Structure and Reproduction Algae | Botany Optional for UPSC PDF Download

Reproduction and Life Cycle Patterns in Algae

There are three main types of reproduction found in algae:

• Vegetative Reproduction
• Asexual Reproduction    
• Sexual Reproduction

Reproduction in Algae

Vegetative Reproduction

Vegetative reproduction is a type of asexual reproduction in algae where new individuals are produced from various parts of the algal thallus. This method does not involve the formation of spores, and there is no alternation of generations. It is the most commonly observed method of reproduction in algae.

Types of Vegetative Reproduction

(a) Cell Division or Fission: This is the simplest form of vegetative reproduction and is commonly observed in unicellular algae such as Chlamydomonas, Synechococcus, and diatoms. In this process, a vegetative cell undergoes mitotic division, resulting in the formation of two daughter cells, each of which develops into a new individual.
(b) Fragmentation: Fragmentation is another method of vegetative reproduction, especially in multicellular filamentous algae like Spirogyra, Ulothrix, Oedogonium, Zygnema, and Cylindospermum. In this process, the filamentous thallus breaks into multiple-celled fragments. Each fragment has the potential to give rise to a new individual. Fragmentation can occur accidentally or due to the formation of separation discs or mechanical forces.
(c) Hormogonia: Hormogonia are structures involved in the vegetative reproduction of blue-green algae. These structures are formed when the trichomes of blue-green algae break into many-celled segments within a sheath. They are often delimited by the formation of heterocysts, separation discs, necridia, or through the death and decay of intercalary cells in the trichome. Hormogonia are commonly found in algae like Nostoc, Oscillatoria, and Cylindosporium.
(d) Formation of Adventitious Branches: Some large thalloid algae have the ability to produce adventitious branches. When these branches are detached from the main plant body, they can develop into new individuals. For instance, adventitious branches resembling protonema can form from the internodes of Chara or stolons of Cladophora glomareta.
(e) Bulbils: Bulbils are tuber-like outgrowths that develop at the tip of rhizoids and on the lower nodes of Chara. These structures store food, and when detached from the parent plant, they have the capacity to grow into new individual plants. The figure provided illustrates vegetative reproduction in algae, with parts A, B, C, and D representing different aspects of this process.
(f) Amylum Stars: Star-shaped clusters of cells containing starch develop on the lower nodes of Chara. These structures are known as amylum stars. When amylum stars are separated from the parent plant, they have the potential to grow into new plants.
(g) Budding: In Protosiphon, structures resembling buds are formed due to the proliferation of vesicles that are delimited from the parental body by a septum. After detachment, these bud-like structures can grow into new plants.

Asexual Reproduction

Asexual reproduction in algae involves the formation of specific types of spores, which can be either naked or enclosed within a wall. This process aims to rejuvenate the protoplast (cell contents) without the need for sexual fusion. Each spore has the potential to germinate into a new individual. Asexual reproduction does not involve alternation of generations.

Various Types of Asexual Spores

(a) Zoospores: Zoospores are motile spores equipped with two, four, or multiple flagella. Depending on the number of flagella, they can be referred to as bi-flagellate, quadri-flagellate, or multi-flagellate zoospores. For instance, biflagellate zoospores are found in organisms like Chlamydomonas, Ulothrix, and Ectocarpus, while quadriflagellate zoospores are present in Ulothrix. Oedogonium contains multiflagellate zoospores. Some organisms, like Vaucheria, have multinucleate and multiflagellate zoospores known as synzoospores. Each zoospore contains a chloroplast and an eye spot, and they can be either haploid or diploid. Zoospores are released from zoosporangia, either through the disintegration of the zoosporangial wall or the formation of an apical pore. After release, zoospores swim briefly, retract their flagella, form cysts, and eventually germinate to form new plants.
(b) Aplanospores: Aplanospores are non-motile spores formed either singly or through the division of the protoplast. They are produced within sporangia during unfavorable conditions, particularly in drought. Examples include aplanospores in Ulothrix and Microspora. Certain algae from semi-aquatic habitats can also produce aplanospores. When these spores closely resemble the parent cell, they are known as autospores (e.g., Scenedesmus, Chlorella). Aplanospores with thickened walls and abundant food reserves are referred to as hypnospores (e.g., Pediastram, Sphaerella). They serve as a survival mechanism during extended periods of desiccation. When favorable conditions return, hypnospores can directly germinate into new individuals or produce zoospores. Hypnospores in Chlamydomonas nivalis are red due to the deposition of haematochrome pigment in their walls.
(c) Tetraspores: Certain diploid algae, such as Polysiphonia, produce a unique type of haploid aplanospore known as tetraspores. These tetraspores are formed within tetrasporangia, where the diploid nucleus undergoes meiotic division to yield four haploid nuclei. Each of these nuclei, along with a small amount of protoplasm, develops into a tetraspore. Upon release, tetraspores germinate to give rise to male and female gametophytes.
(d) Akinetes: Akinetes are specialized spore-like structures formed from vegetative cells of certain filamentous algae. They are characterized by elongated thick walls and contain ample food reserves. Akinetes serve as a mechanism to survive unfavorable conditions. When favorable conditions return, akinetes germinate and develop into new individuals.
(e) Exospores: In some algae, spores are regularly produced at the exposed distal end of the protoplast in a basipetal succession. These spores, known as exospores, aggregate in groups and eventually develop into new colonies. Chamaesiphon is an example of an alga that produces exospores.
(f) Endospores: Endospores are small spores formed as a result of divisions within the mother protoplast. They are also referred to as conidia or gonidia. Once released by the dissolution of the parental wall, endospores germinate directly without undergoing a resting phase and develop into new plants. Dermocarpa is an example of an alga that produces endospores.

