Thursday, 25 October 2018

Reproduction in Algae

In post 3 you have learnt (lint algae vary in size from small microscopic unicellular forms like Chlamydosmonas to large macroscopic multicellular forms like Lammaria. The multicellular Forms show great diversity in their organization and include ligamentous. heterotrichous. tialloid and polysiphonoid forms. In this unit we will discuss the types of reproduction and life cycle in algae taking suitable representative examples from various groups. Algae show all the three types of reproduction vegetative, asexual and sexual. Vegetative method solely depend on the capacity of bits of algae accidentally broken to produce a new one by simple cell division. Asexual methods on the other hand involve production of new type of cells, zoospores.

ln sexual reproduction gametes are formed. They fuse in pairs to form zygote. Zygote may divide and produce a new thallus or it may secrete a thick wall to form a zygospore.

What controls sexual differentiation, attraction of gametes towards each other and determination of maleness or femaleness of gametes? We will discuss this aspect also.

You will see that sexual reproduction in algae has many interesting features which also throw light on the origin and evolution of sex in plants. This will be discussed in the last section of this unit.

Types of Reproduction

Reproductive processes found in various groups of algae can be broadly divided into three types: vegetative, asexual and sexual methods.

Vegetative Reproduction 

The most common type of reproduction in algae is by binary fission. In unicellular prokaryotic algae like Anacystis it is the only method of reproduction found in nature. In filamentous and multicellular forms, the algae may get broken accidently into small pieces, - each developing into a new’ one. The above methods of propagation are known as vegetative reproduction.

Asexual Reproduction 

V/hen vegetative reproduction takes place through specialized cells (other than sex cells), it is described as asexual reproduction.

Anabaena and Nostoc - The cells accumulate rood materials, develop thick walls to become spores or akinetes (Fig. 4.1). Akinetes can withstand dryness (lack of water) and high temperature for a long Lime, but when conditions are suitable they germinate to form new filament.

Reproduction in Algae

Ulothrix - Filamentous algae (like Ulothrix) may reproduce by producing motile cells called zoospores (Fig. 4.2). The protoplast of a single cell divides many times by mitosis to produce several zoospores. Each zoospore has 2-4 flagella with which it swims for some time and then settles by its anterior end. It subsequently divides into a lower cell which becomes the holdfast and the upper cell which by further divisions becomes the vegetative filament. Zoospores are produced in other algae also.

Reproduction in Algae

Asexual reproduction in other algae is described below.

Chlamydomonas - Although this is a unicellular motile algae but it produces zoospores. The parent cell divides inside the cell-envelop and each daughter cell develops two flagella each. These zoospores look exactly like 111e parent cell except they are smaller in size. When the zoospores are fully developed the parent cell wall disolves, releasing them free into tile surrounding water (Fig. 4.3 ).

Reproduction in Algae

Sometimes when there is less water outside, zoospores may lose flagella and round up. These non-motile spores are called aplanospores which develop into thick walled Hypnospores. On moist soil when zoospores cannot be released due to lack of free water, they get embedded within a gelatinous material formed from parent cell wall. Such cells do not have flagella but whenever they become flooded with water they develop flagella and swim away in the water. These gelatinous masses containing thousands of non-motile cells are known as Palmella stage of Chlamydomonas. 

Oedogonlum - Zoospore are produced singly in a cell. Each has one nucleus and a crown of flagella at the apex.

Draparnaldiopsis and Ulva  - Many zoospores are produced from a single cell, as in Ulothrix. They have single nucleus and 2-4 flagella.

