Polysiphonia: Occurrence , Thallus Structure and Reproduction!
The algae of class Rhodophyceae are commonly called red algae due to their red colour Rhodophyceae is represented by about 831 genera and 5250 species.
The distinguishing characters of class Rhodophyceae are as follows:
(i) All members of Rhodophyceae except twenty fresh water species are marine and about two hundred species inhabit land waters. The fresh water species either grow in fast flowing water (e.g., Batrachospermum, Lamanea) or grow in stagnant water with sufficient aeration (e.g.. Asterocystis, Compsopogon).
(ii) Algae of Rhodophyceae are most abundant in the lower intertidal and sub-littoral zones at a depth of 60 meter or more. A few are adapted to a considerable exposure living at or above the high tide level. Some are terrestrial (Porphyridium). Apart from some being epiphytes, several species are distinctly parasites (Ceratocolax, Calleocolax, Pterocladiophila). Some algae have deposit of calcareous substances in their wall.
(iii) Most of the members of Rhodophyceae are red, soft and slimy, the thallus ranges from unicellular to complex multi-axial forms.
(iv) The thallus is unicellular in Porphyridium, filamentous in Goniotrichum, palmelloid in Asterocytis or parenchymatous in Porphyra, Gelidium, Gracillaria. Thallus in Batrachospermum has uniaxial structure and in Polysiphonia it is multi-axial.
(v) The growth is by apical cell in sub-class Florideae and diffused in sub-class Bangioideae.
(vi) The cell wall is differentiated into an outer pectic layer and inner cellulosic layer. The mucilaginous matter of the outer pectic layer consists of agars and carrageenan’s. Cells are generally uninucleate and in some genera they may be multinucleate (e.g., Griffithsia). The shape of centrally situated nucleus is spherical and the peripheral nuclei are elliptical.
(vii) In Bangiodeae the chromatophore is single, star shaped with central pyrenoid, in Florideae the chromatophores are many, discoid and parietal chromatophores without any pyrenoid.
(viii) The main pigments are chlorophyll a and b, α and β carotene, xanthophyll’s and biliproteins. The pigments r-phycoerythrin and r-phycocyanin are responsible for red colour of the thallus.
(ix) The food is stored in the form of floridean starch. There is complete absence of all motile stages either in asexual or sexual phase of life cycle.
(x) The reproduction takes place by asexual and sexual methods.
(xi) The asexual reproduction takes place by mono-spores, neutral spores, carporpores or by tetra spores.
(xii) The non-motile male gametes are called spermatia which are produced in spermatangia.
(xiii) The female reproductive organ called pro-carp consists of carpogonium and trichogyne. The carpogonium is swollen basal part containing female nucleus and the trichogyne is narrow gelatinous receptive part. The pro-carp is sessile in sub-class Bangioideae and stalked in sub-class Florideae.
(xiv) During post fertilization the carpogonium transforms into carposporophyte. In carposporophyte, carposporangia are formed. In carposporangia, carpospores are produced. The carpospores on germination produce gametophytic thallus or alternate diploid thallus.
(xv) Higher forms (e.g., Polysiphonia) produce tetra spores and tetra sporophyte.
F. E. Fritsch (1945) on the basis of the thallus structure and post-fertilization changes classified Rhodophyceae into two sub-classes:
Sub-class I. Bangioideae:
This sub-class includes a single order Bangiales, examples—Bangia, Porphyra, Porphyridium. Bangioideae includes primitive forms of Rhodophyceae. The thalli lack pit connections and apical growth. The reproductive organs are relatively simple. The zygote directly gives rise to carposporangium after division.
Sub-class II. Florideae:
The sub-class Florideae is divided into six orders:
Order 1. Nemalionales:
Order 2. Gelidiales:
Order 3. Cryptonemiales:
Order 4. Gigartinales:
Order 5. Rhodymeniales:
Order 6. Ceramiales:
The algae of sub-class Florideae have uniaxial or multi-axial thallus. The cells are connected by pit connections and the growth of thallus is by apical cell. The reproductive structures are relatively more complex. The zygote divides to make gonimoblast filaments and the terminal cells of gonimoblast filaments make carposporangium.
The order ceramiales is represented by about 160 genera and 900 species. The algae of this group are mostly marine. The thalli are complex multi-axial or polysiphonous.
After fertilization an auxiliary cell is cut off from the supporting cell of the carpogonium. The 2n zygote nucleus divides by mitotic division and one of the diploid nuclei enters supporting cell through auxiliary cell. From supporting cell arise a number of gonimoblast filaments.
