全 文 ：Received 30 Jun. 2003 Accepted 21 Oct. 2003
Supported by the grant from the Science and Technology Commission of Zhejiang Province of China (2001300001).
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Acta Botanica Sinica
植 物 学 报 2004, 46 (4): 457-462
Life History of Monostroma latissimum
HUA Wei-Hua, XIE En-Yi, MA Jia-Hai*
(Fisheries College, Shanghai Fisheries University, Shanghai 200090, China)
Abstract : In the present study the authors observed and reported the morphological characters and
development process in all stages of the life history of Monostroma latissimum Wittrock for the first time
in China. The process of parthenogenesis has also been observed in yearly culture. When the dioecious
macrogametophytes mature, they liberate biflagellate gametes, and when the spherical microsporophytes
mature, they produce quadriflagellate zoospores. Its life history is a periodical heteromorphic alternation of
haploid gametophyte generation and diploid sporophyte generation. In addition, this study indicates the
multiplication of zygotes and cysts by binary fission, two-year-old cysts compared with gametophytes living
for one year in the life history of M. latissimum. The male and female gametes that do not conjugate also
undergo parthenogenesis as they adhere to the substrates. They become unicellular bodies, spherical in
shape, somewhat smaller than the zygotes. Then, the cysts of parthenogenesis mature and liberate a
number of quadriflagellate swarm spores. These spores adhere to substrates and germinate directly into
the leaf fronds. As for zygotes, parthenogenesis of the male and female gametes also can be used for
artificial seeding of M. latissimum in culture.
Key words: Monostroma latissimum ; life history; Chlorophyta
The genus Monostroma belongs to the family
Monos tromaceae of the o rder Ulvales and the clas s
Chlorophyceae, which consists of forty species including
six species (M. nit idum Wittrock, M. ang icava Kjellm, M.
latissimum Wittrock, M. crassifolia Tseng et C. F. Chang,
M. arcticum Wittrock and M. undu latum Vinogradova) in
China (Huang, 1994). M. latissimum is an important source
of food, or chemical derivatives since it contains abundant
nutrient components (Xie et a l., 2002), and many act ive
substances, such as sulphated polysaccharide (Maeda et
al., 1991; Hiraoka et a l., 1992; Harada and Maeda, 1998),
which is useful for blood coagulation and high blood fat. In
a consideration o f production and economy, large-s cale
commercial cultivation of M. latissimum for human food in
China is promising.
To date, internal studies on basic theories and applica-
tion techniques of M. latissimum are relatively little (Chen
and Chiang, 1994; Huang, 1994; Xie et al., 2002). As for the
life history of the genus, there exist a great variety of differ-
ent views in China and abroad. Some think that there is a
single gametophyte (Smith, 1955; Li et al., 1982) or sporo-
phyte generation (Dawes, 1981) without an alternation of
two generations. Others agree that some species undergo
an alternation of isomorphic or heteromorphic generations,
and later workers subsequently separated those of an al-
ternation of isomorphic ones from Monostroma into a new
taxon (Maurice, 1967; Fott, 1971; Kida, 1989).
In this study consisting of field, cultural, and cytologi-
cal observations, the thorough evidence indicates that an
alternat ion of heteromorphic generations occurs in M.
latissimum, which is beneficial not only in the taxonomy of
genus Monostroma but also in the development of artifi-
cially seed commercial cultivation of M. latissimum.
1 Materials and Methods
In April, a number of yellow brown or yellow green ga-
metophytes of Monostroma latissimum Wittrock were at-
tached to the rocks in shore of Zhejiang Province, China.
Specimens were collected at the lowest tides by wading.
The thalli, about 15 cm long , were washed with sterile
seawater, semi-dried (30%-40% moisture), sealed in plas-
tic bags, packed in an ice-box, transported to the labora-
tory of the Fisheries College (Shanghai Fisheries University,
China) and stored at –20 ℃ for further use.
After thawing, the thalli were immersed into filtered and
sterile seawater to dis charge the gametes. This seawater
was enriched with varied amounts of sodium nitrate and
sodium glycerophosphate. Zygotes were obtained on slides
by combining drops of water containing gametes from sexu-
ally d ifferent p lants. Following further cultu re in a illumi-
nating incubator (ZHUJIANG, LRH250G) with a photope-
riod of 12:12 L:D and a photon flux density of 20 to 75 mmol.
Acta Botanica Sinica 植物学报 Vol.46 No.4 2004458
m-2.s-1 at 18 to 28 ℃, the zygotes developed from fused
gametes grew into cysts that matured in early September.