Ectocurpus - Zoospores are produced in sporangia which are of following two types:
  1. Plurilocular Sporangia; The sporangium is made up of many cells and several biflagellate zoospores are produced (Fig.4.4).
  2. Unilocular Sporangia: The sporangium is made up of one cell which prpduces single biflagellate zoospore (Fig.4.4). 
Reproduction in Algae

Sexual Reproduction 

Sexual reproduction in algae like in other organisms involves the fusion of two cells from opposite sex called gametes, resulting in the formation of a zygote. Some basic features of this method of reproduction are as follows:

gametes are always haploid and may or may not be different in morphology, if both [lie sex cells look alike, they could be male called plus (+) or female called minus (-) mating types or strains. gametes can fuse only when one is plus and the oilier is minus.

Both of them + and - may be produced by a single parent. This is called monoccious or hornothallic condition. When they come from different plus or minus thallus types it is called dioccious or heterothallic condition.

There are three types of gametic fusion (fig! 4.5):
  1. Isogamy: When both the gametes are or the same size and morphology.
  2. An isogamy: The two gametes are distinctly different in size or shape, the larger of the two is minus (female) type. 
  3. Oogamy: The female gamete, egg or ovum is big in size and has no flagella hence it is non-motile. Male gametes and flagellated and highly motile. They are also known as antherozoids, spermatoziods or sperms. 
The male gametes are attracted by the female cells because of special hormones called gamones (a volatile hydrocarbon) produced by them. Fusion of the gametes leads to the formation of a zygote. 1f the conditions are unsuitable for growth. the zygote may develop a thick wall and become a resting zygospore. Gametes being haploid, are produced by mitosis in a haploid thallus. if the thallus is diploid as in Fucus the reproductive cells undergo meiosis or reduction division to form haploid gametes.


In haploid thallus, after the fusion of gametes, the diploid zygote undergoes meiosis during germination. however, in diploid algae a zygote may divide mitotically and give rise to a diploid thallus (Fucus). Both haploid and diploid thallus are found in Ulva. They look very similar in size and shape.

Reproduction And Life Cycle

We have given above (lie basic modes of reproduction in algae. Now we take up some specific algal types to illustrate their life cycle in nature. It is to be noted that the life cycle of an alga is very much controlled by environmental factors like temperature, light, seasons, and availability of nutrients, and also salinity, wave action and periodicity of tides in the case of marine forms. Observations made by people during different times from various geographical locations and sometimes experimentally studied under controlled conditions, give us fairly comprehensive if not a complete picture of the life cycle of an alga.

Chlamydomonas 

Sexual reproduction in this alga shows all the three different types depending oit Ihe species (Fig. 4.6). isogamy is found in C.moewusii, C.reinhandii. C.gynogama and C. media Isogamy is of two types:

Reproduction And Life Cycle

in clonal populations (cells obtained by the repeated divisions of a single parent cell) fusion may lake place below gametes which are homothallic or in self compatible strains. For example, fusion occurs between any two cells of C.gynogama and C.media.

in C.moewusii and C.reinhan III fusion of gametes can lake place only when they come from two different unrelated (heterothallic, self-incompatible) strains.

in many isogamous species the parent cell may divide to produce 16 to 64 biflagellate gametes while in sonic the adult cells themselves may directly behave as gametes and fuse.

Anisogamous form of gametic fusion is Found in C.bramii. A female cell divides and produces Four large cells. Each of these cells have two flagella but are less active. 11w male cells are about 8 in number but smaller in size.

Oogamy is the advanced type of sexual reproduction found in C. coccifera. A parent cell discards its flagella and directly becomes a non-motile egg or ovum. While male parent cell by repeated divisions produces sixteen male gametes. These are biflagellate and highly motile.

The process of gametic attraction, fusion and related phenomena have been studied in some detail iii the laboratory. Under proper light condition and carbon dioxide concentration, production of gametes can be initiated by nitrogen starvation. The formation of male or female gametes even in the case of isogamy) is attributed to the varying concentration of gamones produced by them. the attraction between gametes was found due to the presence glycosidic mannose at the tips of the flagella of one strain which in a complementary way binds with the substance present in the flagella of the gamete of the opposite strain. Once this sticking of the flagella of plus and minus gametes takes place, flagella twist about each other bringing the anterior ends of the gametes close. this is followed by cellular and nuclear fusion.