The terminal cell of gonimoblast filament bears carposporangium which forms a diploid carpospore. The complete structure is called as carposporophyte. The carpospore on germination forms tetra sporophyte. The tetra sporangia on tetra sporophytes form haploid tetra spores. The tetra spores on germination form haploid gametophytic plants. Thus, the life cycles are triphasic and diplobiontic.
The order Ceramiales is divided into four families:
Family 1. Ceramiaceae e.g., Ceramium
Family 2. Delesseriaceae e.g., Delesseria
Family 3. Rhodomelaceae e.g., Polysiphonia
Family 4. Dasyaceae e.g., Dasya
The algae of family Rhodomelaceae are polysiphonous. The thallus consists of central siphon and pericentral siphons. The cells of central siphon are uninucleate. The male and female reproductive structures develop on special reproductive branches called as trichoblasts. After fertilization carposporophytes and tetra sporophytes are formed. The life cycles are diplobiontic.
Occurrence of Polysiphonia:
Polysiphonia is a large genus with about 200 species. The genus is represented in India by about 16 species found is southern and western coasts of India. Some common Indian species are P. ferulacea, P. urceolata and P. variegata.
Most of the species are lithophytes i.e., found growing on rocks. Some species are epiphytic, found growing on other plants and algae e.g., P. ferulacea grows on Gelidium pusillum. P. variegata grows on the roots of mangroves. Some species are semi parasitic e.g., P. fastigiata is semiparasiite on Ascophyllum nodosum and Fucus.
Thallus Structure of Polysiphonia:
The thallus is filamentous, red or purple red in colour. The thallus is multi-axial and all cells are connected by pit connections hence, the name given is Polysiphonia. Due to continuous branching and re-branching the thallus has feathery appearance (Fig. 1A). The thalli may reach the length of about 30 cm.
The thallus is heterotrichous and is differentiated into a basal prostrate system and erect aerial system.
The prostrate system (Fig. 1B) creeps over the substratum. Its functions are attachment of the thallus to the substratum and perennation. In many species of Polysiphonia e.g., in P. nigrescens, the prostrate system is well developed and multi-axial in structure. In some species e.g., in P. elongata and P. violacea the multi-axial prostrate system is absent.
The plants remain attached to the substratum by:
(a) Unicellular richly branched rhizoids arising from multi-axial prostrate system.
(b) Rhizoids arising from the erect system, forming, an attachment disc or hapteron.
(c) By the unicellular rhizoids arising in groups from the prostrate system e.g., P. fastgata. The erect aerial system arises from the prostrate system. It is made of multi-axial branched filaments. The main axis and long branches have similar structure.
These are made of a central large filament or central siphon of cylindrical cells. The central siphon is surrounded by a number of pericentral cells or pericentral siphons (Fig. 2 A, B). The number of pericentral siphons varies from species to species. The length of central and pericentral siphons is equal hence, the filaments appear to be divided in nodes and internodes like.
Each pericentral siphon remains connected with central siphons through pit connections. The successive central siphon cells and all peripheral cells are also connected to each other through pit connections. Hence the complete thallus makes a polysiphonaceous structure (Fig. 2 C).
The thallus of Polysiphonia bears two types of branches (a) Short branches (b) Long branches. The branches are lateral and monopodial. The branching starts from the cell lying 2-5 cells below the apical cell.
(A) Short Branches or Trichoblasts:
The short branches or trichoblasts are branches of limited growth. These are uniaxial in structure and lack pericentral siphons. The cells are connected to each other by pit connections. These branches arise on main axis and on long branches in spiral manner. Their cells contain very few chromatophores.
These branches are deciduous, perennial species shed these branches before winter and develop again in spring season. The basal cell of the last trichoblast is retained as scar cell by the pericentral siphon.
Development of Trichoblast:
The trichoblast initial is differentiated from a cell 2-5 cells below the apical cell (Fig. 3 A, B). It starts as a small cell and divides repeatedly to form dichotomously branched, uniseriate multicellular hair like trichoblast (Fig. 4 C, D). The trichoblast may bear male and female reproductive structures or remain sterile.
(B) Long Lateral Branches:
The long lateral branches are branches of unlimited growth are polysiphonous at the base and monosiphonous in terminal parts. These branches develop from the basal cells of short branches. In species like P. violacea they develop as outgrowth from trichoblast initial. They develop along with trichoblast and after few divisions the trichoblasts are pushed aside so they appear to arise from trichoblast dichotomously.
The outgrowth functions as the apical cell of the Long Branch which after repeated division forms the central siphon. The central siphon later on develops pericentral siphons. In species like P. elongata the long branches arise directly from the main axis. The outgrowth develops from a cell 2-5 cells below the apical cell. The outgrowth forms central siphon and later pericentral siphon in normal way.
