Biologia 63/6: 778—780, 2008
Section Botany
DOI: 10.2478/s11756-008-0098-8
Zygote germination in Pleodorina starrii
(Volvocaceae, Chlorophyta)*
Hisayoshi Nozaki
Department of Biological Sciences, Graduate School of Science, University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033,
Japan; e-mail: [email protected]
Abstract: Zygote germination of the anisogamous/oogamous colonial green flagellate Pleodorina starrii was observed.
After the zygotes were transferred to the usual, illuminated conditions from the dark treatment on the agar plate, they
began to germinate. The germinating zygotes gave rise to one or two viable biflagellate gone cells. This type of zygote
germination is rare in the colonial Volvocales and may characterize a certain lineage within the anisogamous/oogamous
members of the colonial Volvocales.
Key words: Chlorophyta; Volvocales; Volvocaceae; Pleodorina starrii; morphology; zygote germination
Introduction
During our research of the male specific “OTOKOGI”
(PlestMID) gene in the anisogamous/oogamous colonial volvocalean Pleodorina starrii (Nozaki et al.
2006a, Nozaki 2008) two heterothallic strains (2000602-P14female and 2000-602-P15male) of P. starrii
were used. These two strains were established in June
2000 from a water sample collected in Lake Sagami,
Kanagawa, Japan (Nozaki et al. 2006b). However, the
induction of sperm packet formation had markedly decreased after five years (Nozaki, unpublished). In order to recover the fertility, F1 progeny (2005-701-F15female, 2005-701-F1-1male and 2005-701-F1-3male)
were produced based on other stains of P. starrii (2001-608-P26female, 2001-608-P21male, and 2001608-P17male) that were established from a different collection in Lake Tsukui, Kanagawa, Japan (Nozaki et al.
2006b). Thus, F2 progenies were obtained from 2005701-F1-5female X 2005-701-F1-1male. During these
studies, zygote germination was successively observed
in P. starrii. In this report the morphological details of
zygote germination in P. starrii are described.
Material and methods
In order to obtain F1 and F2 strains of P. starrii, formation and germination of zygotes were induced as described
by Nozaki et al. (1989, 2006b). Actively growing male and
female cultures in VTAC medium (Nozaki et al. 1989; Kasai et al. 2004) were concentrated and mixed with Pleodorina mating medium. During the subsequent 10 days, sperm
packet formation, conjugation between male and female gametes and zygote maturation occurred. Mature zygotes were
transferred to 1% agar plates (AF-6 medium, Kasai et al.
2004), and put into darkness for 1–3 months at 20–25 ◦C.
After the dark treatment, the zygotes were transferred to
the liquid AF-6 medium under 14:10 h LD and 20–25 ◦C.
Light microscopy was carried out using an OLYMPUS BX60
microscope (KS OLYMPUS, Tokyo, Japan), equipped with
Nomarski interference optics.
Results
Three days after their transfer from darkness to the illuminated condition, the zygotes began to germinate. Initially, part of the outer wall ruptured. The inner wall in
this region protruded and swelled, forming a ellipsoidal
shape (Fig. 1).
Relatively small zygotes occasionally missed the
transverse division (see below) when they germinated.
In the germinating, ellipsoidal zygote, a small hyaline
body, possibly a meiotic product, budded off from the
protoplast within the protruded wall (Figs 1, 2). The
protoplast then grew two equal flagella. As the protoplast squeezed out into the protruding wall, the tip of
the wall ruptured and a single biflagellate gone cell was
released, leaving the empty wall behind (Figs 3, 4).
The protoplast within the zygote frequently underwent a transverse division, forming two protoplasts
of approximately equal size within the expanding zygote wall. A hyaline body could be observed within
the zygote wall (Fig. 5). The two protoplasts became
separated from each other within the wall. The tip of
* Presented at the International Symposium Biology and Taxonomy of Green Algae V, Smolenice, June 26–29, 2007,
Slovakia.
c 2008 Institute of Botany, Slovak Academy of Sciences
Unauthenticated
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Zygote germination in Pleodorina starrii
779
Figs 1–9. Nomarski interference microscopy of zygote germination in the anisogamous/oogamous volvocacean Pleodorina starrii. Note
a hyaline body or meiotic product (arrow). All at the same magnification throughout. 1–4 – Germinating zygotes producing a single
gone cell. 2–4 – Successive stages of a germinating zygote. 5–8 – Germinating zygotes producing two viable gone cells. Initial stage (5),
successive stages of two viable gone cells released from a single germinating zygote (5–8). 9 – Gone colony formation. Note developing
embryo enclosed by gelatinous envelope. India ink preparation.
the protruded wall then ruptured and two biflagellate
gone cells were released separately, leaving the empty
zygote wall behind (Figs 6–8). Each of the two gone
cells had the potential to develop into a new colony
(gone colony).
Irrespective of these two types of zygote germination, the liberated gone cells had the same form
and exhibited the essentially the same mode of gone
colony formation. The gone cell was nearly spherical
in shape, measured 20–26 µm in diameter and contained reddish brown granules provided by the former
zygote (Nozaki et al. 2006b). Before cell division, the
protoplast formed a gelatinous envelope from which the
two flagella projected. The protoplast within the gelatinous envelope then divided successively to form a gone
colony (Fig. 9) as in the asexual reproduction (Nozaki et
al. 2006b). During colony formation, the gelatinous envelope containing a developing embryo moved by means
of the two flagella inherited from the original gone
cell.
Discussion
Zygote germination in Pleodorina was previously observed only in P. japonica (Nozaki et al. 1989; Nozaki &
Ito 1994). Only a single gone cell was produced from the
germinating zygote in P. japonica (Nozaki et al. 1989).
In contrast, a germinating zygote of P. starrii gave rise
to one or two viable gone cells (Figs 1–8). This difference in mode of zygote germination between these two
Pleodorina species may be reflected by its polyphyletic
status: P. japonica is phylogenetically separated from
P. starrii within the anisogamous/oogamous members
of the Volvocaceae (Nozaki et al. 2006b). In most volvocacean algae, only a single viable gone is produced
from a germinating zygote (see Nozaki & Ito 1994).
Unauthenticated
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H. Nozaki
780
However, two species of the Volvocaceae, Eudorina illinoisnesis (Waters 1960; Goldstein 1964) and E. elegans
var. synoica (Nozaki 1986) exhibit two viable gone cells
produced from a single germinating zygote,. Interestingly, these two species of Eudorina and P. starrii occupy closely related phylogenetic positions (Nozaki et
al. 2006b), so this attribute may be important for characterizing a lineage within the anisogamous/oogamous
Volvocacae. However, the modes of zygote germination in other related species (such as Volvox gigas and
Pleodorina indica) have not been observed. Several genera within the ansogamous/oogamous Volvocaceae are
not considered to be monophyletic because of a lack of
shared morphological attributes (Nozaki et al. 2006b).
Thus, the mode of zygote germination may contribute
to the establishment of a natural taxonomic system at
the genus level.
Acknowledgements
This work was supported by Grants-in-Aid for Creative Scientific Research Nos 16GS0304 and 17370087 to HN from
the Ministry of Education, Culture, Sports, Science and
Technology, Japan.
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Received September 1, 2007
Accepted March 17, 2008
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