The zygote secretes a thick ‘vail and accumulates large amount of food materials like starch, lipids and orange- red pigments. It is now known as zygospore which remains dormant till the environmental conditions are favorable for its germination.

It has been shown that during germination of zygospore meiosis takes place followed by mitosis resulting in haplold Chamydomonas cells.

Life Cycle 

Chamydomonas is unicellular. Haploid and reproduces asexually many times by forming zoospores. Under tin favorable environmental conditions it produces gametes which fuse to form diploid zygospores. During germination reduction division takes place, and haploid cells arc formed (Fig. 4.7).

Life Cycle of Chamydomonas

Chlamydomonas is of great interest biologists. Its study has brought to light several interesting features of biological importance, some of which are listed below:
  1. Presence of DNA in the chloroplasts of the alga. 
  2. Presence of cytoplasmic genes. 
  3. Production of genetic mutations— affecting nutrition, photosynthesis and production of mutants without flagella or cell wall.
  4. Discovery of gamones and their role in sexual reproduction. 
  5. Presence of isogamy, anisogamy and oogamy in a single genus. 
  6. Control of reproduction by environmental conditions. 

Ulothrix 

Sexual reproduction lakes place by means of isogamous. Biflagellate gametes. Fusion takes place only between plus and minus mating types. The gametes are from different filaments (heterothallic). The zygote develops a thick wall and remains dormant till the conditions are favorable For germination. When conditions become favorable meiosis takes place and 4-16 haploid zoospores are produced which settle down and give rise w vegetative filaments (Fig. 4.8).

It has been found that Ulothrix produces gametes when grown under long day conditions while short day conditions initiate the formation of zoospores.

Life Cycle 

Look at Fig 4.8 showing the life cycle of Ulothrix.
Which is the diploid stage of the algae? The thallus of Ulothrix is haploid and the diploid stage is represented by the zygote only. We would like to draw your attention to the fact that in some species (Uspeciosa, U.flcca and in U.implexa) the zygote develops into an independent, unicellular lhallus which is diploid in nature. It produces zoospores asexually by meiosis. The zoospores develop into haploid filaments.

Thus in Ulothrix two types of life cycles can be distinguished:

Haplobiontic:

The thallus is haploid and only the zygote is diploid e.g. U.zonata?

Diplobiontic: 

In diplobiontic cycle, the alga consists of a haploid thallus that produces gametes and a diploid unicellular stalked thallus which produces zoospores after meiotic division. The two generations - haploid and diploid, alternate with each other. (Alternation of Generations). Because the two thalli are very different in size and morphology it is known as heteromorphic, diplobiontic life cycle.
Life Cycle of Ulothrix

Ulva 

The life cycle of (lira is shown in Fig. 4.9. Note the thalli sporophyte and gametophyte. Both are morphologically alike. However, the gametophyte is haploid (n) whereas the sporophyte is diploid (2n). The haploid gametophyte produces gametes and [lie diploid sporophyte produces after meiosis zoospores that germinate to form haploid gametophytes.

Life Cycle of Ulva

The gametophytes of Ulva produce gametes which are isogamous or anisogamous. After fusion the zygote is formed which develops into a diploid sporophyte. The life cycle of Ulva is described as isomorphic, diplobiontic type.

Laminaria 

Sexual reproduction in Laminaria is oogamous type.

The nature diploid thalli of sporophytes produce sori or unilocular sporangia on the surface of the lamina. Each sporangium divides by meiosis to give rise to 32 biflagellate zoospores which germinate to form male and female gametophytes (Fig. 4.10).

Life Cycle of Laminaria

The gametophytes of both sexes are microscopic with a few branches and their fertility is controlled by environmental conditions. Any cell of the female gametophyte can develop into an oogonium, the contents of which form a single egg. The egg protrudes out when mature but remains attached to the mouth of the empty oogonial cell.

