Marine Biosystems Research Center, Chiba University, Kamogawa, 299-5502, Japan.
Medical Mycology Research Center, Chiba University, Chiba, 260-8673, Japan.
J Phycol. 2019 Jun;55(3):534-542. doi: 10.1111/jpy.12841. Epub 2019 Mar 28.
The life-cycle system of Ulotrichales, a major order of Ulvophyceae, remains controversial because it is unclear whether the Codiolum phase, a characteristic unicellular diploid generation in ulotrichalean algae, is a zygote or a sporophyte. This controversy inhibits the understanding of the diversified life cycles in Ulvophyceae. To distinguish between zygotes and sporophytes, we have to examine not only whether diploid generations function as sporophytes, but also whether mitosis occurs before meiosis in diploid generations. However, the nuclear behavior in the Codiolum phases is largely unknown, probably because no suitable methods are available. Using fluorescent microscopy with ethidium bromide and transmission electron microscopy of cell-wall-dissected specimens, we report the nuclear behavior in the Codiolum phases of an ulotrichalean alga with a representative life cycle, Monostroma angicava. Each vegetative Codiolum phase had a single polyploid nucleus due to endoreduplication, a type of mitosis without nuclear division. During zoosporogenesis, the nucleus had a structure that would be a meiosis-specific complex. We quantitatively showed that Codiolum phases grew extremely large and produced numerous zoospores. Our results suggest that an event comparable to mitosis occurs before meiosis in the Codiolum phase of M. angicava. This nuclear behavior and the functions (growth and zoospore production abilities) correspond to those of sporophytes. Therefore, the life-cycle system of M. angicava is a heteromorphic haplo-diplontic cycle. This system appears to be widely adopted among other ulotrichalean algae.
Ulotrichales 是 Ulvophyceae 中的一个主要目，其生活史系统仍然存在争议，因为不清楚 Ulotrichalean 藻类中特征性的单细胞二倍体 Codium 相是合子还是孢子体。这种争议阻碍了对 Ulvophyceae 多样化生活史的理解。为了区分合子和孢子体，我们不仅要检查二倍体世代是否作为孢子体发挥作用，还要检查二倍体世代中的有丝分裂是否发生在减数分裂之前。然而，Codium 相的核行为在很大程度上是未知的，可能是因为没有合适的方法。我们使用溴化乙锭荧光显微镜和细胞壁分离标本的透射电子显微镜，报告了具有代表性生活史的 Ulotrichalean 藻类 Monostroma angicava 中 Codium 相的核行为。由于内复制，每个营养 Codium 相都有一个单一的多倍体核，内复制是一种没有核分裂的有丝分裂。在游动孢子发生过程中，核具有一个将成为减数分裂特异性复合体的结构。我们定量地表明，Codium 相生长得非常大，并产生了许多游动孢子。我们的结果表明，在 M. angicava 的 Codium 相中，有丝分裂发生在减数分裂之前。这种核行为和功能（生长和游动孢子产生能力）与孢子体相对应。因此，M. angicava 的生活史系统是异态单倍二倍体循环。这种系统似乎在其他 Ulotrichalean 藻类中广泛采用。
