PREMISE Unique among vascular plants, some species of Selaginella have single giant chloroplasts in their epidermal or upper mesophyll cells (monoplastidy, M), varying in structure between species. Structural variants include several forms of bizonoplast with unique dimorphic ultrastructure. Better understanding of these structural variants, their prevalence, environmental correlates and phylogenetic association, has the potential to shed new light on chloroplast biology unavailable from any other plant group. METHODS The chloroplast ultrastructure of 76 Selaginella species was studied with various microscopic techniques. Environmental data for selected species and subgeneric relationships were compared against chloroplast traits. RESULTS We delineated five chloroplast categories: ME (monoplastidy in a dorsal epidermal cell), MM (monoplastidy in a mesophyll cell), OL (oligoplastidy), Mu (multiplastidy, present in the most basal species), and RC (reduced or vestigial chloroplasts). Of 44 ME species, 11 have bizonoplasts, cup-shaped (concave upper zone) or bilobed (basal hinge, a new discovery), with upper zones of parallel thylakoid membranes varying subtly between species. Monoplastidy, found in 49 species, is strongly shade associated. Bizonoplasts are only known in deep-shade species (<2.1% full sunlight) of subgenus Stachygynandrum but in both the Old and New Worlds. CONCLUSIONS Multiplastidic chloroplasts are most likely basal, implying that monoplastidy and bizonoplasts are derived traits, with monoplastidy evolving at least twice, potentially as an adaptation to low light. Although there is insufficient information to understand the adaptive significance of the numerous structural variants, they are unmatched in the vascular plants, suggesting unusual evolutionary flexibility in this ancient plant genus.

中文翻译：

---

巨大的叶绿体，包括双生体，在古老的维管植物家族卷柏科的适应阴凉的物种中很常见

前提 在维管植物中是独一无二的，一些卷柏物种在其表皮或上部叶肉细胞（单质体，M）中具有单个巨大的叶绿体，物种之间的结构不同。结构变体包括具有独特二形超微结构的多种形式的双生体。更好地了解这些结构变异、它们的流行、环境相关性和系统发育关联，有可能为任何其他植物群所无法提供的叶绿体生物学提供新的线索。方法采用各种显微技术研究了76种卷柏属植物的叶绿体超微结构。将选定物种和亚属关系的环境数据与叶绿体性状进行比较。结果我们划定了五个叶绿体类别：ME（背侧表皮细胞中的单质体），MM（叶肉细胞中的单质体）、OL（寡质体）、Mu（多质体，存在于大多数基础物种中）和 RC（减少或残留的叶绿体）。在 44 个 ME 物种中，有 11 个具有双生体，杯状（凹上区）或双叶状（基底铰链，新发现），平行类囊体膜的上区在物种之间略有不同。在 49 个物种中发现的单质体与阴影密切相关。Bizonoplasts 仅在 Stachygynandrum 亚属的深荫物种（<2.1% 全日照）中已知，但在旧世界和新世界中都存在。结论 多质体叶绿体最有可能是基础的，这意味着单质体和双质体是衍生性状，单质体进化至少两次，可能是对弱光的适应。