Antheridia are produced singly as lateral outgrowths of the male gametophyte. Only one sperm is produced from each antheridium, which is pear shaped and has two flagella of unequal length.

After fertilization the zygote immediately divides mitotically without any resting period and develops into a sporophyte( Fig, 4.10).

Life Cycle in Laminaria

there is a distinct alteration of haploid gametophyte and a dominant diploid sporophyte. Reduction division takes place in the sporangia of sporophyte before the formation of zoospores, which germinate to form the male and female gametophytes. The two dissimilar generations - one simple filamentous gametophyte and the other highly differentiated, complex multicellular thallus - alternate with each other - hence the life cycle is termed heteromorphic alternation of generations.

Fucus 

Fucus has advanced type of reproductive structures, termed as receptales, which are swollen at the tips of branches (Fig. 4.11 A).

Distributed over the surface of each receptacle are small pores, known as ostiolos which lead into the cavities, called conceptacles (Fig. 4.11 B). Each conceptacle may produce only eggs, only sperms or as in some cases both. A thallus may be unisexual - either having male receptacle or only female ones.

structures of Fucus

At the base, inside the conceptacle is a fertile layer of cells which develops into oogonia (Fig. 4.12A and 4.14A). Each oogonium has basal stalk cell and an upper cell which undergoes reduction division and produces eight haploid eggs (4.12 C and D). These are liberated in the conceptacle (Fig. 412E). Some of the cells inside the conceptacle produce unbranched multicellular hairs called paraphyses which emerge out of the ostiole as tufts. Antheridia are produced on branched paraphyses inside (lie conceptacle (Fig 4. I 2B and 4.1 4B). Each antheridium is like a unitocular sporangium which divides melodically and [lien by further divisions produces 64 haploid sperms. The biflagellate sperm has a longer flagellum pointing backwards and a shorter one projecting cowards the Front. It has a single chloroplast and a prominent orange eye spot (Fig.4.l4A).

Fucus

The release of the gametes is connected with the sea tides. At low tide, Fucus Fronds shrink due to loss of water, and when such fronds are exposed to an oncoming tide, the eggs and sperms are released into the surrounding sea water.

The eggs of Fucus are known to attract sperms (Fig.4.13 A and B) by secreting a gamone. Immediately after fertilization a wall is secreted around the zygote. It has been shown that un Fertilized eggs can develop into germlings parthenogenetically if treated with dilute acetic acid.

The diploid zygote germinates b)’ producing a rhizoidal outgrowth on one side. It is later cut by wall formation to form a lower rizoidal cell and apical cell ( Fig. 4. 13C) which by Further divisions (Fig. 4.13 D and E) gives rise to the Fucus fronds.

Fucus

Life Cycle 

Fucus plants are diploid and the haploid stage is represented by gametes only. The life cycle of Fucus is described as diplontic life cycle.

the four besic types of life eyles described above are summerised in Fig-4.15
four besic types of life eyles in alagae
When the dominant phase is the haploid gametophyte the life cycle is termed as hapontic life cycle. in this cycle diploid state or saprophyte is represented by zygote which produces spores by meiosis that develop into gametophytes.

in diplontic cycle the main or dominant phase is the diploid sporophyte The zygote directly germinates into a sporophyte. Later meiosis takes place producing haploid ganietes that fuse to form the zygote. In the diplontic algae it is to be noted (lint 110 free living haploid thalii are Found.

When both the gametophyte and the sporophyte are equally developed and look morphologically similar, we have isomorphic alternation of generations. However, if gametophyte is underdeveloped compared to the sporophyte the life cycle is known as heteromorphic alternation.

Origin And Evolution of Sex

Origin of Sex 

The basic feature of sex is the fusion of two cells— gametes which are of two types, male (plus) and female (minus).