Sexual Reproduction

With the exception of members belonging to the Cyanophyceae class, sexual reproduction is a common feature among all algae. During sexual reproduction, gametes combine to form a zygote, as illustrated in Figure 2.3. This process involves the fusion of gametes originating from different parents, resulting in the creation of a novel genetic makeup.

There are five main types of sexual reproduction in algae, distinguished by the structure, physiological behavior, and complexity of their reproductive organs:
(a) Autogamy: Autogamy involves the fusion of gametes that originate from the same parent cell. After fusion, they form a zygote. In autogamous reproduction, there is no introduction of new characteristics. An example is seen in the diatom Amphora normani.
(b) Hologamy: Hologamy occurs when vegetative cells from different strains, denoted as + and -, act as gametes and fuse to form a zygote. While this process may not be the most efficient in terms of multiplication, it generates new genetic combinations. Chlamydomonas is an example of an alga that reproduces through hologamy.
(c) Isogamy: Isogamy involves the union of two gametes that are morphologically and physiologically similar. After fusion, they give rise to a zygote. These gametes, known as isogametes, are typically flagellate. Examples include Chlamydomonas eugametos and Ulothrix.
(d) Anisogamy: Anisogamy differs from isogamy in that the uniting gametes are morphologically and physiologically distinct. The smaller, more active gamete is referred to as the microgamete (male), while the larger, less active one is the macrogamete (female). An example is Chlamydomonas braunii. In some cases, there can be physiological anisogamy, where gametes share morphological similarities but have physiological differences. Zygnema and Spirogyra are examples of organisms that exhibit physiological anisogamy.
(e) Oogamy: Oogamy represents an advanced form of sexual reproduction where fertilization occurs between a small, motile (or non-motile in the Rhodophyceae class) male gamete, called a sperm or antherozoid, and a large, non-motile female gamete, known as an egg or ovum. Male gametes develop within structures called antheridia, while female gametes develop within oogonia. Examples of algae that reproduce through oogamy include Oedogonium, Vaucheria, Chara, Laminaria, Sargassum, Polysiphonia, and Batrachospermum.

Life Cycle Patterns in Algae

Algae have four main patterns of life cycle, i.e, Haplontic Life Cycle, Diplontic Life Cycle, Diplohaplontic Life Cycle, Triphasic Life Cycle.

Haplontic Life Cycle

In the haplontic life cycle, the plant body primarily exists as a haploid gametophyte, while the diploid sporophyte stage is limited to the zygote. The gametophytic plant generates haploid gametes within specialized structures called gametangia. When these gametes fuse, they form a zygote, which is the sole diploid stage, representing the sporophytic phase of the life cycle.
The zygote subsequently undergoes meiotic division, producing four meiospores. These meiospores then develop into haploid plants, thereby completing the cycle. This alternation of generations can be understood by considering the chromosome numbers, as depicted in Figure 2.4.

This type of life cycle is also referred to as a monogenic life cycle and is prevalent among various organisms, including many Chlorophyceae such as Chlamydomonas, Ulothrix, Oedogonium, Spirogyra, Chara, as well as all members of Xanthophyceae.

Diplontic Life Cycle

In the sporophytic life cycle, the dominant phase is the sporophyte, which constitutes the main plant body and develops specialized sex organs. These sex organs produce gametes through the process of meiosis. However, it's important to note that the gametes themselves represent the gametophytic stage in this cycle.
Upon their formation, the gametes undergo fertilization immediately, resulting in the formation of a zygote. Unlike in the haplontic life cycle, the zygote in this cycle does not undergo meiosis; instead, it gives rise to a new sporophytic plant body directly, as illustrated in Figure 2.5.
This type of life cycle is commonly observed in many members of Bacillariophyceae, as well as in some members of Chlorophyceae like Cladophora glomerata. Additionally, certain brown algae such as Fucus and Sargassum, which belong to the Phaeophyceae group, also exhibit this type of life cycle.