The cortical cells arise from outer pericentral cells by pericentral division. The central cells divide anticlinally and surround the pericentral cells. The cortical cells are parenchymatous in nature and form several layers. The cortical cells may be present in lower part of thallus e.g., P. mollis or throughout the thallus e.g., P. crassiuscula.
Cell Structure of Polysiphonia:
The cells of central and pericentral siphons are cylindrical and elongated. The cell wall is differentiated into outer pectic and inner cellulosic layer. The cell contains a large central vacuole which is delimited by a membrane tonoplast. The cytoplasm is present between the cell wall and the central vacuole. The cell contains a number of red discoid chromatophores which lack pyrenoids.
The chromatophores contain pigments chlorophyll a, chlorophyll d, a carotene, (3 carotene, r-phycoerythrin and r-phycocyanin. The chromatophores are parietal in position (Fig. 2A). The central siphon cells and pericentral siphon cells posses single peripheral nucleus. The cytoplasm contains granules of floridean starch as food reserve.
Growth of Polysiphonia:
The growth takes place by the dome shaped apical cell located on the tip of central siphon. The apical cell cuts many cells on lower side by transverse divisions which form the central siphon. Some of the lower cells divide vertically to form pericentral cells.
Reproduction in Polysiphonia:
Polysiphonia is mainly heterothallic. In the life cycle of Polysiphonia three kinds of thalli are found. These are:
(a) The gametophytic thalli which are haploid free living and dioecious. The male sex organs spermatangia are formed on male gametophytic plant and the female sex organs carpogonia are formed on female gametophytic plant.
(b) The carposporophytes are diploid, depend upon the female gametophyte. They develop after fertilization from zygote and later bear carposporangia. The carposporangia form diploid carpospores.
(c) The tetrasporophytic plant which is formed by germination of diploid carpospores is diploid and independent. Then plant bears tetrasporangia which form four haploid tetraspores which again give rise to male and female gametophytic plants.
In life cycle of Polysiphonia both asexual and sexual reproduction takes place. The life cycle is example of triphasic alternation of generation.
Sexual reproduction is oogamous type and plants are dioecious i.e., male and female sex organs are produced on different male and female gametophytic plants.
The male sex organs, spermatangia or antheridia develop on fertile trichoblasts present on tips of male gametophytic plant. The male trichoblast when only 2-3 celled divides dichotomously. In most of the species one branch remains sterile and the other bears spermatangia, in some specie both branches become fertile. The sterile branch may divide again to form fertile trichoblasts.
The cells of fertile uniaxial trichoblast except the 2-3 divide periclinally to form pericentral cells. The pericentral cells form spermatangial mother cells on outer-side (Fig. 4B). Each spermatangial mother cell cuts off 2-4 sporangia on outer side. The complete structure makes cone shaped cluster of spermatangia (Fig. 4 A).
The mature spermatangium is a globular or oblong, unicellular structure. Its cell wall is differentiated into three layers, inner refractive middle, gelatinous and outer thick layer. The uninucleate protoplast of spermatagnium forms a male gamete or spermatium. The spermatium is non-motile and is released through an apical pore in the spermatangium (Fig. 4 C).
The female sex organ of Polysiphonia is called as carpogonium. (Fig. 5 F). The carpogonium develops on trichoblast on female gametophytic plant. The trichoblast initial arises from a cell, 2-4 cells behind the apical cell.
It develops into 5-7 celled female trichoblast. The three lower cells form 5 pericentral cells of which there is one adaxial, two lateral and two abaxial cells (Fig. 5 C-E). These cells surround the central cell. The adaxial cell called supporting cell, forms a basal sterile filament initial, a lateral sterile filament initial and a curved four celled carpogonial branch.
The basal swollen flask shaped cell of the carpogonial branch functions as carpogonium or egg cell and the upper tubular elongated part is called trichogyne (Fig. 5 C). The lateral sterile filament initial divides to form two celled lateral sterile filament. The pericentral cells surrounding carpogonium form outgrowths to cover the carpogonium. The sterile sheath around carpogonium is called pericarp (Fig. 5 F).
The spermatia are carried to the trichogyne of carpogonium through water currents. The spermatium adheres to the trichogyne by the mucilage around it. The walls between spermatium and the trichogyne dissolve. The male protoplasm enters carpogonium through trichogyne. After fertilization of male and female nuclei, a diploid zygote cell is formed.
Post fertilization changes:
After fertilization many changes take place within and around the female reproductive structure. The basal sterile initial divides to form basal sterile filaments which are 2-4 celled. The lateral sterile initials divide to make lateral sterile filaments which are 4- 10 celled.