What factors lead (o the fusion of cells as such is not clear but fusion brings about mixing of two different (but related) genomes together, one probably compensating for the deficiencies of (Ile other. This particular feature is a biological advantage for the survival of the species. it is no wonder that almost all organisms developed r sexual method of reproduction.

Origin of Sex in Ulothrix

Even in the case of prokaryotic cyanobacteria, and also in other bacteria different mechanisms were discovered (para-sexual mechanisms) whose essential feature is exchange or mixing of genes or complete genoines between a donor and a recipient.

In all eukaryotic algae as in all plants and animals fusion of cells is the method by which sexual reproduction takes place. The question is how this fusion of cells originated and further how this phenomenon was preserved and refined during evolution. The study of the sexual processes found in the present day algae provide some answers to the above questions.

In lower algae like Chlamydomonas, Ulothrix and others asexual reproduction takes place through motile swarmer called zoospores. in Ulothrix depending on the number of divisions that a cell undergoes at least two types of zoospores are produced. small micro zoospores and large micro zoospores The micro zoospores often fail to germinate to produce new plants, probably due - to deficiency or low level of some vital substances needed for cell division and growth. However. such swarmer are found to fuse in pairs occasionally and then develop into Ulothrix salient. It appears that macro zoospores are self sufficient and do not require any such fusion.

In many algae one can not make out any difference in structure between a zoospore and a gamete. except for their behavior - a zoospore directly develops into a filament whereas a gamete needs fusion with another gamete for further development. If certain type of zoospores - small micro zoospores can behave like gametes, at times gametes which fail to ruse may behave like zoospores and develop directly into thallus - a phenomenon called parthenogenesis reported to be present in diverse organisms. Such observations indicate that gametes are modified zoospores and gametic fusion originated through accidental fusion of small and weak zoospores. As such fusions in general help by genetic mixing, lo acquire characters useful for biological survival, [he essential features of sex were retained and improved further during evolution.

Evolution of Sex 

Isogamy, fusion of identical gametes seems to be the earliest state of sex. However, the morphologically similar gametes may he different in origin, arising from two different gametic mating types, plus and minus strains (heterotliallic).

The simplest early state appears to be the fusion (not any more accidental but regularised) of morphologically similar gametes, perhaps arising from the same thallus - homothallic isogamy. This is improved further by heterothallic isogamous fusion, ¡n which though gametes looked morphologically similar but with genetically and biochemical differences to encourage fusion of opposite mating types, plus and minus only.

Anisogamy constitutes an intermediary state as it may involve fusion of gametes with distinct size difference. Although both gameties are flagellated, the bigger one may be less active than the smaller male gamete. Further refinement ultimately led to oogamy - which is the most common and the only form of sexual reproduction in higher halloid algae. Oogamy is characterised by big non-motile egg and a small motile spermatozoid. The gametes may be produced in oogonia and antheridia. The oogonia ni ay produce only a few eggs (eight) or as in sonic algae a single egg, while the number of sperms formed is always very large.

Generally, the eggs are liberated into the surrounding water but there is a tendency to retain the egg inside the oogonium itself, where fertilization also takes place. The zygote or oospore may develop further inside the empty oogonium.

It is to be noted that the above account of the origin and evolution of sex is entirely based on the study of reproductive process of various algae. Biologists in recent years discovered that in algae, sex has genetic and biochemical basis. In Chlamydomonas gametes produce a volatile substance that attracts the gametes of the opposite sex. The eggs of Fucus, , Laminaria, Oedogonium and many other algae have been shown to produce species-specific chemicals to attract the spermatozoids. Such chemicals are known by a collective name ‘gamones or pheromones’ or sex hormones.

In algae, several other processes connected with reproduction like gametogenesis, chemotaxis of gametes, adhesion and fusion of gametes of opposite sex - are known to be controlled by pheromones.

No comments:

Post a Comment