Protists (Protistans) and Life cycles

Protists are generally unicellular, but sometimes multicellular species.

Protists are all eukaryotic, their DNA is enclosed in a nucleus inside the cell unlike bacteria, which, whilst unicellular, are prokaryotic and have no nucleus to enclose their DNA.

Some protists are heterotrophs, some are autotrophs and a few species are both!

Protists are usually grouped into three subcategories: plant-like protists, animal-like protists , and fungus-like protists.

Most live in water, or places that have a watery film.

HAPLOID

DIPLOID

Sporesn

Meiosis

Mitosis

Gametes (?)

Fusion (syngamy)

Mitosis

Zygote2n

A Basic Life Cycle Structure

Is this what humans do?

No, I don’t think so.

Plant-like Protists (Algae) 6 phyla s

1. Euglenas (Euglenophyta)

Some are autotrophs when sunny and heterotrophs when dark. Unicellular, found mostly in fresh water. Some have a flagella

Red spot shields the photoreceptor

2. Diatoms (Chrysophyta)

Examples in the lab. Fig 23.16c in text book

Cyclotella stelligera

Diatoms make 20% to 25% of global organic carbon fixation.

Plant-like Protists (Algae) 6 phyla

4. Green Algae (Chlorophyta)

Mostly unicellular, but some form colonies, and a few are multicellular. Live in fresh water, salt water, and a few live on land. Fig 23.21 of text book

3. Dinoflagellates (Pyrrophyta)Ocean dwelling, unicellular, covered by stiff plates, two flagella, bioluminescent (biochemical reaction to oxygen). Fig 23.17 of text book.

Protoperidinium steinii

Ulva on the sea shore

Flagella are ~10 microns long.

Generation time is ~5 hours

Chlamydomonas

Dormant Chlorella nivalis

The red “resting” stage

A thick-walled resistant zygote develops.

Meiosis and Germination

Haploid cell (- strain)

Haploid cell (+ strain)

Mitosis occurs. Whether the resulting cells develop into spores or gametes depends on environmental conditions.

More spores are produced.

More spores are produced.

ASEXUAL REPRODUCTION

–+

SEXUAL REPRODUCTION:

Mainly when nitrogen levels are low and light is of a certain quality and intensity, the cells develop into gametes.

Cytoplasmic Fusion

Diploid Stage

Haploid Stage

Page 389 Fig 23.22

Nuclear Fusion

+–Gametes of different mating types meet

Life cycle of Chlamydomonas

Zygote (cross-section)

isogamy

Prizes!Prizes!

Prizes!PrizesPrizes!!

One difference and two important features

What is different about the Chlamydomonas life cycle compared to the basic life cycle?

HAPLOID

DIPLOID

Fusion (syngamy)Meiosis

Sporesn

Zygote2n

Gametes (?)

Mitosis

Mitosis

Mitosis

Important life cycle features of Chlamydomonas

+ and - gametes look the same

In Chlamydomonas the diploidphase is a “resting” stage – no mitosis of the diploid phase

“Sexual” reproduction induced by environmental conditions

The life cycle of Ulva

Both the spores and the zygote develop into a multicellular thallus

Important Feature

How does this suit Ulva’s niche?

What’s the function of sexual reproduction?

Sexual reproduction introduces variations in the details of traits among offspring.

This variation is the FEEDSTOCK of natural selection

Don’t tell me that’s all there is to sex!Sure … it does just fine for Chlamydomonas and Ulva

Differentiation of the sexes is a characteristic that enables specialization in:

(a) Ensuring fertilization(b) Producing and distributing successful progeny

In some plant types producing and distributing large numbers is achieved by spores

 In plants the generation with sexual reproduction is called the gametophyte and the asexual generation is the sporophyte, either of which may dominate the life cycle, and there is also alternation of the haploid and diploid states.

Alternation of Generations

The occurrence within the life cycle of an organism of two or more distinct forms (generations), which differ from each other in appearance, habit, and method of reproduction.

The phenomenon occurs in some protists, certain lower animals (e.g. parasitic flatworms), and in plants.

The malaria parasite (Plasmodium), for example, has a complex life cycle involving the alternation of sexually and asexually reproducing generations.

Large leaf-like structures called blades and frequently air-filled sacs called air bladders and root-like structure called a holdfast. Not closely related to land plants. Cells contain pigments such as chlorophyll c and fucoxanthin. They also lack plasmodesmata and starch production. Fig 23.20 of text book.

The pigment phycoerythrin reflects red light and absorbs blue. Blue light penetrates water to a greater depth than light of longer wavelengths and so these pigments allow red algae to photosynthesize and live at somewhat greater depths than most other "algae". Fig 23.18 of text book.

