Fungus-like protists Feb 1, 2000

Oomycetes are water molds, a primarily aquatic group of fungi.

Oomycetes have a filamentous body

- individual filament called hypha (pl. Hyphae)

- Mass of interwoven filaments is mycelium

Oomycetes cell walls are compose of cellulose or cellulose-like polysaccharides

- recent discoveries lead systematists to group oomycetes with some algae (perhaps red or some chrysophytes) than to other groups of algae

Hyphae forming the "body" of the oomycete is diploid

--also coenocytic few or no cell walls

sexual reproduction involves the formation of an oogonium (female gametangium)

Oogonium is a cell segregated by a cross wall

- cell enlarges becoming spherical

- nuclei of oogonium undergo meiosis

- separate cells form around the haploid nuclei (eggs)

- Antheridia = male gametangia

- antheridia are single cells in which nuclei undergo meiosis, forming haploid nuclei

- antheridium grows toward oogonium

- antheridia penetrate oogonia with fertilization tubes

- Syngamy results in oospore

- this form of reproduction is called oogamy because the female gamete is nonmotile and larger than the male.

oospore is the resting/ dispersal stage

- oospore (germination) results in of new diploid hyphae

Asexual reproduction

- hyphal tip segregated by crosswall

- this terminal cell = sporangium

- many nuclei in sporangium and

- each cell forms 2 flagella , making them motile

- Cells called zoospores,

- when zoospores retract flagella ,

- The cell where zoospores form is called a zoosporangium

-some oomycetes are terrestrial parasites

- obligate means they live only on a particular host...

Pythium debaryanum causes damping- off of seedlings

--fungus enters seeds or seedling

- pectolytic enzymes secreated by fungus

- cause loss of tissue structure in seedling

- special hypha called haustorium

- Plant seedling often invaded at soil line

- tissues become discolored

- seedling falls over

- use chamomile tea to keep this from happening 4 tea bags to 2C water

Genus Phytophthora includes many plant pathogens

Phytophthora infestans causes late blight in solanaceae potatoes and tomatoes

- when infected tubers are planted the fungus grows through the plant as it grows above ground

- sporangia project through the stomates

- spores are dispersed

- fungus over winters in tuber.

- when the tubers are cut up and replanted the next spring they bring the fungus infection along.

In Ireland the population grew from 4.5 million in 1800 to 8.5million in 1845

- the peasants relied on potatoes

- adults consumed 9-13 pounds of potatoes per day

- in the summer of 1846

- genetic uniformity

- potatoes are reproduced asexually

- 800,000 people died of starvation

- many others migrated to the USA

- example of how plants and fungus can have a huge effect on human population and demographics and why everyone should understand the lives of plants

Plasmopara viticola causes downy mildew of grape

--the disease and grape V. riparia native to N america

--American grapes resistant to the disease.

--Fungus selects for survival of plants with resistance

Grapes very important in the french economy

--In 1865 a root aphid was introduced

-- aphid resistant N. A. grape rootstock ....

-- disease caused the near collapse of wine industry

In 1882 , Alexix Millardet made the first fungicide

Acrasiomycota cellular or communal slime mold

-abundant in moist forests

- life cycle similar to myxomycota

- aggregate of amoebas

- form a pseudoplasmodium, or slug or grex

- feed by phagocytosis (engulfing)

- form a stalk made of cellulose.

- some amoebas form spores,

- the stalk gets too big and falls over and

- spores germinate into amoebae

- Sexual reproduction is rare, two amoebas will unite to form a macrocyst.

- nuclei unite and form a tough cell wall,

-within which they digest other amoebas

- macrocyst nucleus goes through meiosis

Main differences:

1)It's a cellular "republic" made of separate amoebas

2)Most of the life cycle is haploid

3)sex not necessary and seems uncommon

4)produce cellulose

Myxomycota slime molds remember are :

1) multinucleate mass of protoplasm in a slime sheath

2)main body is diploid

3)spores are formed in a sporangium by partitioning nuclei into diploid cells which undergo meiosis

Chytridiomycota "chytrids"

750 spp mostly aquatic but a wide variety of forms. a common characteristic between spp. is that the zoospore has a single posterior flagellum. Pg 250 text.

- haploid Zoospore germinates and produces coenocytic mycelia

-gametangia form

- separate male and female gametangia

- gametes anisogamous (female larger)

- male and female gametes unite and form diploid zygote

-zygote forms diploid mycelium

--diploid mycelium produces meiosporangium and /or mitosporangium

- meiosporangium makes haploid zoospores

- mitosporangium yields diploid zoospores

- sporangia are resistant

- do exhibit alternation of generations sporophyte and gametophyte.

Chytrids and oomycota can be lumped in the "Flagellated fungi" (an artificial group) because of the presence of motile forms.