At 25 to 27 ℃ and 128 to 160 mmol.m-2.s-1, the mature
cys ts could release a lo t of swimming zoos pores which
soon attached to the nylon silk previously placed in the
water box. Subsequently, the nylon silk, with all stationary
cells from the zoospores, was transferred to an aerated cul-
ture with a pho toperiod of 12:12 L:D and a photon flux
density of 96 to 128 mmol.m-2.s-1 at 16 to 18 ℃.
To study asexual reproduction of M. latissimum, a single
gametophyte per dish was cultured to release monosexual
gametes (female or male). The following culture o f mono-
sexual gametes was similar to that o f zygotes described
above.
Dimensions of gametes and zoospores were determined
from osmium-fixed material (Maurice, 1967).
A stereo microscope (OLYMPUS, BH2) equipped with
OLYMPUS PMCBAD and C35AD2 cameras was us ed to
observe and record the growth progress.
2 Results
2.1 Morphology and cytology ( Figs.1–4 )
The thalli were green to yellow green in color, more than
20 cm in height, soft and lubricous, 19–25 mm thick, mem-
branous and with wrinkled margin. In surface view, the cells
were polygonal, square or oval in shape, mostly grouped in
two. In cross section at the middle part, the cells were elon-
gated and perpendicular to the plane of the thallus, with a
space between both ends of the cells and the external mem-
brane where the length was about 5 mm.
2.2 Development of germlings from zygotes and
zoosporogenesis (Figs.1–12)
Mos t gametes were pos it ively pho to tactic when
released, and concentrated on the surface o f water illumi-
nated from above. Gametes tended with t ime to become
less responsive to overhead lighting and mated to become
zygotes that are global, 4.2–4.5 mm in diameter. While the
cell wall was forming, zygotes stopped swimming, took fla-
gella back into the body and sett led at the bottom of the
dish. Following further divisions, zygotes developed into
cysts at the end of June. The cysts growth, due most likely
to h igher water temperatu re, was retarded in July and
August, compared with growth in the past month. Zygotes
at prophase and metaphase of growth multiplied by binary
fission (Figs.8, 9).
In early September the mature, reproductive cysts turned
yellow green or yellow brown, 35–55 mm in diameter, with a
more transparent wall, 5 mm thick. The release of zoospores
was preceded by cellular changes lasting about seven days.
These changes included a granular appearance of the chlo-
roplast and the initiation of the vaulted tubercle on the cyst
walls. After moving slowly for three to four hours, the ma-
jority of zoospores released through round discharge pores
in a dish illuminated from above. The minority in the cysts
still remained active for more than 6 h. Following the com-
plete release of zoos pores, the cysts composed o f empty
cell walls could attach to the substrates for a long period
(Fig.11), and some immature cysts could mature and pro-
duce zoospores in the following year.
2.3 Development of germlings from zoospores and game-
togenesis ( Figs.1–6, 13–24 )
Zoospores were pyriform, 7.5–10 mm long, 3 mm broad,
with a hyacinthine stigma and a chloroplast occupying more
than half of the cell lumen. There were four flagella (10.5 to
15 mm long) in the front of the zoospores. The time from
release to attachment observed in 80% of the zoospores
was 4 h. After 24 h virtually all attached zoospores became
spherical, 5–5.4 mm in diameter. In one to two days the
attached cells began to elongate. The first cell division oc-
curred after two to th ree days. It took nine to ten days to
develop into the 32–64-celled stage. On the 20th day the
macroscop ically v is ib le germlings developed from
zoos pores reached abou t 1 cm leng th , which were
monostromate and membranous.
Early ontogeny of the zoos pores followed one of the
basic types in this study. In one type, zoospores devel-
oped and became uniseriate or biseriate filaments (Fig.17).
In the o ther type, zoospores d irect ly grew in to the
monos tromate and membranous gametophytes. Bes ides,
following the development of a zoospore, some could only
be a germling (Fig .18), and the o thers were divided into
multicellular aggregates that cont inuously developed a
group of germlings with many blades and rhizoids (Figs.
19–22).
The most apparent morphological character happened
during the formation of gametangia: a single chlorop last
divided into four granular and parietal parts in March. In
early April fully mature parts of the female and male game-
tophytes were yellow green and yellow brown, respectively,
and green areas remained at the immature stage (Figs.1, 2).
At the release of gametes, the gametangium walls became
thinner and more transparent, and finally diss olved and
broke. The gametangia without gametes would, unlike the
cysts, disappear and would not leave the membrane com-
posed of empty cell walls. The gametes res pectively from
the female and male thallus showed the same morphology,
but different dimensions, with the female gamete bigger
than the male one. A stigma was hyacinthine similarly to


HUA Wei-Hua et al.: Life History of Monostroma latissimum 461
the zoospore. The length of the two flagella was 1.6 to 1.7
times longer than that of the gamete body.
