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. 2016 Nov 23;11(11):e0167148.
doi: 10.1371/journal.pone.0167148. eCollection 2016.

A New Morphological Type of Volvox from Japanese Large Lakes and Recent Divergence of this Type and V. ferrisii in Two Different Freshwater Habitats

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A New Morphological Type of Volvox from Japanese Large Lakes and Recent Divergence of this Type and V. ferrisii in Two Different Freshwater Habitats

Hisayoshi Nozaki et al. PLoS One. .

Abstract

Volvox sect. Volvox is characterized by having unique morphological characteristics, such as thick cytoplasmic bridges between adult somatic cells in the spheroids and spiny zygote walls. Species of this section are found from various freshwater habitats. Recently, three species of Volvox sect. Volvox originating from rice paddies and a marsh were studied taxonomically based on molecular and morphological data of cultured materials. However, taxonomic studies have not been performed on cultured materials of this section originating from large lake water bodies. We studied a new morphological type of Volvox sect. Volvox ("Volvox sp. Sagami"), using cultured materials originating from two large lakes and a pond in Japan. Volvox sp. Sagami produced monoecious sexual spheroids and may represent a new morphological species; it could be clearly distinguished from all previously described monoecious species of Volvox sect. Volvox by its small number of eggs or zygotes (5-25) in sexual spheroids, with short acute spines (up to 3 μm long) on the zygote walls and elongated anterior somatic cells in asexual spheroids. Based on sequences of internal transcribed spacer (ITS) regions of nuclear ribosomal DNA (rDNA; ITS-1, 5.8S rDNA and ITS-2) and plastid genes, however, the Volvox sp. Sagami lineage and its sister lineage (the monoecious species V. ferrisii) showed very small genetic differences, which correspond to the variation within a single biological species in other volvocalean algae. Since V. ferrisii was different from Volvox sp. Sagami, by having approximately 100-200 zygotes in the sexual spheroids and long spines (6-8.5 μm long) on the zygote walls, as well as growing in Japanese rice paddies, these two morphologically distinct lineages might have diverged rapidly in the two different freshwater habitats. In addition, the swimming velocity during phototaxis of Volvox sp. Sagami spheroids originating from large lakes was significantly higher than that of V. ferrisii originating from rice paddies, suggesting adaptation of Volvox sp. Sagami to large water bodies.

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Conflict of interest statement

The authors have declared that no competing interests exist.

Figures

Fig 1
Fig 1
Light microscopy of Volvox sp. Sagami strains 13-614-Vx13 (A-C), 13-614-Vx15 (D-G) and 14-614-Vx04 (H and I). (A) Asexual spheroid with daughter colonies (d). (B-E) Part of asexual spheroids. (B) Side view of anterior cells, showing elongate-ellipsoidal or spindle cell shape, stigma (s) and pyrenoid (p) in the chloroplast. (C) Top view of somatic cells interconnected by cytoplasmic bridges (b). (D) Optical section of top view of cells surrounded by individual sheaths (asterisks). Stained with methylene blue. (E) Optical section of developing embryo during late stage of inversion. Note that gonidia (g) of the next generation are evident. (F) Monoecious sexual spheroid with eggs (e) and sperm packets (sp). (G) Side view of sperm packet (sp) in monoecious sexual spheroid. (H) Mature zygote with short and acute spines developing on the walls. (I) Sexual spheroid with mature zygotes.
Fig 2
Fig 2. Maximum likelihood (ML) tree of Volvox sect. Volvox based on ITS region of nuclear ribosomal DNA (ITS-1, 5.8S rDNA, and ITS-2) (S1 Table).
Bootstrap values from ML (left) and maximum parsimony (right) analyses are shown on the branches.
Fig 3
Fig 3. Maximum likelihood (ML) tree of Volvox sect. Volvox and other colonial Volvocales based on rbcL and psbC genes (S1 Table).
Bootstrap values from ML (left) and maximum parsimony (right) analyses are shown on the branches.
Fig 4
Fig 4. Swimming velocities in various strains of Volvox sp. Sagami. and V. ferrisii Isaka et al.
Swimming velocities during phototaxis in each culture were measured by tracing 20 individual Volvox spheroids. Two media (AF-6/3 and AF-6) were used for each strain. For statistical tests, see S4 Table.

References

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