Ectocarpus

 Ectocarpus

( Gr. Ektos-External, Karpus-Fruit)
                                                  Division :Phaeophyta
                                                  Class : Isogenerate
                                                  Order : Ectocarpales
                                                Family : Ectocarpaceae
                                                 Genus : Ectopcarpus
Distribution and Habitat:
This marine alga is most primitive of all the brown algae. The species are of worldwide in
distribution but found in abundance in colder seas of temperate and polar regions. The plants
occur attached to the rocks and stones in littoral and sub littoral zones of oceans. Majority of its
species are lithophyte but a few are epiphytes.
E. tomentosus, E. coniferus grow on large algae like Fucus and Laminaria. E. arabicus grows
epiphytically on leaves of Sargassumtenerimum. E. fasciculatus is an epizoic species that grows on
fins of fishes.
In India, the genus is represented by 16 species – 9 freely growing species and 7 epiphytic
ones.
Thallus structure:
The thallus is macroscopic, multicellular, filamentous, branched. It shows heterotrichous
habit differentiated into two parts (i) the prostrate or creeping portion and (ii) the erect or
projecting portion.
The prostrate portion is irregularly and more or less profusely branched. It is firmly
attached to the substratum by rhizoids. In epiphytic species, it sticks to the substratum.
The erect portion consists of a tuft of feely waving branched filaments arising from the
prostrate branches. Each filament is several inches long and one cell in thickness (monosiphonous)
The main axis bears lateral branches. The branches arise just beneath the septa.
Occasionally, due to evection, which pushes the branch upwards and the main axis to one side,
apparent dichotomy may result.
The branches end either in a point or taper into a series of narrow, elongated, tapering,
hyaline, colourless, hair like trichomes.
In some cases, the older portions of the main axis of the thallus may be surrounded by the
descending rhizoid-like branches and thus become corticated. These branches arise from the
lower cells of the lower branches.
Cell Structure:
Filaments of Ectocarpus are made up of uninucleate, rectangular or cylindrical cells, placed
one above the other. The cell wall is firm and distinguishable into an inner cellulosic and outer
gelatinous layer containing a characteristic pectic substances called align or alginic acid and
fucoides.
The protoplast is richly vacuolated and distinguishable into a single nucleus and cytoplasm
and one or more chromatophores. The chromatophores are variable in form. Accordingly, to the
species they may be disc-shaped or ribbon-like. The chromatophores contains a naked projecting
pryrenoid.
Each chromatophores is golden brown in colour due to presence of characteristic brown
carotenoid pigment – fucoxanthin or phycoxanthin in addition to Chlorophyll – a, c, carotene and
Xanthophylls. The reserve food material is in form of oil, mannitol and laminarin, but not starch.
The presence of brown pigmentation, the fucosan in the form of fucosan vesicles is a
unique feature of Pheaophycean cell. The cells of Ectocarpus possess certain characteristic
vacuoles called physodes, which contain polyphenols and are thought to be lysosomal in function.
Growth:
The growth is apical in the prostrate system. But in the erect system the growth is
intercalary and diffuse due to intercalary meristem. Short meristematic cells are found at the base
of each upright branch. The cells of meristem cut off segments both above and below. The cells
formed below give rise to ordinary photosynthetic cells from which also branches are produced.
The cells formed above increase the length of the terminal hair. This method of adding new cells is
called as trichothallic growth.
Reproduction:
In the life cycle of Ectocarpus, these are two types of plants, namely, the diploid
sporophyte and haploid gametophyte. The sporophyte and gametophyte are morphologically
similar except for the reproductive structures they produce.
Asexual Reproduction:
The diploid sporophyte reproduces asexually by biflagellate zoospores. Zoospores are
produced in sporangia. The diploid sporophyte produces two types of sporangia”: (1) Plurilocualar
sporangia and (ii) Unilocular sporangia.