2.4 Asexual reproduction
After being released , unfert ilized gametes from both
sexes adhered and changed in little spherical unicellular
bodies with an apparent stigma and a single chlorop last,
but the survival rate was lower than that of zygotes. With
further culture, the survival unicellular bod ies all devel-
oped into cysts. Our observation showed that ontogeny of
those survival in culture also included a progression from
cysts through zoospores to membranous plants, similar to
zygotes. Asexual reproduction is rather useful for an artifi-
cial seeding.
2.5 Periodicity of growth
At the end of spring (in April and May) vegetative areas
of heterothallic gametophytes began gradually to produce
mature, reproductive gametangia, which could release iso-
morphic female and male gametes. All fused gametes in
mating became uninucleate zygotes through plasmogamy
and karyogamy. Following further development zygotes
and unfused gametes grew into cysts. With the rise of air
temperature, cysts passed the hot summer in dormancy. In
early autumn (in September) fu lly mature cysts released
zoos pores that developed into gametophytes . Immature
cysts began maturing and releasing zoospores in the next
autumn. So, the period of M. latissimum growth is cyclic.
3 Discussion
3.1 Taxonomic feature
The blade marg in of the gametophytes has wrinkled.
Cells are polygonal and grouped in two in surface view, but
are elongated and ovate in transverse section. The vegeta-
tive cells, except the rhizoid cells of the proximal portion of
the thallus, can all change into gametangia. While the ga-
metes are released out of the thallus and the gametangium
walls dissolve and disappear, the thallus becomes smaller
and smaller, and finally dis appears . However, after the
zoospores have been released, the thick cyst walls do not
immediately disappear but still remain attached for a long
period.
3.2 Early ontogeny of gametophytes
During on togeny of M. la ti ssimum there is no t a
ves icular stage. Generally, the top cell at the 3–4-celled
stage divides in all directions to become a multicellular and
membranous plant. Moreover, in some cases, the zoospores
develop into uniseriate or diseriate filaments composed of
more than 10 cells , with cons iderable res emblance to
Enteromorpha and some species of Ulva (Maurice, 1967).
The similarity in early on togeny indicates that the th ree
genera have a certain development relationship each other
during their course of evolution, according to the theory of
recapitulation (Christopher, 1980).
Besides, as described before, a single zoospore can di-
rectly develop in a single juven ile p lant, or indirectly in
many ones through a cell aggregate stage.
3.3 Anisogamy
It was shown, in Marine Phycology (1961), that sexual
reproduction of Monostroma is isogamic. In the present
study, it is observed that homomorphic female gametes
and male ones are different from each other in many aspects,
such as dimensions, locations of ch loroplast and stigma,
and some physiological functions. Sexual reproduction of
M. lat i ssimum shou ld therefo re be regarded as an
anisogamy.
3.4 Alternation of generations
In consideration of the life history of Monostroma, Bold
and Whynne (1978) designated it as a single and haploid
generation because they only found a haploid gametophyte
generation. However, the release of gametes and zoospores
from morphologically different plants, and the difference of
ontogeny of plants grown from zygotes and zoospores in
M. latissimum in this study, are interpreted as evidence for
an alternation of heteromorphic generat ions. The life his-
tory of M. lati ssimum includes haploid macrogametophyte
and dip loid microsporophyte generat ions , and the spe-
cies has the capab ility o f asexual reproduct ion . Similar
results were obtained by Kida (1989), but the present study
has some new find ings. For example, gametangium walls
disso lve and d isappear when gametes are released, while
cyst walls are still attached long time after the zoospores
release. Zygotes and cysts could mult ip ly by binary
fission. Three styles o f early ontogeny o f gametophytes
have been found and part of immature cysts could persist
for two years.
Figs.11-24. The life history of Monostroma latissimum. 11. A mature cyst in September, ×400. 12. Zoospores liberation, ×400.
13. Zoospores, × 1 000. 14. One-celled sporeling adhered to the net, × 600. 15. Two-celled germling developed from a zoosp ore,
×500. 16. Six-celled germling developed from a zoospore, ×500. 17. A sporeling at the nine-celled stage of which the top cell is already
longitudinally divided, ×400. 18. A sporeling at the thirty-two celled stage, ×200. 19-22. Further development of sporelings derived
from a zoospore, ×200. 23. A juvenile thallus of 22 days old, ×200. 24. Juvenile thallus adhered to net, ×40.
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Acta Botanica Sinica 植物学报 Vol.46 No.4 2004462
(Managing editor: WANG Wei)
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