(Thallus Structure, Reproduction, Post Fertilization Changes and Life Cycle)
Polysiphonia- Systematic Position
Distribution of Polysiphonia
Ø Polysiphonia is marine red algae belongs to the class Rhodophyceae.
Ø They are abundantly present in the Atlantic Coasts of America.
Ø Polysiphonia platycarpa is present in the Indian coastal regions.
Ø Polysiphonia violacea is an epiphytic alga grown on some brown algae such as Fucus vesiculosus.
Ø Some species of Polysiphonia are parasitic (example: Polysiphonia fastigiata).
Thallus Structure of Polysiphonia
Ø As the name suggests, the thallus of Polysiphonia is siphonous with many interconnected filaments (siphons).
Ø The thallus color of Polysiphonia is varied from red to purple.
Ø The red coloration is due to the abundance of the pigment r-phycoerythrin.
Ø Other pigments such as chlorophyll-a, β-carotenes, Xanthophyll and r-phycocyanins are also present.
Learn more: Pigmentation in Algae
Ø The thallus of Polysiphonia consists of two systems:
(1). Prostrate system
(2). Erect system
(1). Prostrate System
Ø The prostrate system is creeping and basal.
Ø It is attached to a suitable substratum.
Ø The attachment is facilitated by well-developed rhizoids.
Ø Each rhizoid is single celled, thick-walled with swollen distal ends.
Ø These swollen distal ends of each rhizoids act as the attachment disc.
(2). Erect system
Ø The erect system is developed from the prostrate system.
Ø The erect system is vertical and profusely branched.
Ø The thallus of the erect system consists of several parallel filaments called siphons.
Ø The cells of each siphon are placed end to end and are connected by Plasmodesmata.
Ø Each cell of the siphon are genetically related.
Ø There are three types of siphons: (a) Central siphons, (b) Pericentral siphons and (c) Cortical siphons.
(a). Central siphons:
$. The central siphon is located in the center of the thallus.
$. Cells of the central siphon are elongated and comparatively larger.
(b). Pericentral siphon:
$. Peripheral siphon surrounds the central siphon.
$. Cells of the pericentral siphon are developed by the longitudinal division of the central siphon cells.
$. There are 4 – 24 pericentral siphons are present.
$. The number of pericentral siphons varies in different species.
$. The cells of pericentral siphon are comparatively smaller and narrow.
(c). Cortical cells (cortical siphon):
$. The cortical siphon is formed only in the older branches.
$. The cells of the pericentral siphon divide periclinally to produce many cells.
$. These cells are smaller than the pericentral cells.
Ø Plasmodesmatal connections are also present between the cells of central, pericentral and cortical siphons.
Ø Only a single siphon (central siphon) will be present in the apical portion of the thallus.
Ø The thallus of Polysiphonia shows apical growth which is facilitated by an apical cell.
What are Trichoblasts?
Ø Trichoblasts are small mono-siphonous branches of the thallus with limited growth.
Ø Trichoblasts are developed from the apical initial cell.
Ø The apical initial cell undergoes an oblique division to form a trichoblast initial cell.
Ø The trichoblast initial undergoes many rounds of division to form the multicellular, uniseriate and dichotomously branched filaments called the Trichoblasts.
Ø The cells of Polysiphonia possess two-layered cell wall.
Ø The outer layer is pectic and the inner layer is cellulosic.
Ø Each cell is uninucleate with a viscous protoplast.
Ø The central portion of the cell possesses a large vacuole.
Ø The chromophores which bear the pigments are discoid.
Ø The reserved food material in Polysiphonia is Floridean starch.
Ø The floridean starch is distributed in the cytoplasm as small granules around the nucleus.
Ø Floridoside is also a food reserve in Polysiphonia.
Reproduction in Polysiphonia:
Ø Polysiphonia reproduces by means of vegetative, asexual and sexual methods.
Ø Vegetative reproduction is by fragmentation of the thallus.
Ø Sexual reproduction is oogamous type.
Ø Asexual reproduction is by the production of Tetraspores.
Sexual reproduction in Polysiphonia:
Ø The life cycle of Polysiphonia is completed in three phases.