Diplohaplontic Life Cycle

In the diplohaplontic life cycle, both haploid and diploid phases are equally significant and are represented by two distinct vegetative individuals. These phases differ primarily in chromosome number and their respective functions. The haploid gametophytic plant reproduces through sexual means, while the diploid sporophytic plant reproduces through asexual processes. This alternation between two distinct vegetative individuals is facilitated by sporogenic meiosis and the fusion of gametes, as depicted in Figure 2.6.

There are two types of diplohaplontic life cycles:

• Isomorphic or Homologous Diplohaplontic Type: In this type, both sporophytic and gametophytic plants are morphologically similar and exist as free-living entities. The gametophytic plant (haploid) produces gametes, engages in sexual reproduction, and forms zygotes. These zygotes then germinate directly into sporophytic (diploid) plants. The sporophytic plants, in turn, produce haploid zoospores through meiosis, which can develop into new gametophytic plants. This type of life cycle is observed in organisms such as Cladophora, Ulva, and Draparnaldiopsis from the Chlorophyceae group, as well as Ectocarpus from the Phaeophyceae group.
• Heteromorphic or Heterologous Diplohaplontic Type: In this type, both sporophytic (diploid) and gametophytic (haploid) plants are morphologically dissimilar. Typically, the sporophyte is more complex and elaborate, while the gametophyte is simpler and smaller. For example, in Laminaria of Phaeophyceae, the gametophytic plant body consists of tiny filaments that produce gametes. These gametes fuse to form zygotes, which directly germinate into sporophytic plants. The sporophytic plant body in Laminaria is macroscopic and can grow to several meters in length. The sporophytic plants bear zoosporangia and produce zoospores through meiotic division. These haploid zoospores, upon germination, give rise to haploid gametophytic plants.

This diplohaplontic life cycle can exhibit morphological diversity in the relationship between the two phases, with some cases showing dominance of the gametophyte, as seen in Cutlaria, while others, like Laminaria, feature a more elaborate sporophyte.

Triphasic Life Cycle

In this type of life cycle, there is a succession of three distinct generations, consisting of both haploid and diploid phases.
This cycle can be categorized into two types:

• Haplobiontic Type: In the haplobiontic type, the gametophytic (haploid) phase is more elaborate, dominant, and longer-lasting than the sporophytic (diploid) phase. The sporophytic phase is represented solely by the zygote, making it a triphasic cycle. Two successive haploid generations are interrupted only by the diploid zygote stage, highlighting its triphasic nature. This type of life cycle is observed in primitive members of Rhodophyceae, such as Batrachospermum and Nemalion.
  In Batrachospermum, the gametophytic plant body develops sex organs and produces male (spermatium) and female (egg) gametes. The fusion of these gametes results in the formation of a zygote. The zygote immediately undergoes meiosis to produce another haploid generation known as the carposporophyte. The carposporophyte develops carposporangia, which produce haploid carpospores. These carpospores germinate and develop into new, free-living gametophytic plants. Thus, in this cycle, three phases are present: (i) haploid carposporophyte, (ii) haploid gametophyte, and (iii) diploid zygote.

• Diplobiontic Type: In the diplobiontic type, there is one gametophytic phase and two sporophytic phases, indicating its triphasic nature. The sporophytic phase is more elaborate and persists for a longer duration than the gametophytic phase. This type of life cycle is observed in Polysiphonia, a member of Rhodophyceae.
  In Polysiphonia, the gametophytic phase is represented by two types of gametophytic plants: male and female plants, bearing permatangium and carpogonium, respectively. These structures later develop sperms and eggs, respectively. Male and female gametes, i.e., sperm and egg, fuse to form a zygote (2n). The zygote develops into a diploid carposporophytic phase, during which diploid carpospores are formed in the carposporophyte.
  Upon germination, these carpospores give rise to diploid tetrasporophytic plants. The tetrasporophytic plant produces diploid tetrasporangia, each of which produces four tetraspores (n) through meiotic division. These tetraspores are liberated by the splitting of the sporangial wall. Out of the four tetraspores, two produce male gametophytes, and the other two develop into female gametophytes. In this cycle, three phases are present: (i) haploid gametophyte, (ii) diploid carposporophyte, and (iii) diploid tetrasporophyte.

Affinity with Fungi

There are certain similarities or affinities between algae and fungi:

• Thalloid Structure: Both algae and fungi exhibit a thalloid (non-vascular) structure, lacking the complex structures found in higher plants.
• Simple Sex Organs: The sex organs in both algae and fungi are relatively simple and lack protective structures like the protective jacket walls seen in higher plants.
• No Embryo Formation: Neither algae nor fungi undergo embryo formation after sexual union, which is in contrast to higher plants where embryonic development occurs.