Mukai Hideo, Saito Yasunori, Watanabe Hiroshi
Department of Biology, Faculty of Education, Gunma University, Maebashi, Gunma 371, Japan.
Shimoda Marine Research Center, The University of Tsukuba, Shimoda, Shizuoka 415, Japan.
J Morphol. 1987 Sep;193(3):263-276. doi: 10.1002/jmor.1051930305.
The mode of sexual reproduction and embryogenesis was compared in 3 species of Botrylloides: B. simodensis, B. lenis, and B. violaceus. In all species, a testis and an egg (occasionally 2 eggs), the former being anterior to the latter, mature in the mantle on either side of a zooid. The egg is surrounded by 2 follicular layers and is attached by a vesicular follicle stalk (oviduct) to the atrial brood pouch. The egg is ovulated into the brood pouch, where it is fertilized and undergoes embryogenesis. The egg of B. simodensis is heavily yolked and measures about 180 μm in diameter. The course of embryogenesis in this species is that typical of ascidians. A mature tadpole larva is produced and shed in about 5 days; then, the mother zooid degenerates. The larva is smallest of the three species and has 8 ampullae. The metamorphosed oozooid bears a single bud on the right side only. Extraembryonic nutrition seems to be very limited. Both Botrylloides lenis and B. violaceus are species which display extreme examples of viviparity. Their eggs are devoid of yolk granules, measuring about 90 μm in diameter in the former species and 60 μm in the latter. The course of embryogenesis is similar in these 2 species. The neurula stage is characterized by a spherical vesicular shape owing to precocious differentiation of the embryonic pharynx, whose ectoderm becomes vacuolated. At the posterior end of the neurula, the mesodermal cells are located in a mass, from which the tail is extended later. In B. lenis, embryogenesis takes about 20 days. At the neurula stage of the embryo, the mother zooid becomes a mantle sac as a result of visceral disintegration. During further embryogenesis, the growth of buds of successive generations in the colony is characteristically arrested. A swimming larva of this species is somewhat larger than that of B. simodensis. It has 14-24 ampullae, and the oozooid carries a single bud on its right side. In B. violaceus, the gestation period lasts for more than a month. At the early gastrula stage of the embryo, the body of the mother zooid fully disintegrates. Only the brood pouch bearing the embryo survives and remains connected with the colonial vascular system. In this species, sexual reproduction does not affect the growth of buds in the colony. The swimming larva is gigantic, being furnished with 24-34 ampullae, and the oozooid always bears 3 buds, 2 on the right side and one on the left side.
对3种菊海鞘属物种进行了有性生殖和胚胎发生模式的比较，这3种物种分别是：下田菊海鞘、柔菊海鞘和紫菊海鞘。在所有物种中，一个精巢和一个卵（偶尔为2个卵，精巢位于卵的前方）在个员一侧的外套膜中成熟。卵被2层滤泡包围，并通过一个泡状滤泡柄（输卵管）附着于围鳃育卵囊。卵排入育卵囊，在那里受精并进行胚胎发生。下田菊海鞘的卵富含卵黄，直径约180μm。该物种的胚胎发生过程是典型的海鞘类胚胎发生过程。约5天后产生并排出成熟的蝌蚪形幼虫；然后，母体个员退化。该幼虫是这3个物种中最小的，有8个壶腹。变态后的初出芽体仅在右侧有一个芽。胚外营养似乎非常有限。柔菊海鞘和紫菊海鞘都是胎生的极端例子。它们的卵没有卵黄颗粒，前一种物种的卵直径约90μm，后一种约60μm。这2个物种的胚胎发生过程相似。神经胚阶段的特征是由于胚胎咽部的早熟分化而呈球形囊状，其外胚层变得空泡化。在神经胚的后端，中胚层细胞聚集成团，尾部随后从中延伸出来。在柔菊海鞘中，胚胎发生约需20天。在胚胎的神经胚阶段，由于内脏解体，母体个员变成一个外套囊。在进一步的胚胎发生过程中，群体中连续几代芽体的生长通常会停止。该物种的游泳幼虫比下田菊海鞘的稍大。它有14 - 24个壶腹，初出芽体在其右侧有一个芽。在紫菊海鞘中，妊娠期持续一个多月。在胚胎的原肠胚早期，母体个员的身体完全解体。只有携带胚胎的育卵囊存活下来，并与群体血管系统相连。在这个物种中，有性生殖不影响群体中芽体的生长。游泳幼虫很大，有24 - 34个壶腹，初出芽体总是有3个芽，右侧2个，左侧1个。
