Zhou Na, Wilson Keen A, Andrews Mary E, Kauffman Jeffery S, Raff Rudolf A
Department of Biology, Indiana University, Bloomington, Indiana 47405, USA.
J Exp Zool B Mol Dev Evol. 2003 Dec 15;300(1):58-71. doi: 10.1002/jez.b.46.
Heliocidaris erythrogramma is a direct-developing sea urchin that has evolved a modified ontogeny, a reduced larval skeleton, and accelerated development of the adult skeleton. The Orthopedia gene (Otp) encodes a homeodomain transcription factor crucial in patterning the larval skeleton of indirect-developing sea urchins. We compare the role of Otp in larvae of the indirect-developing sea urchin Heliocidaris tuberculata and its direct-developing congener H. erythrogramma. Otp is a single-copy gene with an identical protein sequence in these species. Expression of Otp is initiated by the late gastrula, initially in two cells of the oral ectoderm in H. tuberculata. These cells are restricted to oral ectoderm and exhibit left-right symmetry. There are about 266 copies of Otp mRNA per Otp- expressing cell in H. tuberculata. We tested OTP function in H. tuberculata and H. erythrogramma embryos by microinjection of Otp mRNA. Mis-expression of Otp mRNA in H. tuberculata radialized the embryos and caused defects during larval skeletogenesis. Mis-expression of Otp mRNA in H. erythrogramma embryos did not affect skeleton formation. This is consistent with the observation by in situ hybridization of no concentration of Otp transcript in any particular cells or region of the H. erythrogramma larva, and measurement of a level of less than one copy of endogenous Otp mRNA per cell in H. erythrogramma. OTP plays an important role in patterning the larval skeleton of H. tuberculata, but this role apparently has been lost in the evolution of the H. erythrogramma larva, and replaced by a new patterning mechanism.
红斑刻肋海胆是一种直接发育的海胆，它进化出了一种经过改良的个体发育过程、简化的幼虫骨骼以及加速发育的成体骨骼。正位基因（Otp）编码一种同源结构域转录因子，对间接发育海胆的幼虫骨骼模式形成至关重要。我们比较了Otp在间接发育的海胆瘤海胆及其直接发育的同属红斑刻肋海胆幼虫中的作用。Otp在这些物种中是单拷贝基因，蛋白质序列相同。Otp的表达在原肠胚后期开始，最初在瘤海胆的口外胚层的两个细胞中。这些细胞局限于口外胚层，呈现左右对称。瘤海胆中每个表达Otp的细胞约有266个Otp mRNA拷贝。我们通过显微注射Otp mRNA来测试Otp在瘤海胆和红斑刻肋海胆胚胎中的功能。在瘤海胆中错误表达Otp mRNA使胚胎呈辐射状，并在幼虫骨骼发生过程中导致缺陷。在红斑刻肋海胆胚胎中错误表达Otp mRNA不影响骨骼形成。这与原位杂交观察结果一致，即在红斑刻肋海胆幼虫的任何特定细胞或区域中未检测到Otp转录本，并且测量得出红斑刻肋海胆中每个细胞内源性Otp mRNA水平低于一个拷贝。Otp在瘤海胆幼虫骨骼模式形成中起重要作用，但在红斑刻肋海胆幼虫的进化过程中，这一作用显然已经丧失，并被一种新的模式形成机制所取代。