5. Red Algae (Rhodophyta)

Porphyra species

6. Brown Algae (Phaeophyta)

Ascophyllum nodosum

Alginic acid: used in toothpastes, soaps, ice cream, tinned meats, fabric printing. It forms a stable viscous gel in water, and is as a binder, stabilizer, emulsifier, or moulding agent.

Fig 23.19 Page 386

10 cmgametophyte (n)

female gametes

male gametes

zygote

Diploid Stage

Haploid StageFertilization

Life cycle of Porphyra

Diploid thallus and haploid thallus have different morphology, size and habitat

+and gametesare different in size and morphology

How does this life cycle differ from that of Ulva?

sporophyte (2n)

germinating spore (n)

Meiosis

(Protozoa) - 4 phylaAnimal-like Protists

1. Sarcodines (Sarcodina)

2. Flagellates (Mastigophora)

Food is surrounded by pseudopods and stored in a food vacuole

Parasite causing gasteroenteritis. Giardia lamblia trophozoites, are released from cysts by contact with stomach acids and attach to the epithelium of the small intestine. Scanning electron microscope photograph. 

Image by Arturo Gonzalez, CINVESTAV, Mexico

Amoeba

These protists move by beating cilia that also help it capture food.

Paramecium is an example

3. Ciliates (Ciliophora)

4. Apicomplexa (Sporozoa) Parasites!

Named for a complex of organelles located at the apex of the cell used to break through tissues and cells of the host. Sexual and asexual stages and often need two or more hosts to complete life cycle.

Plasmodium - malaria

Oocysts: mosquito midgut Sporozoites Merozoite: in a

red blood cell

1. Water Molds & Mildews (Oomycota)

Saprolegnia mixta -

Saprophytes and parasites.

"Oomycota" means "egg fungi," and refers to the large round oogonia, or structures containing the female gametes, (picture)

Fungus-like Protists

Slime moulds have structural adaptations

Slime moulds have structural adaptations and life cycles that enhance their ecological role as decomposers

2 - Plasmodial slime moulds or true slime moulds, are a large single-celled mass with thousands of nuclei called a plasmodium formed when individual flagellated cells swarm together and fuse into one large bag of cytoplasm with many diploid nuclei.

http://www.hiddenforest.co.nz/slime/intro.htmClive Shirley

A model organism for bio-medical research.

Characteristics of Dictyostelium

Many molecular and cellular processes of communication appear to have remained unchanged throughout evolution.

Subsequent processes depend on cell-cell communication

Up to 100,000 cells signal each other by releasing a chemo-attractant and aggregate to form a mound.

3 - Cellular slime moulds spend most of their lives as separate single-celled amoeboid protists, but upon the release of a chemical signal, individual cells aggregate into a great swarm, known as a pseudoplasmodia and eventually muticellular slugs.

Dictyostelium

Fig. 23.3, p. 375

1 Stalked, spore-producing structure releases spores.

MITOTIC CELL DIVISION

2 Spores give rise to free-living amoebas that feed, grow, and reproduce by mitotic cell division.

3 When food gets scarce, the amoebas stream together to form an aggregate that crawls like a slug.

AGGREGATION

4 The slug may start developing at once into a spore-bearing structure, or it may migrate elsewhere first.

eitherorMIGRATING

SLUG STAGE

CULMINATION

MATURE FRUITING BODY

But what’s wrong with this life cycle?

The life cycle of a cellular slime mold,

Dictyostelium

Fig. 23.3, p. 375

1 Stalked, spore-producing structure releases spores.

MITOTIC CELL DIVISION

3 When food gets scarce, the amoebas stream together to form an aggregate that crawls like a slug.

AGGREGATION

4 The slug may start developing at once into a spore-bearing structure, or it may migrate elsewhere first.

eitherorMIGRATING

SLUG STAGE

CULMINATION

MATURE FRUITING BODY

2 Spores give rise to free-living amoebas that feed, grow, and reproduce by mitotic cell division.

1. Plants accumulate matter and make growth

2. Plant growth is an organized process following rules of anatomy, morphology and physiology

3. Plants actively maintain their heat and water balance

4. Plants have a life cycle with reproduction and dispersal

5. Evolution is a constant process

Protists are not plants – but they follow the same 5 principles in their own ways

For unicellular organisms growth (cell division) asexual reproduction

Many protists show distinctive physiological adaptations – though some have distinct morphology

Protists live in water, or in damp environments and tend to actively

avoid dry (+/or low nutrient) conditions

Life cycles are adapted to suit ecological conditions

Many protists are specialists – adapted to very distinctive environments