Laboratory of Terrestrial Microbiology and Systematics, Graduate School of Global Environmental Studies, Kyoto University, Yoshida-Honmachi, Kyoto, 606-8501, Japan.
Department of Biological Sciences, Florida Gulf Coast University, 10501 FGCU Blvd. South, Ft. Myers, Florida, USA.
J Phycol. 2022 Aug;58(4):612-625. doi: 10.1111/jpy.13256. Epub 2022 Jun 18.
The recently described genus Rhizonema is among the most important cyanobacterial partners in lichen symbioses, but its morphological characterization in the genus diagnosis-true branching of the T-type-appears at odds with several published figures showing false branching. We investigated cyanobiont branching and cell division with light microscopy in two basidiolichens from Florida and one from Japan, including aposymbiotically cultured material of the latter. Mycobiont species identities (Cyphellostereum jamesianum, Dictyonema darwinianum, and D. moorei) and photobiont genus identity (Rhizonema) were corroborated with ITS and rbcLX sequences, respectively. Single and paired false branching occurred commonly in all three strains examined. False branches developed adjacent to necridic cells or heterocytes, or by separation of vegetative cells at compression folds in the trichome. Non-transverse cell divisions, usually oblique, were observed in two of the three Rhizonema strains examined. T-type true branches sometimes arose from such divisions, although oblique growth from the branch cell often resulted in ambiguous branch junctions. Additionally, Y-type true branches appeared to grow from contorted filaments. In cultured material, a kind of pseudo-branch sometimes arose from single- or several-celled segments liberated from trichome apices. The segments attached secondarily to filaments and grew there as apparent branches. We conclude that Rhizonema is a genus of considerable morphological flexibility, with multiple modes of branching possible in a single strain. While true branching or non-transverse divisions, when observable, may help distinguish Rhizonema from the phenotypically similar Scytonema, false branching occurs commonly in both genera, and therefore cannot be used to distinguish them.
最近描述的 Rhizonema 属是地衣共生体中最重要的蓝藻类伙伴之一，但它在属诊断中的形态特征——T 型真分支——与几个已发表的显示假分支的图片不一致。我们用光学显微镜研究了来自佛罗里达州的两个担子菌地衣和一个来自日本的地衣的蓝藻共生体分支和细胞分裂，包括后者的无共生培养材料。通过 ITS 和 rbcLX 序列分别证实了菌物共生种的身份（Cyphellostereum jamesianum、Dictyonema darwinianum 和 D. moorei）和光生物共生属的身份（Rhizonema）。在所有三种受检菌株中，单分支和双分支假分支都很常见。假分支是在邻近死细胞或异形细胞的地方发育的，或者是在藻丝的压缩褶皱处通过营养细胞的分离而发育的。在三种受检的 Rhizonema 菌株中，观察到非横向细胞分裂，通常是斜分裂。在其中两种 Rhizonema 菌株中观察到 T 型真分支有时是从这种分裂中产生的，尽管分支细胞的斜向生长通常会导致分支结的不明确。此外，Y 型真分支似乎是从扭曲的丝状体中生长出来的。在培养材料中，有时会从藻丝顶端释放的单细胞或几个细胞段中产生一种假分支。这些段附着在丝体上，在那里生长为明显的分支。我们的结论是，Rhizonema 是一个具有相当形态灵活性的属，在单个菌株中可能存在多种分支模式。虽然真分支或非横向分裂，如果可观察到，可能有助于将 Rhizonema 与表型相似的 Scytonema 区分开来，但在两个属中都经常出现假分支，因此不能用于区分它们。