Both types of sporangia are borne on the same plant. The plurilocular sporangia produce
diploid zoospores, which germinate to produce the diploid thalli, the unilocular sporangia produce
haploid zoospores, which germinate to produce haploid plant body or gametophyte.
Plurilocular Sporangia:
Plurilocularsporangia is a multicellular spore-producing asexual reproductive body of
Ectopcarpus.
Plurilocular sporangia are elongated, cone-like multicellular structures which may be
sessile or stalked. They are borne singly on the diploid sporophyte plants at the end of the lateral
branchlets.
The terminal cell of the branchlet function as the sporangial mother cell. It enlarges and
becomes spherical and accumulates numerous chromatophores.
The sporangial initial now undergoes repeated transverse division to form a vertical row of
6-12 cells. Now, the vertical divisions sets in all the cells, beginning with the median cells of the
row. The nuclear divisions involved are mitotic.
As a result of repeated transverse and vertical divisions an elongated, multicellular,
cone-like structure in formed. It consists of several hundred of small cubical cells arranged in
20-40 transverse tiers. It is the plurilocular sporangia.
The protoplast of each cells metamorphoses into a single, biflagellate zoospore. Since, the
nuclear division are mitotic, the resultant zoospores are diploid.
Each zoospore is a biflagellate, pear-shaped structure with an eyespot and a brown
chromatophore. The two flagella are inserted laterally and are of unequal lengths. The longer
tinsel flagellum is directed forward in motion. The shorter whiplash flagellum is directed
backwards.
The mature sporangium dehisces by formation of an apical aperture. The septa between
the compartments disappear and zoospores pass out through the apical aperture.
The liberated zoospores swim for a while and settle down on solid substratum by attaching
themselves by their anterior ends. The zoospores withdraw their flagella, become rounded and
secretes a membrane around it.
The zoospore germinates by producing a tubular prolongation which subsequently form
the prostrate system of the diploid sporophyte. The diploid zoospores produced in Plurilocular
sporangia thus serve to duplicate the sporophyte generation. They play no role in alternation of
generation. Thus, these sporangia are also called neutral sporangia.
Unilocular Sporangia:
Unilocularsporangia is an unicellular spore producing asexual reproductive body of
Ectopcarpus. It is one-celled and is thus called the unilocular sporangium.
These sporangia develop singly at the tips of lateral branchlets. The terminal cell of the
branchlet enlarges in size and functions as sporangial initial. The sporangial initial increases
considerably in size with increase in chromatophores and cytoplasm and become globose or
ellipsoid form.
The diploid nucleus of the unilocular sporangia undergoes meiosis or reduction division to
form four haploid nuclei. These haploid nuclei further divide by mitosis to produce 32-64 daughter nuclei. The nuclear divisions are not accompanied by wall formation, and as such the sporangium remains unilocular.
The protoplast of the sporangium cleaves into as many daughter protoplast as the number
of nuclei. Each daughter protoplast metamorphoses into a pyriform, uninucleate, haploid,
biflagellate zoospore called meiozoospore or gonozoospore.
Each zoospore has a chromatophore and an eyespot. The two flagella are laterally inserted
and are unequal. The anterior flagellum is longer, pantonematic and directed forward. The
posterior flagellum is short, acronematic and directed backwards.
The zoospores are liberated enmass through a small aperture formed at the distal end of
each sporangium. They remain inactive for 30-60 seconds and then become free.
They swim for sometime and settles on the substratum with their anterior end. They
withdraw their flagella, become rounded and germinates within 2-3 hours and produces the
alternate haploid plant or gametophyte which is concerned with sexual reproduction.
Sexual Reproduction:
The haploid sexual plants or gametophytes are similar to the asexual plants or
sporophytes. But, they produce gametes in a large, elongated, conical sex organ called the
gametangium. The gametangium resemble the plurilocular sporangia in their structure and
development, hence they are called plurilocular gametangia.
The terminal cell of the lateral branchlet of the gametophyte gets enlarged. It then
undergoes repeated transverse divisions to produce a vertical row of cells. This is followed by
longitudinal and transverse divisions resulting in formation of several hundred small cubical cells.
