last day- introduced the ‘protists’ – eukaryotes that are not plants, fungi or animals - have...
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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…
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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
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Radiolarians have tests made of silica, long ‘axopodia’
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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
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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
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Some red algae harvested as foode.g. nori from Porphyra
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Green algae (Chlorophyta) may be single-celled, colonial, multi-nucleate or relatively large & complex
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More than 7,000 spp., most in fresh water but also marine or other habitats
Chlamydomonas nivalis in glacier snow
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Most have complex life cycles, some with alternation of generations
- Chlamydomonas undergoes sexual reproduction only in harsh conditions
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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’!)
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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
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Entamoebas are parasitic - Entamoeba histolytica causes amoebic dysentery - causes up to 100,000 deaths per year
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‘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
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Mostly diploid, produces fruiting bodies & spores when conditions are harsh- spores produce haploid flagellated or ameboid cells
which fuse to form diploid stage again
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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
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Amoebas can also fuse to form zygote- zygote consumes other amoebas to become ‘giant cell’- forms resistant wall, later produces new amoebas
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Kingdom Fungi
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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
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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
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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
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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
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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
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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
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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
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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
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Nuclei of fungal hyphae & spores of most species are haploid, except for transient diploid stages that form during sexual life cycles
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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…)
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- later, karyogamy occurs when nuclei fuse- diploid nucleus then undergoes meiosis to form spores
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Many fungi reproduce asexually - processes of asexual reproduction in fungi vary widely
10 m
yeast cells budding
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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
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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
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Very early fossils (420-400 MYA) show mycorrhizal associations
fossil from Rhynie Chert, early Devonian, Scotland
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Five different phyla generally recognized within Fungi
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Genetic evidence suggests microsporidians also are fungi (or closely related)- intracellular parasites, no functional mitochondria
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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)
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May be saprobes, parasites or mutualists- appear to be a major cause of worldwide decline of amphibians
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Phylum Zygomycota – Zygomycetes
About 1000 spp., including many molds on food & other saprobes, also parasites & some commensals (neutral symbionts)
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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.
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If environmental conditions deteriorate, zygomycetes may reproduce sexually - plasmogamy of opposite mating types produces a zygosporangium (tough & resistant)
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Initially, zygosporangium is heterokaryotic- if conditions improve karyogamy occurs, then meiosis
to produce genetically diverse spores
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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.
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Some zygomycetes can aim & shoot their spores in appropriate direction
Pilobolus