Download DISCICRISTATA (EXCAVATA)

DISCICRISTATA (EXCAVATA)
Phylum Euglenozoa
Introduction
There is little agreement on what name should be used for the Infrakingdom of protists with disc
shaped mitochondria: Discicristata, Discomitochondria, or Euglenozoa are all being considered
by various groups that work on the taxonomy of these organisms. Among those the have disc
shaped mitochondria there is one characteristic, a kinetoplast, that unites some of the protists.
This unusual, self-replicating structure, consists of a mass of DNA contained within an enlarged
mitochondria. They’re usually found at the base of the flagellum and it’s assumed they are
involved in flagellar function. Kinetoplasts are found in both Euglena and Trypanosoma and the
two are often combined with other kinetoplast protists in the phylum Euglenozoa.
Euglena
Fig. 1.
Anatomical features of Euglena. © BIODIDAC
The taxonomic position of Euglena seems to be forever shifting, in part because it seems to be
an intermediate between plants and animals. There’s a chloroplast capable of photosynthesis.
In the absence of light, or in euglenoid species that don’t have chloroplasts, they can survive as
heterotrophs and absorb nutrients across the cell membrane using pinocytosis. Even when they
use light they still require certain amino acids for optimal growth. With the recent use of
mitochondrial and membrane structure as a taxonomic tool for protist identification, rather than
being photosynthetic or not, Euglena finds itself closely allied to a variety of other flagellates
with a kinetoplast, although it’s too small to see in these minute protist. What type of
mitochondrial cristae does Euglena have?
Euglena is a freshwater species that may form green scums in ditches and small, warm bodies
of water. They are usually between 50 and 100 µm in length, you will need to make your
observations under high power. If available, use the oil immersion lens to make your
observations of the prepared slides. (Don’t use oil immersion with wet mounts!) It is also
important that you carefully align your microscope. If too much light floods through the optics
you won’t be able see the delicate flagellum of the stigma (eyes pot). Once you have your
microscope aligned lower the brightness of the light and you’ll find you can see quite a bit of
detail.
Discicristata (Excavata)
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Digital Zoology LabManual © Houseman
The spindle shaped body is covered in a flexible pellicle with underlying myonemes which
allows the species to use its characteristic euglenoid movement to squeeze and push its way
through tight spaces or swim. Quite often when Euglena are placed in wet mounts they will
remain stationary and squirm and wriggle showing this type of movement, although in this case
it really isn’t getting them anywhere.
The large flagellum is anchored in the base of the reservoir. Although you can’t see it a second
small flagellum is also present at fuses with the larger near its base. Watch closely, while
swimming does Euglena use the flagellum to push or pull itself through the medium?
As a photosynthetic organism Euglena is phototaxic and will orient to light using the stigma
located on one side of the reservoir wall. The stigma contains pigments, which give it a red to
orange color under the microscope. How does Euglena know which direction to swim to
orient itself properly to the light source?
The chloroplasts and the stored starch-like food in the paramylum reserves are the large
greenish granules that appear in the cytoplasm. The water expulsion vesicle is located near the
reservoir into which it opens, and the nucleus tends to lie in middle or towards the back of the
body.
Trypanosoma
Fig. 2. Anatomical features of a trypanosome.
© BIODIDAC
Trypanosoma gambiense is the causative organism of human sleeping sickness in large parts
subsaharan Africa. A second species, Trypanosoma cruzi spreads Chaga’s disease in south and
Central America. Trypanosomes live in the blood of their vertebrate host and are consumed in
the blood meals of their insect vectors, the tsetse fly in Africa and blood feeding bugs
(Hemiptera) in the Americas. The life cycle is complete when an infected insect takes a blood
meal from another vertebrate. As they feed, a small amount or fecal material is released and
when the irritation causes scratching, the parasite enters the vertebrate host. Adult trypanosomes
are between 10 and 70 µm in length and live n the vertebrate hosts plasma, and are not found
inside the blood’s cellular components.
Locate a specimen between the blood cells and move up to oil immersion to better see the details
of this small parasitic protist. The organism resembles a curved leaf of short blade of grass with
one end being blunt, or rounded and the other being drawn out to a point. The flagellum is fused
to the body and starts from the anterior blunt end of the organism. As it extends back along the
length of the body wrapped in a thin membrane holding it to the body forming the undulating
membrane. At the “pointy” distal end the flagellum extends beyond the body and functions in
the same was a flagella in other protists. Look closely at the anterior end and you’ll see a dark
staining body, the kinetoplast. The other prominent feature is the large central nucleus.
Trypanosomes feed by absorbing nutrients directly across their body wall.
Discicristata (Excavata)
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