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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 foode.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 topand 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
(a) Hyphae adapted for trapping and killing prey
(b) Haustoria
Fungal hypha Plant cell wall
Haustorium
Plant cell plasma membrane
Plant cell
Hyphae may be specialized to feed on animals
- or as haustoria, which may form associations with plant roots called mycorrhizae
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
Many fungi reproduce asexually - processes of asexual reproduction in fungi vary widely
10 m
yeast cells budding
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
Very early fossils (420-400 MYA) show mycorrhizal associations
fossil from Rhynie Chert, early Devonian, Scotland
Five different phyla generally recognized within Fungi
Genetic evidence suggests microsporidians also are fungi (or closely related)- intracellular parasites, no functional mitochondria
25 m
4 m
Hyphae
Flagellum
Phylum Chytridiomycota - Chytrids
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 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
around several haploid nuclei by a septum.
2
Rhizopus
growing
on bread
ASEXUAL
REPRODUCTION
Mycelium
Dispersal and
germination
MEIOSIS
KARYOGAMY
PLASMOGAMY
Key
Haploid (n)
Heterokaryotic (n + n)
Diploid
Sporangium
Diploid
nuclei
Zygosporangium
(heterokaryotic)
100 m
Young
zygosporangium
(heterokaryotic)
SEXUAL
REPRODUCTION
Dispersal and
germination
Mating
type (+) Mating
type ()
Gametangia with
haploid nuclei
50 m
Sporangia
A heterokaryotic
zygosporangium
forms, containing
multiple haploid
nuclei from the two
parents.
3
4
5
6
7
9
This cell develops a
rough, thick-walled
coating that can resist
dry environments and
other harsh conditions
for months.
When conditions are favorable,
karyogamy occurs, followed by
meiosis. The zygosporangium
then breaks dormancy,
germinating into a
short sporangium.
The sporangium
disperses genetically
diverse, haploid spores
The spores
germinate and
grow into new
mycelia.
8 Mycelia can also reproduce
asexually by forming sporangia
that produce genetically identical
haploid spores.
Some zygomycetes can aim & shoot their spores in appropriate direction
Pilobolus