PARAMECIA: MORPHOLOGY, FEEDING AND CLASSIFICATION - BIOLOGY - 2025

The paramecium are organisms belonging to the genus Paramecium. They are protists and have a large number of cilia - appendages that facilitate locomotion of the individual. They are usually considered "model species", therefore they have been widely studied.

There is abundant knowledge about its biology, ultrastructure, physiology, and genetics. The species of this genus are common inhabitants in freshwater environments and ponds with decomposing organic matter. Their feeding is heterotrophic.

Morphology

Organisms belonging to the phylum Ciliophora are characterized by having cilia and two types of nuclei, distinguishable from each other. Paramecium has one macronucleus and two or more micronuclei.

They are quite complex organisms both in their structure and in their function. Within the group there are free-living individuals, commensals and parasites. Specifically, the paramecia species are free-living.

Although the different species of paramecia vary among themselves, their average length is 150 µm and width 50 µm. The variation of the size depends mainly on the availability of food and the moment of the life cycle in which it is found.

Vacuoles

Paramecia have two contractile vacuoles located on the aboral surface. These vacuoles are located at two ends of the body and drain their fluids to the outside.

Waste that is not digested can be discharged through the anal pore, which is ventral and subterminal. There are specialized structures for the consumption of matter (food); These openings are called a cytostome.

The cytoplasm contains numerous mitochondria. In some colonies of Paramecium found in nature there are also a significant number of endosymbionts. Also, there are ribosomes.

The nuclei are one of the most relevant characteristics of the Paramecium. The macronucleus is active (50-60 µm long and 20-30 µm wide), in contrast to the micronuclei (3 µm in diameter), which are not.

Feeding

They are heterotrophic organisms. Among its most frequent prey are algae and bacteria. In some cases they can consume other protozoa.

Near the feeding cleft, the paramecia have an organ with a large number of cilia in it. This structure helps to create a current that favors the entry of food particles into the mouth of the single-celled organism.

Taxonomic classification

Paramecia belong to the phylum Ciliophora and the class Oligohymenophorea. As the name of the group indicates, they are ciliated organisms.

Regarding the internal relations of gender, in 1921 the researcher Woodruff divided gender into two groups based on the shape of each organism. Slipper-shaped individuals belong to the aurelia group, and those who resemble a cigarette belong to the bursaria group.

Later, in 1969 and 1992, Jankowski proposed a division into three groups called putrinum, woodruffi and aurelia. According to him, the taxonomic rank of this classification was of subgenera.

To propose this classification, the morphology, size and shape of the cell, particularities of the nucleus, among others, were used as essential characteristics.

The taxonomic validity of the groups described above has been doubtful and questioned. A recent study aimed to clarify these conflicts and, through the use of molecular tools, they sought to resolve the phylogenetic relationships of the group.

The small subunit of rRNA revealed that the bursaria group does not form a monophyletic group. In contrast, the species assigned to aurelia are related and the phylogeny supports the existence of this group as monophyletic.

Distribution

Its distribution is worldwide. To explain the wide distribution range of the species, several hypotheses have been proposed.

It is speculated that dispersal occurs through water to insects, birds, and other animals with long-distance migration patterns, including man.

It is also possible that the oldest species of paramecia were distributed worldwide before the separation of the continents.

This hypothesis does not require extensive migration. Recent evidence supports the first hypothesis, which requires a recent and continuous migration.

Reproduction

Binary fission

They can reproduce asexually by a mechanism called fission. Paramecium grows gradually when it has access to food.

When it reaches the maximum size it divides into two halves, which gives rise to two identical individuals. The process occurs in an interval of about five hours at the optimum temperature of 27 ° C.

During this process the two micronuclei undergo a process of mitosis. The macronucleus does not divide mitotically.

Conjugation

This process is considered as a source of sexual recombination of hereditary elements. Conjugation involves the pairing of two cells that undergo a series of sexual processes within a couple of hours, physically joined by their oral surfaces. The macro nucleus fragments.

Autogamy

In autogamy you don't need a second individual. In contrast, the nuclei of the same organism come together, reminiscent of a traditional conjugation.

The nuclei undergo a meiotic process, of which only one nucleus remains; the rest are destroyed. The single resulting nucleus divides by mitosis. The new haploid nuclei unite and give rise to a new diploid nucleus.

If a heterozygous individual (Aa) divides by autogamy, some of his descendants will be homozygous dominant (AA) and others will be homozygous recessive (aa).

Cytogamy

Cytogamy is a hybrid process between conjugation and autogamy. The union of two organisms occurs, as occurs in conjugation, but the exchange of genetic material does not occur. The union of nuclei occurs between the nuclei of the same individual (as occurs in autogamy).

Hemixis

It is a process of fragmentation and division of the macronucleus without activity of the rest of the micronuclei. Several authors consider that the species that undergo this process are abnormal or pathological individuals. They generally degenerate until they die.

This process cannot be considered a normal step in the individual's life cycle. On the contrary, it must be classified as an aberrant state.

Macronuclear regeneration

The disintegrated products of the old macronuclei carry out a regeneration process. In short, old nuclei give rise to new nuclei, possibly by a non-mitotic process.

The fragmented chunks are segregated equally among the offspring individuals formed by fission.

References

1. Beale, G., & Preer Jr, JR (2008). Paramecium: genetics and epigenetics. CRC Press.
2. Marshall, AJ, & Williams, WD (1985). Zoology. Invertebrates (Vol. 1). I reversed.
3. Strüder-Kypke, MC, Wright, ADG, Fokin, SI, & Lynn, DH (2000). Phylogenetic relationships of the genus Paramecium inferred from small subunit rRNA gene sequences. Molecular phylogenetics and evolution, 14 (1), 122-130.
4. Wichterman, R. (2012). The biology of Paramecium. Springer Science & Business Media.
5. Johri, P., Krenek, S., Marinov, GK, Doak, TG, Berendonk, TU, & Lynch, M. (2017). Population genomics of paramecium species. Molecular biology and evolution, 34 (5), 1194-1216.