(1). Gametophytic phase (separate male and female plant)
(2). Carposporophytic phase
(3). Tetrasporophytic phase
Ø The life cycle of Polysiphonia consists of THREE plants
(1). Male Plant
(2). Female Plant
(3). Tetrasporophytic Plant
Ø These three plants are morphologically similar.
Ø The male plants bear antheridia which produce male gamete called spermatium (plural: spermatia).
Ø The female plant produces female sex organ called carpogonium (plural: carpogonia).
Ø The tetrasporophytic plant is the asexual plant which produces haploid tetraspores.
Structure and Development of Antheridium in Polysiphonia
Ø The antheridium is the male sex organ.
Ø They are produced on the thallus of male plants.
Ø Antheridia produce the male gametes called spermatia.
Ø The antheridia are produced in clusters on the apical portion of some specialized branches.
Ø Each antheridial branch consists of a central trichoblast.
Ø The central trichoblast produces many lateral pericentral cells.
Ø Each pericentral cell develops many antheridial mother cells.
Ø In each antheridial mother cell, two to four antheridial cells are developed.
Ø These antheridial cells are called spermatangia.
Ø Each spermatangia produce a single spermatium or male gamete.
Ø Each spermatia (male gamete) is non-motile and uninucleate.
Ø The male gametes reach the female parts through the help of water current.
Ø After the liberation of spermatium, new spermatangia may develop on the old spermatangium.
Structure and Development of Carpogonium in Polysiphonia
Ø The female sex organ of Polysiphonia is called carpogonium.
Ø The carpogonium is developed on the thallus of a female plant.
Ø The young stage of carpogonium is called the pro-carp.
Ø The pro-carp is developed from a special cell of a reduced trichoblast.
Ø The pro carp consists of a row of 3 or 4 cells called carpogonial filament.
Ø Each cell of the carpogonial filament are connected by the plasmodesmata.
Ø The terminal cell of the carpogonial filament is narrow and elongated with a swollen base.
Ø The terminal narrow portion of this cell is called trichogyne and the swollen basal part is called carpogonium.
Ø The carpogonium possess the haploid female nucleus (female gamete).
Ø The cell below the carpogonium in the carpogonial filament is called the supporting cell.
Fertilization in Polysiphonia
Ø The fertilization in Polysiphonia is affected by the water current.
Ø The non-motile male gamete (spermatium) reaches the trichogyne of the carpogonium.
Ø The trichogyne is sticky. Thus the spermatium firmly sticks to the surface of trichogyne.
Ø The wall between the point of contact between trichogyne and spermatium dissolves and the male nuclei migrate into the basal carpogonium.
Ø The male nucleus fuse with the female nucleus situated at the base of the carpogonium to form the diploid zygote.
Ø After the fertilization, the trichogyne disintegrates and falls off.
Post-Fertilization Changes in Polysiphonia
Ø Polysiphonia is characterized some specific and advanced events in its life cycle.
Ø These events occur after the fertilization and hence called the ‘Post-Fertilization Changes’.
Ø The post fertilization changes lead to the development of cystocarp.
Ø The cystocarp produce diploid carpospores.
Ø The supporting cell of the carpogonium produces an auxiliary cell on its upper side and just below the carpogonium.
Ø The auxiliary cell is haploid since it is developed from the haploid supporting cell.
Ø The auxiliary cell remains connected with the carpogonium with a tubular cytoplasmic connection.
Ø During this time, the diploid zygote in the carpogonium divides mitotically into two nuclei.
Ø Among these two nuclei, one migrates into the auxiliary cell through the tubular connection.
Ø The haploid nucleus in the auxiliary cell now degrades.
Ø Thus after the complete degradation of the haploid nuclei of the auxiliary cell, the cell now occupy the diploid nuclei migrated from the carpogonium.
Ø The diploid nucleus received by the auxiliary cell divide mitotically.
Ø They give rise to the development of gonimoblast initials.
Ø The gonimoblast rise from the upper part of the auxiliary cell.
Ø The gonimoblast contain many filaments, each cell of the filament possesses a diploid nucleus in it.
Ø The pericentral cell close to the supporting cell produce many vegetative cells.
