ORIGEN Y EVOLUCION DE LA FAMILIA

In the early days of biology, all organisms were classified as either plants or animals. In year 1969, Robert H. Whittaker, an American taxonomist, described a five-kingdom classification. The five kingdoms are Monera (prokaryotes), Protista, Fungi, Plantae and Animalia. His classification is based on two main criteria - the level of organization of the organisms and their method of nutrition. He recognized two levels of organization - prokaryotes (which are without nucleus) and eukaryotes (which are with nucleus). Eukaryotes are further subdivided into unicellular and multicellular eukaryotes. The methods of nutrition were: heterotrophic (which are further subdivided into ingestive and absorptive) and autophototrophic.

Monera : Prokaryotic cellular organization.

Protista : Unicellular eukaryotes.

Fungi : Multicellular eukaryotes which feed heterotrophically by absorption. Plantae : Multicellular eukaryotes which are photoautotrophic.

Animalia : Multicellular eukaryotes which feed heterotrophically by ingestion.

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creating a level of classification larger than a kingdom – a new category called a domain. The two domains of prokaryotes are Bacteria and Archaea. The domain Eukarya encompasses all of the kingdoms of eukaryotes. The

three-domain system of biological classification was introduced by Carl Woese on the basis of differences in the

signature sequences of 16S/18S rRNA genes of eubacteria, archaebacteria and eukaryotes.

History of taxonomic concepts

Linnaeus, 1735 2 Kingdoms Animalia and Vegetabilia Haeckel, 1866 3 Kingdoms Protista, Plantae and Animalia

Chatton, 1937 2 Empires Prokaryota and Eukaryota

Copeland, 1956 4 Kingdoms Monera (prokaryotes), Protista, Plantae and Animalia Whittaker, 1969 5 Kingdoms Monera, Fungi, Protista, Plantae and Animalia

Woese et al, 1977 6 Kingdoms Eubacteria, Archaea, Protista, Fungi, Plantae and Animalia Woese and Fox, 1999 3 Domain systems Eubacteria, Archaea and Eukaryotes

6.3

Protists

The German zoologist Ernst Haeckel first used the term protista. Protists are mostly unicellular (some are colonial and multicellular)eukaryotic organisms. Most eukaryotes are single-celled organisms. Protists are difficult to characterize because of the great diversity of the kingdom. These organisms vary in body form, nutrition and reproduction. Protists are capable of sexual, as well as asexual reproduction. They can be free-living, or may live symbiotically with another organism. The symbiosis can be mutualistic, where both partners benefit, or parasitic, where the protist uses its host as a source of food or shelter while providing no advantage to the other organism. Protists are more nutritionally diverse than other eukaryote groups. Some protists are photoautotrophs. Some are heterotrophs, absorbing organic molecules or ingesting larger food particles. Still others, called mixotrophs, combine photosynthesis and heterotrophic nutrition.

Despite the great diversity evident in this kingdom, scientists have been able to classify the protists into several groups. The protists can be classified into one of three main categories, animal-like, plant-like and fungus-like. Grouping into one of the three categories is based on an organism’s mode of reproduction, method of nutrition and motility. The animal-like protists are known as the protozoa, the plant-like photosynthetic protists are the algae, and the fungus-like protists are the slime molds and water molds.

6.3.1

Protozoan protists

The protozoa are all unicellular heterotrophs, mostly free living. They obtain their nutrition by ingesting other organisms or dead organic materials. The word ‘protozoa’ comes from the Latin word for ‘first animals’.

Features of protozoa:

• Unicellular (single celled) organisms. • Aquatic, free-living or parasitic organisms. • Nutrition is heterotrophic.

• Locomotion with the help of pseudopodia, cilia or flagella. The protozoans are grouped into various phyla based on their modes of locomotion. They may use cilia, flagella or pseudopodia. Some protozoans are non motile. The protozoans that use pseudopodia to move are known as amoeboid protozoans, those that use flagella are called flagellated protozoans, those that use cilia are known as the ciliated protozoans and those that do not move are called the sporozoans.

• Both asexual and sexual reproduction occur among the protozoa. One of the simplest and most common forms of asexual reproduction is binary fission, a mitotic process. Other forms of asexual reproduction are budding and multiple fission (or schizogony). In multiple fission, a large number of daughter cells form from the division of a single protozoan.

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A generalized life cycle of bryophytes

During sexual life cycle, sporophyte produces spores in sporangium. Haploid spores formed by meiosis begins the gametophytic generation. The spore germinates to form a gametophyte, which is more diverse in bryophytes than in any other group of plants. The gametophyte bears antheridia and archegonia. Antheridia produce sperms, and archegonia produce eggs. Sexual reproduction in bryophytes requires free water, because the sperm must swim to the egg. Sperms released from an antheridium do not swim randomly; rather, they are attracted by a gradient of a still-unidentified chemotactic substance(s) produced by an archegonium. A sperm fertilizes an egg and forms a diploid zygote, thus beginning the sporophytic generation of the life cycle. Bryophytes have heteromorphic alternation of generation; that is, the sporophyte and gametophyte are distinctly different.

Mature sporangium Spores Sperms (Motile) Gametophyte Gametophyte Sporophyte Archegonium Antheridium Archegonium Fertilization Embryo Capsule

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Figure 6.9 Sexual life cycle in Funeria. The spore germinates and produces a protonema (not shown in the figure) and the leafy stage of the gametophyte develops from the protonemal stage. The male and female sex organs, the antheridia and the archegonia respectively, are produced on the gametophytic plant. Haploid sperm are released from the antheridia and fertilize with a haploid egg present in an archegonium. The zygote develops into the diploid sporophyte. Typically sporophyte is differentiated into a foot, seta and spore forming capsule.

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