher cyclorrhaphan flies from distinct serosal and amniotic epithelia. The underlying genetic mechanism of this evolutionary transition is unknown. Amnioserosa development of Drosophila correlates with novel expression characteristics of the homeobox gene zerknüllt (zen), including a broad zen expression domain in the syncytial blastoderm and the complete absence of postgastrular zen expression. Here we examine the functional significance of these features by altering the activity profile of zen in Megaselia (a lower cyclorrhaphan fly with distinct serosal and amniotic epithelia) and Drosophila, and by examining in Megaselia the function of u-shaped group (ush-group) genes, which in Drosophila maintain the amnioserosa after gastrulation when zen is no longer expressed. In Megaselia, loss of postgastrular zen expression abrogates serosa development but allows amnion development. Ectopic expression of zen in early Megaselia embryos allows serosa formation but perturbs amnion development. Megaselia homologues of u-shaped group genes are not essential for serosa formation but mediate germband retraction and dorsal closure. Finally, ectopic postgastrular zen expression in Drosophila causes an enlargement of amnioserosa cells and interferes with the morphogenetic functions of the amnioserosa. Our results suggest that the origin of the amnioserosa involved the loss of postgastrular zen expression from extraembryonic tissue, that the early broad expression domain of Drosophila zen evolved afterwards, and that the ush-group genes ancestrally played a role in morphogenetic functions of the amnion.
羊膜质是一种胚胎外上皮，在高等直翅目昆虫中从不同的浆膜和羊膜上皮进化而来。这种进化转变的潜在遗传机制尚不清楚。果蝇的羊膜质发育与同源盒基因 zerknüllt（zen）的新表达特征相关，包括合胞胚层中广泛的 zen 表达域和完全缺乏原肠胚后 zen 表达。在这里，我们通过改变 Megaselia（一种具有不同浆膜和羊膜上皮的低级直翅目昆虫）和果蝇中 zen 的活性谱，以及检查 Megaselia 中 u 形组（ush-group）基因的功能，来研究这些特征的功能意义，在果蝇中，当 zen 不再表达时，ush-group 基因维持原肠胚后羊膜质的形成。在 Megaselia 中，原肠胚后 zen 表达的缺失会破坏浆膜的发育，但允许羊膜的发育。早期 Megaselia 胚胎中 zen 的异位表达允许浆膜的形成，但扰乱了羊膜的发育。果蝇中 u 形组基因的 Megaselia 同源物对于浆膜的形成不是必需的，但介导原肠胚缩回和背侧闭合。最后，果蝇中原肠胚后 zen 的异位表达导致羊膜质细胞的扩大，并干扰了羊膜质的形态发生功能。我们的结果表明，羊膜质的起源涉及到胚胎外组织中原肠胚后 zen 表达的丧失，随后果蝇 zen 的早期广泛表达域进化而来，并且 ush-group 基因在原肠胚的形态发生功能中具有祖先作用。