Ø These vegetative cells ultimately develop into the diploid carpo-sporophytic plant.
Ø The carpogonium and trichogyne completely disintegrate.
Ø The diploid nucleus in the gonimoblast initial divides mitotically.
Ø Each of these nuclei will migrate into separate gonimoblasts.
Ø Each gonimoblast now contain a diploid nucleus.
Ø The terminal cell of each gonimoblast initial become swollen and functions as the carposporangium.
Ø The cell content of each carposporangium is differentiated into a single diploid carpospore.
Ø During this period, the supporting cell, auxiliary cell, carpogonial filament and sterile filaments fuse together to form an irregularly shaped nutritive cell called the placental cell.
Ø The entire structure consisting of the gonimoblast filaments, carposporangium, carpospores and placental cell are completely surrounded by young vegetative filaments.
Ø These vegetative filaments ultimately form the cup-shaped cystocarp.
Ø The cystocarp is actually the second phase of the life cycle of Polysiphonia called carposporophyte.
Ø The wall of the cystocarp is called pericarp and it contains an opening called osteole.
Ø The entire cystocarp is nutritionally dependent on the haploid gametophytic plant.
Ø Carpospores are liberated from the carpospores and they pass out through the osteole of the cystocarp.
Ø Each cystocarp many contain 50 – 60 carpospores.
Ø The carpospores germinate to form a new diploid phase called tetrasporophyte.
Ø The tetrasporophyte is the third phase of the life cycle of Polysiphonia.
Ø The tetrasporophyte resembles the gametophytic plant in their morphology.
Tetrasporophytic phase of Polysiphonia
Ø The carpospores germinate immediately into a diploid plant called tetrasporophyte.
Ø The tetrasporophyte is a free living plant and it is similar to the gametophyte.
Ø The tetrasporophyte is a diploid plant which does not bear sex organs.
Ø It is an asexual plant which reproduces by haploid spores called tetraspores.
Ø The tetraspores are produced in tetrasporangia.
Ø The diploid nucleus of each of the tetrasporangium divides meiotically to produce for tetraspores.
Development of tetrasporangium in Polysiphonia
Ø The tetrasporangium is developed from the pericentral cells.
Ø These pericentral cells divide vertically to form an outer cell and an inner cell.
Ø The outer cell divide to produce two or more cover cells.
Ø The inner cell functions as the tetrasporangial mother cell.
Ø The tetrasporangial mother cell divides transversely to produce a lower stack cell and upper tetrasporangial cell.
Ø The tetrasporangial mother cell enlarges and functions as the tetrasporangium.
Ø The nucleus of the tetrasporangium divides meiotically to produce 4 haploid tetraspores.
Ø The sporangial wall ruptures and the tetraspore are liberated.
Ø The tetraspores germinate into the haploid gametophytic plant.
Ø Out of the four tetraspores produced from a single tetrasporangium, two develop into male plant and the other two developed into female plant.
Polysiphonia Life Cycle
Learn more: Life Cycles in Algae
Ø The life cycle of Polysiphonia is triphasic consists of three phases.
Ø These three phases are:
(1). Haploid gametophytic phase
(2). Diploid carposporophytic phase
(3). Diploid tetrasporophytic phase
Ø Thus the life cycle is with two diploid and one haploid phase and thus the life cycle of Polysiphonia is haplodiplobiontic.
Ø The gametophytic plant is haploid and they produce male and female gametes.
Ø The male gamete is called spermatium and female gamete is called carpogonium.
Ø Fusion of spermatium with carpogonium results in the formation of diploid zygote.
Ø From the diploid zygote, gonimoblast filaments with carposporangia arise.
Ø Each carposporangia produce single diploid carpospores.
Ø The carpospores producing diploid structure represent the second phase of the life cycle of Polysiphonia called Carposporophyte.
Ø The carpospores are germinated to produce the third phase called tetrasporophyte.
Ø Tetrasporophyte produce sporangium called tetrasporangium.
Ø Each tetrasporangium produces four haploid tetraspores.
Ø The tetraspores germinate into the haploid gametophytic phase and the lifecycle is completed.