Muroran Marine Station, Field Science Center for Northern Biosphere, Hokkaido University, Muroran 051-0013, Japan.
Marine Biosystems Research Center, Chiba University, Kamogawa 299-5502, Japan.
Biol Lett. 2023 Oct;19(10):20230313. doi: 10.1098/rsbl.2023.0313. Epub 2023 Oct 18.
The emergence of sporophytes, that is, diploid multicellular bodies in plants, facilitated plant diversification and the evolution of complexity. Although sporophytes may have evolved in an ancestral alga exhibiting a haplontic life cycle with a unicellular diploid and multicellular haploid (gametophyte) phase, the mechanism by which this novelty originated remains largely unknown. Ulotrichalean marine green algae (Ulvophyceae) are one of the few extant groups with haplontic-like life cycles. In this study, we show that zygotes of the ulotrichalean alga , which usually develop into unicellular cysts, exhibit a developmental variation producing multicellular reproductive sporophytes. Multicellular development likely occurred stochastically in individual zygotes, but its ratio responded plastically to growth conditions. Sporophytes showed identical morphological development to gametophytes, which should reflect the expression of the same genetic programme directing multicellular development. Considering that sporophytes were evolutionarily derived in Ulotrichales, this implies that sporophytes emerged by co-opting the gametophyte developmental programme to the diploid phase. This study suggests a possible mechanism of sporophyte formation in haplontic life cycles, contributing to the understanding of the evolutionary transition from unicellular to multicellular diploid body plans in green plants.
孢子体的出现，即植物中具有二倍体多细胞体的出现，促进了植物的多样化和复杂性的进化。尽管孢子体可能是在具有单倍体生活史的祖先藻类中进化而来的，该藻类具有单细胞二倍体和多细胞单倍体（配子体）阶段，但这种新颖性起源的机制在很大程度上仍然未知。Ulotrichalean 海洋绿藻（Ulvophyceae）是少数具有类似单倍体生活史的现存群体之一。在这项研究中，我们表明，通常发育成单细胞胞囊的 Ulotrichalean 藻类的受精卵表现出一种发育变异，产生多细胞生殖孢子体。多细胞发育可能在单个受精卵中随机发生，但它的比例对生长条件具有可塑性响应。孢子体表现出与配子体相同的形态发育，这应该反映了指导多细胞发育的相同遗传程序的表达。考虑到孢子体在 Ulotrichales 中是进化而来的，这意味着孢子体是通过将配子体发育程序共同适应二倍体阶段而出现的。这项研究提出了一种在单倍体生活史中形成孢子体的可能机制，有助于理解从单细胞到多细胞二倍体体计划的绿色植物的进化转变。
