Polysiphonia: Thallus Structure, Reproduction, Post Fertilization Changes and Life Cycle


Polysiphonia Post Fertilization Changes

Polysiphonia
(Thallus Structure, Reproduction, Post Fertilization Changes and Life Cycle)

Polysiphonia- Systematic Position

Division: Rhodophyta
Class: Rhodophyceae
Order: Ceramiales
Family: Ceramiaceae

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).

Habit of PolysiphoniaØ  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.

Thallus Structure of Polysiphonia

(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.

Thallus Structure of Polysiphonia(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.

Cell structure

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.

Antheridium of Polysiphonia

image source: wikipedia

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.

What is Cystocarp?

image source: wikipedia

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.

Tetrasporophyte definition

image source: cc wikipedia

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.

Tetrasporophyte of Polysiphonia

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.

diplobiontic life cycle of polysiphonia

Haplo-diplobiontic Life Cycle (Eg. Polysiphonia)


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