Last day- introduced the
‘protists’ – eukaryotes that
are not plants, fungi or
animals
- have covered Excavata,
Chromalveolata
(Alveolates & Stramenopiles),
now move on to Rhizaria…
Foraminiferans , Cercozoans & Radiolarians
are somewhat distant relatives grouped as Rhizaria
- all have threadlike pseudopodia (so ‘amoebas’ of sorts)
‘Forams’ have shells
(‘tests’) with
calcium carbonate
- pseudopodia extend
through pores for
swimming, feeding
- may have symbiotic
algae in test
- important fossils
Radiolarians have tests made of silica, long ‘axopodia’
Next eukaryotic ‘supergroup’ to be covered is…
the Archaeplastida, which includes the red and green
algae (plus the land plants…)
- all descended from protist that swallowed a cyanobacterium
Red algae (Rhodophyta) are larger multicellular algae,
often in warmer seas
Absorb green & blue light well (in deeper water), due to
phycoerythrin, an accessory pigment
- no flagellated stages, gametes depend on currents
Some red algae
harvested as food
e.g. nori from
Porphyra
Green algae (Chlorophyta) may be single-celled, colonial,
multi-nucleate or relatively large & complex
More than 7,000 spp.,
most in fresh water
but also marine or
other habitats
Chlamydomonas nivalis
in glacier snow
Most have complex life cycles, some with alternation of
generations
- Chlamydomonas undergoes sexual reproduction only in
harsh conditions
The last big group of
eukaryotes, the Unikonta,
divides into the ‘protists’
known as Amoebozoans
(‘slime molds’,
gymnamoebas, &
entamoebas)…
…and the Opisthokonts
that includes fungi &
animals (not ‘protists’!)
Amoebozoans have
lobe-shaped pseudopodia
- includes gymnamoebas,
entamoebas, & ‘slime
molds’
Gymnamoebas are common
in water and soil
- usually engulf prey
(bacteria or protists)
- no sex
Entamoebas are parasitic
- Entamoeba histolytica causes amoebic dysentery
- causes up to 100,000 deaths per year
‘Slime molds’ formerly thought to be close to fungi
Plasmodial slime molds form brightly colored network
- multinucleate ‘supercell’
- cytoplasm flows back & forth
- engulfs food particles with pseudopodia
Mostly diploid, produces fruiting bodies & spores when
conditions are harsh
- spores produce haploid flagellated or ameboid cells
which fuse to form diploid stage again
Cellular slime molds forage as solitary amoebas, aggregate
(but do not fuse) when times tough
- some cells form stalk (and die), other cells crawl to top
and release spore
Dictyostelium
Amoebas can also fuse to form zygote
- zygote consumes other amoebas to become ‘giant cell’
- forms resistant wall, later produces new amoebas
Kingdom Fungi
Fungi often overlooked, though they play an important
role in ecosystems as decomposers
- recycle vital chemical elements back to environment in
forms other organisms can assimilate
Most plants depend on mutualistic fungi to help their roots
absorb minerals and water from the soil
- cultivated for centuries for food, to produce antibiotics &
other drugs, to make bread rise, & to ferment beer & wine
Fungi are heterotrophs that acquire their nutrients by
absorption.
- absorb small organic molecules from the surrounding
medium.
- use exoenzymes to break down food outside body
Ecological roles as decomposers (saprobes), parasites, &
mutualistic symbionts
- saprobic fungi absorb nutrients from dead organisms or
organic compounds
-parasitic fungi absorb nutrients from cells of living hosts
- mutualistic fungi also absorb nutrients from hosts, but also
benefit their partner in some way
Extensive surface area & rapid growth adapt fungi for
absorptive nutrition
- most species are multicellular
- vegetative bodies of most are constructed of tiny filaments
called hyphae, form an interwoven mat called a mycelium
Hyphae have cell walls made mainly of chitin
- most fungi multicellular, hyphae divided into cells by
cross walls, or septa
- those without septa called coenocytic.
