Abstract
We sequenced the 18S ribosomal RNA gene of seven isolates of the enigmatic marine amoeboflagellate Reticulamoeba Grell, which resolved into four genetically distinct Reticulamoeba lineages, two of which correspond to R. gemmipara Grell and R. minor Grell, another with a relatively large cell body forming lacunae, and another that has similarities to both R. minor and R. gemmipara but with a greater propensity to form cell clusters. These lineages together form a long-branched clade that branches within the cercozoan class Granofilosea (phylum Cercozoa), showing phylogenetic affinities with the genus Mesofila. The basic morphology of Reticulamoeba is a roundish or ovoid cell with a more or less irregular outline. Long and branched reticulopodia radiate from the cell. The reticulopodia bear granules that are bidirectionally motile. There is also a biflagellate dispersal stage. Reticulamoeba is frequently observed in coastal marine environmental samples. PCR primers specific to the Reticulamoeba clade confirm that it is a frequent member of benthic marine microbial communities, and is also found in brackish water sediments and freshwater biofilm. However, so far it has not been found in large molecular datasets such as the nucleotide database in NCBI GenBank, metagenomic datasets in Camera, and the marine microbial eukaryote sampling and sequencing consortium BioMarKs, although closely related lineages can be found in some of these datasets using a highly targeted approach. Therefore, although such datasets are very powerful tools in microbial ecology, they may, for several methodological reasons, fail to detect ecologically and evolutionary key lineages.
Highlights
The genus Reticulamoeba was created in 1994 by the distinguished protozoologist Karl Grell, in which study a single species, R. gemmipara was described [1]
We think this unlikely as this pattern only relates to these three lineages, and distinctive sequence signatures in variable regions along the SSU of isolates 1–3 show a mosaic distribution across the three isolates suggesting a partly reticulate evolutionary history of these lineages
The difficulty of PCR-amplifying those strains that differed from other granofilosea in having bi-directionally streaming granules on their reticulopodia was a puzzle that was only resolved with intensive PCR attempts using a variety of primers and targeting short amplicons, and eventually largely sequencing by ‘walking’ along the SSU rDNA molecule
Summary
The genus Reticulamoeba was created in 1994 by the distinguished protozoologist Karl Grell, in which study a single species, R. gemmipara was described [1] This was followed in 1995 by a second species description, R. minor [2]. Grell observed that networks from different individuals can fuse with each other, forming ‘feeding communities’, at least in R. gemmipara He describes a bi-flagellate stage, which is initially roundish in shape, becoming more irregular. These ‘swarmers’ or ‘zoospores’ have short anterior and long posterior flagella, and swim by active beating of the anterior flagellum, the posterior trailing behind. The main differences between R. gemmipara and R. minor are a) the flagellate and amoeboid stages of the latter are smaller, b) flagellate formation in R. minor occurs by fission of the amoeboid stage, resulting in two, four, or more zoospores, whereas in R. gemmipara zoospores are formed by unequal fission (budding) from the edge of the amoeboid cell
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