The sterile filaments are of nutritive nature. The supporting cell divides transversely to form an auxiliary cell between itself and the carpogonium. A tubular protoplasmic connection is established between auxiliary cell and carpogonium (Fig. 6A, B).
The diploid zygote nucleus divides mitotically and forms two diploid nuclei of which one nucleus remains in the carpogonium and the other nucleus migrates into the auxiliary cell. The auxiliary eel which contains one haploid nucleus receives this diploid nucleus. The haploid nucleus of the auxiliary cell degenerates and it then contains diploid nucleus only.
The trichogyne at this time degenerates, the carpogonium, auxiliary cell and supporting cell fuse and form irregular shaped placental cell. The diploid nucleus of the auxiliary cells divides mitotically forming many diploid nuclei in the placental cell.
A number of gonimoblast initials arise from the placental cell and each initial receives a diploid nucleus from placental cell. Each gonimobalst initial forms a two celled gonimoblast filament or gonimalobe.
The lower cell of gonimoblast filament can also give rise to new gonimoblast filaments. All the gonimoblast filaments make a compact mass and this structure arising from diploid zygote cell is V called the carposporophyte (Fig. 6 B-D).
This is diploid sporophytic phase in life cycle of Polysiphonia and it is dependent upon the gametophytic haploid phase. The carposporophyte or cystocarp or gonimocarp is made of many gonimoblast filaments attached on the placental cell which remain covered by Sterile pericarp. (Fig. 6 B-D).
It is urn shaped structure. The terminal cell of the gonimoblast filament Carpogonium develops into a carposporangium which forms a single diploid carpospore. The diploid carpospores are liberated through the ostiole of carposporophyte (Fig. 6E-F). The catpospores are carried away by water and germinate on suitable substratum.
The carpospore develops a wall around itself and then divides by mitotic division to make a small lower cell and the larger apical cell. The two celled filament divides to make four celled filament.
The lowermost cell of the filament differentiates into rhizoidal cell and the uppermost cell makes the apical cell. The apical cell divides transversely to make central siphon cell which divide periclinally to make pericentral cells. The germination of diploid carpospore results in the formation of diploid tetrasporophytic plant (Fig. 6G-I).
The tetra sporophytes are free living diploid plants in the life cycle of Polysiphonia. Morphologically these plants are similar to haploid gametophytic plants but they do not bear male or female sex organs like gametophytic plants. Some pericentral cells of tetrasporophytic plant function as tetra sporangial initials. These are smaller than other pericentral cells and only one in each tier.
The tetra sporangial initial divides by vertical division to make an outer cover cell (Fig. 7 A-C) and the inner sporangial mother cell. The cover cell divides further to make two or more cover cells.
The sporangial mother cell divides by transverse division to make a lower stalk cell and the upper sporangial cell. The sporangial cell enlarges and makes tetra sporangium. The branches bearing tetra sporangia become twisted and swollen and are called stichidia.
The diploid nucleus of tetra sporangium divides meiotically forming four haploid nuclei followed by the division of protoplast. The four uninucleate segments develop into four haploid tetra spores or meiospores which are arranged tetrahedrally.
The tetra spores on maturity are liberated by splitting of sporangial wall accompanied by lifting of the cover cell. Two of the four tetra spores germinate to make haploid male gametophytic plant and the two make haploid female gametophytic plants (Fig. 7 D-I). Hence the asexual reproduction in Polysiphonia take place by means of haploid tetra spores which are formed on tetrasporophytic plant.
Alternation of Generation:
The life cycle of Polysiphonia exhibits triphasic alternation of generation. In the life cycle three distinct phases occur.
1. Gametophytic phase.
2. Carposporophyte phase.
3. Tetra sporophyte phase.
Polysiphonia is dioecious plant. The male gametophytic plants and the female gametophytic plants are distinct. The haploid male gametophytic plant bears sex organs spermatangia which produce haploid spermatia. The haploid female gametophytic plant bears sex organs carpogonium.
The fertilization takes place in situ and diploid zygote nucleus is formed. The zygote develops in second phase of life cycle, the carposporophyte is dependent upon female gametophytic plant. The carporophyte is um shaped structure and forms diploid carpospores in carposporangia.
The carpospores germinate to make diploid tetrasporophytic plants. The tetrasporophytic plant bear tetra sporangia.
The diploid tetra sporangial nucleus divides meiotically to form four haploid tetra spores which again make gametophytic male and female plants. In life cycle of Polysiphonia two diploid phases carposprophyte and tetra sporophyte alternate with one haploid gametophytic phase. The life cycle of Polysiphonia can be called as triphasic diplobiontic with isomorphic alternation of generation (Figs. 8, 9).