- septa generally have pores large enough for organelles,
nuclei to pass through
Nematode
Hyphae
25 m
Hyphae may be specialized
to feed on animals
(a) Hyphae adapted for trapping and killing prey
- or as haustoria,
which may form
associations with
plant roots called
mycorrhizae
Fungal hypha
(b) Haustoria
Haustorium
Plant
cell
wall
Plant cell
Plant cell
plasma
membrane
Fungi produce spores through sexual or asexual life cycles
- one reproductive structure may release trillions of spores
(e.g. puffballs)
- dispersed by wind or water, spores germinate if they land
in a moist place where there is food
Nuclei of fungal hyphae & spores of most species
are haploid, except for transient diploid stages that form
during sexual life cycles
Sexual reproduction: hyphae from 2 genetically distinct
mycelia release pheromones (signaling molecules)
- union of the cytoplasm of two parent mycelia known as
plasmogamy
- leads to heterokaryotic stage with 2 different nuclei in
same cell, but they do not fuse (yet…)
- later, karyogamy occurs when nuclei fuse
- diploid nucleus then undergoes meiosis to form spores
10 m
yeast cells budding
Many fungi reproduce asexually
- processes of asexual reproduction in fungi vary widely
Fungi & animals more closely related to each other than to
plants or most other eukaryotes
- fungi evolved from a unicellular, flagellated protist
- members of Opisthokonta, including animals, fungi, &
closely related protists, possess flagella
- the lineages of fungi that diverged earliest (the chytrids)
have flagella
Animals & fungi likely diverged about a billion
years ago, based on genetic differences
- oldest undisputed fossils only 460 million years
old, likely that first fungi were unicellular, did
not fossilize well
fossil from Rhynie Chert,
early Devonian, Scotland
Very early fossils (420-400 MYA) show mycorrhizal
associations
Five different phyla
generally recognized
within Fungi
Genetic evidence suggests microsporidians also are fungi
(or closely related)
- intracellular parasites, no functional mitochondria
Phylum Chytridiomycota - Chytrids
Hyphae
25 m
Flagellum
4 m
Chytrids ubiquitous in lakes, streams, soil
- have flagellated zoospores
- chitin in cell walls, absorptive nutrition, similar enzymes
- coenocytic hyphae (some unicellular)
May be saprobes, parasites or mutualists
- appear to be a major cause of worldwide decline of
amphibians
Phylum Zygomycota – Zygomycetes
About 1000 spp., including many molds on food & other
saprobes, also parasites & some commensals (neutral
symbionts)
Life cycle of Rhizopus stolonifer, black bread mold,
is typical
- hyphae are coenocytic, septa only where reproductive
cells are formed
In asexual phase, hundreds of haploid spores develop in
sporangia at tips of upright hyphae.
If environmental conditions deteriorate, zygomycetes may
reproduce sexually
- plasmogamy of opposite mating types produces a
zygosporangium (tough & resistant)
Initially, zygosporangium is heterokaryotic
- if conditions improve karyogamy occurs, then meiosis
to produce genetically diverse spores
1
2
Mycelia have
various mating types
(here designated +,
with red nuclei, and ,
with blue nuclei).
Neighboring mycelia of different
mating types form hyphal extensions
called gametangia, each walled off
Key
Haploid (n)
around several haploid nuclei by a septum.
Heterokaryotic (n + n)
Diploid
3
A heterokaryotic
zygosporangium
forms, containing
multiple haploid
nuclei from the two
parents.
PLASMOGAMY
Rhizopus
growing
on bread
Mating
type (+)
Gametangia with
haploid nuclei
Mating
type ()
100 m
9 Mycelia can also reproduce
asexually by forming sporangia
that produce genetically identical
haploid spores.
8
Young
zygosporangium
(heterokaryotic)
The spores
germinate and
grow into new
mycelia.
SEXUAL
REPRODUCTION
Dispersal and
germination
Sporangia
7
The sporangium
disperses genetically
diverse, haploid spores
Sporangium
ASEXUAL
REPRODUCTION
MEIOSIS
Zygosporangium
(heterokaryotic)
KARYOGAMY
Diploid
nuclei
4
This cell develops a
rough, thick-walled
coating that can resist
dry environments and
other harsh conditions
for months.
Dispersal and
germination
50 m
Mycelium
5 When conditions are favorable,
6 The zygosporangium
then breaks dormancy,
germinating into a
short sporangium.
karyogamy occurs, followed by
meiosis.
Pilobolus
Some zygomycetes can aim & shoot their spores in
appropriate direction