These cells are arranged in 24 to 40 transverse tiers.
The protoplast of each cell metamorphoses to a pyriform, biflagellate gamete with
laterally inserted flagella. They are similar to zoospores but are smaller in size. These gametes are
liberated in water through a terminal or lateral pore.
Most of the species of Ectocarpus are dioecious i.e., the fusion occurs between the
gametes from different plants. A few species are monoecious. Sexual fusion between the gametes
varies in different species. It may be isogamous, physiological anisogamy or morphological
anisogamy.
Isogamy:
Goebel reported isogamy in E. globifer. The fusing gametes are similar in every respect.
They look alike and behave alike. Fusion occurs between isogametes coming from the different
plant or same plant or even the same gametangium.
Berthold reported physiological anisogamy in E. siliculousus. It is dioecious. The fusing
gametes are morphologically identical but differ in their sexual behaviour.
Few gametes are less active and are called as female gametes. They become passive and
motionless after a short time. Some gametes are more active and are called as male gametes.
Several active male gametes cluster around a single passive female gamete with their long,
forward directed flagella. This is called clump formation.
The female gamete are known to produce a volatile sexual attractant (Sirenine or
Ectocarpine) which causes clumping of the male gametes. This is believed to be first authentic
report of chemical characterization of sex hormone in plants.
Soon, the anterior anchoring flagellum of one of these gametes contracts. Its body is
brought in contact with that of the female gamete. Finally, the two gametes fuse to form the
zygote. The remaining active gametes swim away.
Despite the morphological similarity between the two fusing gametes, there exists
physiological anisogamy.
Morphological Anisogamy:
In E. secundus, the fusing gametes are of unequal size but both are motile. They are
produced in two kinds of gametangia borne on same plant.
The smaller male or microgamete are produced in the mircrogametangium with smaller,
cubical, pale-coloured loculi each containing a small yellow chromatophore.
The larger female or macrogametes are produced in megagametangia with larger loculi;
each containing many deep brown chromatophores.
After liberation, the female gamete or macrogamete comes to rest. The active
microgametes attach themselves to the female gamete by their longer flagellum. Ultimately one
of them fuses with it to form a zygote.
Thus, the fusing gametes are unequal in size but both are motile. Their fusion is called
morphological anisogamy.
In E. padinae, there are three types of plurilocular gametangia. The largest or the
megagametangia and smallest or the microgametangia, represent the oogonia and antheridia
respectively. They produce eggs and sperms and on fusion form oospore. The medium sized or
meiogaetangia give rise to medium-sized gametes. They do not fuse but germinate
parthenogenetically into a new plant.
Germination of zygote:
The zygote germinates without going into a resting stage. There is no zygotic meiosis. The
zygote develops into a diploid sporophyte
Alternation of Generation:
In the life history of Ectocarpus, there is a regular alternation of generation, between the
diploid sporophyte and the haploid gametophyte.
The sporophyte or asexual plant bears two types of sporangia: the plurilocular and
unilocular sporangia. The plurilocular sporangia produce diploid zoospores, which develop into the
sporophyte. Thus, these neutral spores are not concerned with alternation of generations.
The nucleus in the unilocular sporangium undergoes meiosis or reduction division and
produce haploid zoospores or meiozoospores. The meiozoospores, on germination give rise to
haploid gametophytic plant.
The gametophytic plant bear the plurilocular gametangia, which are similar to the
plurilouclar sporangia of the diploid plants. Each compartment of the plurilocular sporangia
produces single pyriform gamete. The gametes form different plants fuse to form zygote. The
diploid zygote grows into a diploid sporophytic plant.
Thus, in the life cycle of ectocarpus, two distinct generations, the sporophyte and the
gametophyte, regularly alternate with each other. Since, these two plants are morphologically
similar, Ectocarpus is said to exhibit isomorphic alternation of generation.