The study of RNA sequences offers new insights into the evolution of diversity among the Eucarya.
One-celled eucaryotes (formerly called Protista) possess
a diversity of locomotor adaptations: pseudopods for amoeboid locomotion,
flagella, and cilia. Some are non-motile.
Photosynthesizing eucaryotes that do not develop from
multicellular embryos are called Algae; a diversity of photosynthetic pigments exists among
brown algae (Phaeophyta), green algae (Chlorophyta), red algae
(Rhodophyta), and several groups of microscopic algae.
The Eucarya are now classified into about five major groups: Unikonta (including
amoebozoans, animals, and fungi), Excavata (Diplomonads, etc.), Rhizaria, Chromalveolata (a large
and possibly heterogeneous group including ciliates, brown algae, and many others),
and Archaeplastida (a group of photosynthetic organisms including red algae, green algae, and plants).
Different locomotor adaptations have arisen among the Eucarya:
- Amoeboid locomotion using protoplasmic extensions called pseudopods.
Amoebozoans, one of the largest and most diverse group of one-celled eycaryotes, use
broad, lobe-like pseudopods; their body changes shape continually as it moves.
Another group, the Rhizaria, use thin, needle-like pseudopods. Most of these organisms
are predators, engulfing their prey by phagocytosis.
- Flagella: Flagellated cells move by beating a long, whip-like flagellum
with a "9 + 2" grouping of microtubules, or sometimes more than one flagellum.
A few species show both flagellar and amoeboid locomotion.
- Cilia: Some cells are covered with thousands of hairlike cilia whose
rhythmic beating controls both locomotion and feeding. Cilia have the same
"9 + 2" internal structure as flagella.
Chloroplasts and other plastids are thought to have originated by endosymbiotic capture of
photosynthetic Cyanobacteria containing chlorophyll a.
- Brown algae have chloroplasts that also contain chlorophyll c and certain yellowish
pigments (carotenes and xanthophylls) not found in any Archaeplastida, leading to the hypothesis that
these chloroplasts were captured independently. Dinoflagellates and a few other unicellular algae
have similar pigments (and similar flagella) and are thought to be related to brown algae for this reason.
- Red algae have chloroplasts that also contain chlorophyll d; they also have certain
phycobilin and phycoerythrin pigments not found in any other eucaryotes (but found in Cyanobacteria).
- Green algae and plants have chloroplasts that also contain chlorophyll b,
xanthophylls, and α- and β-carotenes. Cell walls contain cellulose and pectin.
Starch is the main storage product.
- A few eucaryotes have plastids that no longer contain chlorophyll. Some "chromoplasts" contain
other pigments; some "leucoplasts" contain stored food (starch).
Unicellular protists
Photosynthetic protists ("algae")
CLASSIFICATION OF THE EUCARYA
Starting in 2004, whole genome sequences became available for many eucaryotes.
As a result, the classification of Eucarya changed dramatically, and is now based on similarities
in RNA sequences. The following major groups (subdomains) of Eucarya are now recognized:
- UNIKONTA:
Most organisms in this group are motile, either by amoeboid locomotion or by a
single posterior (rear-facing) flagellum (or both). This large group includes the
Amoebozoa, the slime molds, the Fungi (Mycota), the choanoflagellates, and the
Animal Kingdom.
- EXCAVATA:
This small group includes the Diplomonads such as the parasite Giardia.
Two smaller groups, the Parabasalids, and the photosynthetic Euglenoids, may also
belong here. Many experts consider this group close to the ancestry of all Eucarya
(possibly excluding Unikonta).
- RHIZARIA:
These one-celled predators all secrete a hard shell enclosing a cell body from which
radiate numerous thin, needle-like pseudopods that are used to trap and engulf their prey.
- Radiolaria: shell is usually a beautiful sphere made of silica (SiO2).
- Foraminifera: shell is coiled, perforated with many tiny holes, and
usually made of calcium carbonate (CaCO3).
- CHROMALVEOLATA:
This large group is often subdivided into two subgroups:
- ALVEOLATA, with bubble-like spaces (alveoli) just beneath the plasma membrane.
Included here are:
- Ciliata, whose entire cell surface is
clothed in cilia. Paramecium is a familiar example.
All ciliates have two nuclei (macronucleus, micronucleus) and a unique type of
sexual conjugation in which two cells exchange micronuclei.
- Apicomplexa, a group of parasites that use a sharp apical complex
to penetrate host cells. Reproduction by spores. The malaria parasite Plasmodium belongs to this group.
- STRAMENOPILES,
characterized by two unequal flagella (one simple, one with feathery branches) and by
chloroplasts containing chlorophylls a and c and several unique xanthins and
other pigments not found in the Archaeplastida. This group is hypothesized to have originated
by a symbiotic capture of plastids, independently of the Archaeplastida.
Included are:
- Brown algae (Phaeophyta), often large, structurally complex; ecologically dominant
in temperate and colder marine waters.
- Dinoflagellates, unicellular, marine and planktonic,
with two flagella at right angles to one another.
Mitosis unusual: no histones, no centrioles, no spindle fibers; nucleolus and nuclear envelope
remain visible throughout mitosis.
- Diatoms, especially abundant in plankton; the major primary producers
in most marine (and several freshwater) ecosystems.
- ARCHAEPLASTIDA:
These organisms are all photosynthetic and all have plastids containing chlorophyll a
and either chlorophyll b or chlorophyll d, but never chlorophyll c.
Included here are the following:
- Red algae (Rhodophyta), containing chlorophylls a and d,
plus other pigments resembling those of Cyanobacteria. No "9 + 2" organelles
(centrioles or flagella); no motile cells, not even gametes. Unique storage
products include "floridean starch" in cell walls. Ecologically dominant
in tropical marine waters.
- Green algae (Chlorophyta), containing chlorophylls a and b,
xanthophylls, and α- and β-carotenes. Cellulose, pectins, and starch are also present.
Ecologically dominant in freshwater ecosystems, but many are also marine.
- Plants (Kingdom Plantae), sharing cellulose, starch, and all the pigments found
in green algae, but differing from algae in developing from multicellular embryos.
- REVIEW:
Study guide and vocabulary
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