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Reproduction
Sexual phenomena occur widely among protozoa, and sexual processes may precede certain phases of asexual reproduction, but embryonic development does not occur; protozoa do not have embryos. The essential features of sexual processes include a reduction division of the chromosome number to half (diploid number to haploid number), the development of sex cells (gametes) or at least gamete nuclei, and usually a fusion of gamete nuclei.
Fission
The cell multiplication process that produces more individuals in protozoa is called fission. The most common type of fission is binary, in which two essentially identical individuals result. When a progeny cell is considerably smaller than the parent and then grows to adult size, the process is called budding. Budding occurs in some ciliates. In multiple fission, division of the cytoplasm (cytokinesis) is preceded by several nuclear divisions, so that a number of individuals are produced almost simultaneously. Multiple fission, or schizogony, is common among the Apicomplexa and some amebas. If the multiple fission is preceded by or associated with union of gametes, it is called sporogony.
The foregoing types of division are accompanied by some form of mitosis. However, this mitosis is often somewhat unlike that found in metazoans. For example, the nuclear membrane often persists through mitosis, and the microtubular spindle may be formed within the nuclear membrane. Centrioles have not been observed in nuclear division of ciliates; the nuclear membrane persists in micronuclear mitosis, with the spindle within the nucleus. The macronucleus of ciliates seems simply to elongate, to constrict, and to divide without any recognizable mitotic phenomena (amitosis).
Sexual Processes
Although all protozoa reproduce asexually, and some are apparently exclusively asexual, the widespread occurrence of sex among protozoa testifies to its importance as a means of genetic recombination. Gamete nuclei, or pronuclei, which fuse in fertilization to restore the diploid number of chromosomes, are usually borne in special gametic cells. When gametes all look alike, they are called isogametes, but most species have two dissimilar types, or anisogametes.
In animals meiosis usually occurs during or just before gamete formation. Such is indeed the case in Ciliophora and some flagellated and amebic groups. However, in other flagellated groups and in Apicomplexa, the first divisions after fertilization are meiotic (zygotic meiosis), and all individuals produced asexually (mitotically) in the life cycle up to the next zygote are haploid. Most protozoa that do not reproduce sexually probably are haploid, although demonstration of ploidy is difficult in the absence of meiosis. In some amebas (foraminiferans) haploid and diploid generations alternate (intermediary meiosis), a phenomenon widespread among plants.
Fertilization of an individual gamete by another is syngamy, but some sexual phenomena in protozoa do not involve syngamy. Examples are autogamy, in which gametic nuclei arise by meiosis and fuse to form a zygote within the same organism that produced them, and conjugation, in which an exchange of gametic nuclei occurs between paired organisms (conjugants). We describe conjugation further in the discussion of Paramecium.
Encystment and Excystment
Although separated from their external environment only by their delicate plasma membrane, unicellular forms are amazingly successful in habitats frequently subjected to extremely harsh conditions. Survival under harsh conditions surely is related to the ability to form cysts; dormant forms marked by possession of resistant external coverings and a complete shutdown of metabolic machinery. Cyst formation is also important to many parasitic forms that must survive a harsh environment between hosts. However, some parasites do not form cysts,apparently depending on direct transfer from one host to another.
Reproductive phases such as fission, budding, and syngamy may occur in cysts of some species. Encystment has not been found in Paramecium, and it is rare or absent in marine forms. The conditions stimulating encystment are incompletely understood, although in some cases cyst formation is cyclic, occurring at a certain stage in the life cycle. In most free-living forms, adverse environmental change favors encystment. Such conditions may include food deficiency, desiccation, increased environmental osmotic pressure, decreased oxygen concentration, or change in pH or temperature.
During encystment a number of organelles, such as cilia or flagella, are resorbed, and the Golgi apparatus secretes cystwall material, which is carried to the surface in vesicles and extruded.
Although the exact stimulus for excystation (escape from cysts) is usually unknown, a return of favorable conditions initiates excystment for those protozoa in which the cysts are a resistant stage. In parasitic forms the excystment stimulus may be more specific, requiring conditions similar to those found in the host.
MAJOR PROTOZOAN TAXA
The evolution of a eukaryotic cell was followed by diversification into many clades, some of which contain both unicellular and multicellular forms. Clades of this type include the Opisthokonta, Viridiplantae, and the red algal clade, traditionally the phylum Rhodophyta. Rhodophyta is considered a plant clade because its members have plastids, are not heterotrophic, and lack flagellated stages (no motile sperm) in the life cycle. The clades we discuss further contain some members traditionally considered protozoans, so Viridiplantae and Opisthokonta are included, but Rhodophyta is not.
The Opisthokonta is a clade characterized by a combination of flattened mitochondrial cristae and one posterior flagellum on flagellated cells, if such cells exist. Recent protein sequence comparisons among taxa have also identified a short sequence of amino acids from one protein (elongation factor 1-alpha) that is shared by both unicellular and multicellular clade members.
The Opisthokonta contains metazoans and fungi as well as some unicellular taxa traditionally considered protozoans. The best-known unicells in this group are the microsporidians and choanoflagellates. Microsporidians are intracellular parasites now recognized as specialized fungi. Choanoflagellates are solitary or colonial protozoans considered the most likely sister taxon to the metazoans. They are used to test hypotheses of how multicellular animals arose, specifi-cally to identify features of the most recent common ancestor of animals and their closest unicellular relatives. We discuss them with sponges because of the strong resemblance between choanoflagellate cells and sponge choanocytes.
The Opisthokonta also contains less well-known unicells such as ichthyosporeans (animal parasites sometimes called DRIPs), nucleariid amebas, corallochytreans, and ministeriid amebas.
Useful External Links
- Reproduction in Protozoa (With Diagram) | Zoology by Zoology Notes
- Asexual reproduction in protozoa and invertebrates by ScienceDirect
- Sexual mode of reproduction in protozoa by Vedantu
