Types of Reproduction:
Reproductive methods found in different groups of algae can be generally categorized into three groups: vegetative, asexual and sexual methods
1) Vegetative Reproduction:
The most common kind of reproduction in algae is by binary fission. In unicellular prokaryotic algae such as Anacysts is only technique of reproduction found in nature. In filamentous and multi-cellular forms, algae may get broken accidently in small pieces, -each developing into the new one. Methods of propagation are called as vegetative reproduction.
2) Asexual Reproduction:
When vegetative reproduction occurs through specialized cells (other than sex cells), it is explained as asexual reproduction.
Anabaena and Nostoc:
Cells collect food material; develop thick walls to become spores or akinetes. Akinetes can endure dryness (lack of water) and high temperature for long time, but when conditions are appropriate they germinate to form new filaments.
Filament algae (such as Ulothrix) may reproduce by producing motile cells known as zoospores. Protoplast of the single cell divides several times by mitosis to make many zoospores. Every zoospore contains 2-4 flagella with which it swims for some time and then settles by anterior end. It subsequently separates into lower cell that becomes holdfast and upper cell which by further divisions becomes vegetative filament. Zoospores are made in other algae also.
Though this is unicellular motile algae it produces zoospores. Parent cell separates inside cell-envelope and every daughter cell develops 2 flagella each. These zoospores look exactly like parent cell except they are smaller in size. When zoospores are completely developed parent cell wall dissolves releasing them free in surrounding eater. At times when there is less water outside, zoospores may lose flagella and round up. The non-motile spores are known as aplanospores that develop in thick walled hypnospores.
On moist soil when zoospores cannot be released because of lack of free water, they get embedded within gelatinous material formed from parent cell wall. These cells don't have flagella but whenever they turn into flooded with water they grow flagella and swim away in water. The gelatinous masses having thousands of non-motile cells are called as palmella phase of Chlamydomonas.
Zoospores are made singly in cell. Everyone has nucleus and crown of flagella at apex.
Draparnaldiopsis and Ulva:
Several zoospores are made from single cell, as in Ulothrix. They contain single nucleus and 2-4 flagella.
Zoospores are produced in sporangia that are of following 2 kinds:
i) Plurilocular Sporangia: Sporangium is composed of several cells and several biflagellate zoospores are produced.
ii) Unilocular Sporangia: Sporangium is composed of one cell that produces single biflagellae zoospore
3) Sexual Reproduction:
Sexual reproduction in algae like in other organisms engages fusion of 2 cells from opposite sex call gametes, resulting in formation of zygote. Few basic features of this method of reproduction are as follows:
Gametes are always haploid and may not be dissimilar in morphology. If both sex cells look alike, they could be male called plus (+) or female called minus (-) mating kinds or strains. Gametes can fuse only when one is plus and other is minus. Both of them + and - may be made by single parent. This is known as monoecious or homothallic. When they come from dissimilar plus or minus thallus types it is known as dioecious or heterothallic condition. There are 3 kinds of gametic fusion.
i) Isogamy: When both gametes are of same size morphology
ii) Anisogamy: Two gametes are definitely different in size or shape, larger of two is minus (female) type.
iii) Oegamy: Female gamete, egg or ovum is big in size and contain no flagella therefore it is non-motile. Male gametes are flagellated and highly motile. They are also called as antherozoids, spermatozoids or sperms.
Male gametes are attracted by female cells due to special hormones known as gamones (volatile hydrocarbon) made by them. Fusion of gametes leads to formation of zygote. If conditions are inappropriate for development, zygote may develop the thick wall and become the resting zygospore. Gametes being haploid are made by mitosis in haploid thallus. If thallus is diploid as in focus reproductive cells experience meiosis or reduction division to create haploid gamets. In haploid thallus, after fusion of gametes, diploid zygote undergoes meiosis in germination. Though, in diploid algae zygote may divide mitotically and give rise to diploid thallus (focus). Both haploid and diploid thallus are discovered in Ulva. They look very similar in size and shape.
Origin and Evolution of Sex:
Origin of Sex:
Basic feature of sex is fusion of 2 cells - gametes that are of 2 kinds male (plus) and female (minus). What factors lead to fusion of cells as such is not clear but fusion brings about mixing of 2 different (but related) genomes one most likely compensating for deficiencies of other. Particular feature is biological benefit for survival of species. It is no wonder that almost every organisms evolved sexual method of reproduction.
Even in case of prokaryotic cyanobacteria and also in other bacteria various mechanisms were found (para-sexual mechanisms) whose necessary feature is exchange or mixing of genes or complete genomes between donor and recipient. In every eukaryotic algae as in all plants and animals, fusion of cells is method by which sexual reproduction occurs. In lower algae such as Chlamydomonas, Ulothrix and other asexual reproduction occurs through motile swarmers known as zoospores. In Ulothrix depending on number of divisions which cell undergoes, at least 2 kinds of zoospore are produced, small microzoospores and large marozoospores microzoospores often fail to germinate to produce novel plants, perhaps because of lack or low level of few vital substances required for cell division and development. Though such swarmers are found to fuse in pair irregularly and then grow into Ulothrix filaments. It seems that macrozoospores are self plenty and don't need such fusion. In several algae one can't distinguish any difference in structure between the zoospore and gamete, except for the behavior - a zoospore directly grows in filament while gamete requires fusion with another gamete for further growth. If certain kind of zoospores - small microzoospores can act like gametes, sometimes gametes that fail to fuse may act like zoospores and grow directly in thallus -phenomenon known as parthenogenesis reported to be present in different organisms. Such observations point to those gametes is altered zoospores. As such fusions generally assist by genetic mixing, to get characters helpful for biological survival, necessary characteristic of sex were retained and enhanced further in evolution.
Evolution of Sex:
Isogamy, fusion of identical gametes appears to be earliest state of sex. Though, morphologically similar gametes may be dissimilar in origin, arising from 2 different gametic mating types, plus and minus strains (heterothallic). Simplest early state seems to be fusion (not any more accidental but regularized) of morphologically similar gametes, maybe arising from same thallus - homothallic isogamy. This is enhanced further by heterothallic isogamous fusion, in which although gametes looked morphologically like they have genetical and biochemical differences to support fusion of opposite mating types (plus and minus only). Anisogamy comprises the intermediary state as it may engaged fusion of gametes with different size difference. Though both gametes are flagellated, bigger one may be less active than smaller male gamete. Additional refinement finally led to oogamy - that is most common and only form of sexual reproduction in higher thalloid algae. Oogamy is described by big non-motile egg and small motile spermatozoid. Gametes may be made in oogonia and antheridia. Oogonia may make only few eggs (eight) or as in few algae a single egg, while number of sperms created is always very large. Usually, eggs are liberated into surrounding water but there is tendency to retain egg inside oogonium itself, where fertilization also occurs. Zygote or oospore may grow further inside empty oogonium.
It is to be noted that above account of origin and evolution of sex is completely based on study of reproductive procedure of different algae. Biologists in current years found that in algae, sex has genetic and biochemical basis. In Chlamydomonas gametes make volatile substance which attracts gametes of opposite sex. Eggs of Laminaria, fucus,Oedogonium and several other algae have been illustrated to make species specific chemicals to attract spermatozoids. These chemicals are known by collective name 'gamones or pheromones or sex hormones. In algae, many other procedures related with reproduction such as gametogenesis, chemotaxis of gametes, adhesion and fusion of gametes of opposite sex are known to be handled by pheromones.
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