Lauzon R J, Chang W T, Dewing L S
Department of Pediatrics, Albany Medical College, New York 12208, USA.
Microsc Res Tech. 1996 Jun 15;34(3):218-27. doi: 10.1002/(SICI)1097-0029(19960615)34:3<218::AID-JEMT4>3.0.CO;2-L.
Botryllus schlosseri is a clonally modular ascidian in which asexually derived adults (zooids) exhibit developmental synchrony. At the conclusion of the blastogenic (asexual) cycle every 5 days at 21 degrees C, all zooids within a colony die simultaneously in 24 hours and are replaced by a new asexual generation of zooids. This cyclical process, called takeover, involves the selective destruction of the zooid's visceral tissues which include the pharynx, esophagus, stomach, intestine, endostyle, neural complex and heart, whereas bud tissues and mesenchymal components (muscle and blood cells) remain unaffected. Ultrastructural analysis indicates that the most prevalent form of cell death occurs by apoptosis, although necrotic changes are also observed in several tissues (i.e., stomach and intestine). Blood-derived macrophages and neighboring cells subsequently engulf visceral tissues, reducing the zooid to the size of a small vesicle. Here, we have tested the possibility that acid phosphatase, a hydrolase whose presence is associated with cell death in several invertebrate systems, could account for some of the regressive changes observed during takeover. Our observations indicate that acid phosphatase (AP) activity was selectively localized in the gut of parent zooids during the growth phase of the cycle, with the stomach exhibiting the most intense histochemical staining on tissue sections. As zooid regression progressed during takeover, stomach AP staining gradually disappeared. Other visceral tissues never became AP-positive. Therefore, this hydrolase appears to play a minimal role in zooid death. In order to characterize genes whose expression pattern was selectively altered during takeover, we have carried out differential mRNA display analysis. We report on two genes, 790.3 and 790.4, that are down- and upregulated, respectively, during this process. Collectively, these findings indicate that the takeover phase of blastogenesis in Botryllus involves modulated gene expression.
葡萄贝螅是一种克隆模块化海鞘，其中无性繁殖产生的成体（游动孢子）表现出发育同步性。在21摄氏度下每5天的出芽（无性）周期结束时，群体内的所有游动孢子会在24小时内同时死亡，并被新一代无性繁殖的游动孢子所取代。这个循环过程称为更替，涉及选择性破坏游动孢子的内脏组织，包括咽、食管、胃、肠、内柱、神经复合体和心脏，而芽组织和间充质成分（肌肉和血细胞）则不受影响。超微结构分析表明，尽管在几个组织（如胃和肠）中也观察到坏死变化，但最普遍的细胞死亡形式是凋亡。血液来源的巨噬细胞和邻近细胞随后吞噬内脏组织，使游动孢子缩小到小囊泡的大小。在这里，我们测试了酸性磷酸酶（一种水解酶，其存在与几个无脊椎动物系统中的细胞死亡相关）是否可以解释在更替过程中观察到的一些退行性变化的可能性。我们的观察表明，在周期的生长阶段，酸性磷酸酶（AP）活性选择性地定位于亲代游动孢子的肠道中，胃在组织切片上表现出最强的组织化学染色。随着更替过程中游动孢子的退化，胃AP染色逐渐消失。其他内脏组织从未变为AP阳性。因此，这种水解酶似乎在游动孢子死亡中起最小作用。为了表征在更替过程中表达模式被选择性改变的基因，我们进行了差异mRNA显示分析。我们报告了两个基因，790.3和790.4，它们在此过程中分别下调和上调。总的来说，这些发现表明葡萄贝螅出芽生殖的更替阶段涉及基因表达的调节。
