Transcript Eukaryotes to Multicellular

Chapter 9
Eukaryotic Cells and
Multicellular Organisms
Overview
• Review of key structural and genetic features of eukaryotic cells.
Remind yourself of these by your own reading.
• The origin of cells with eukaryotic organization, some 2.5 Bya,
facilitated the evolution of multicellularity.
– Multicellularity dependent on origin of 02-rich atmosphere.
– Multicellularity was dependent on the evolution of compartmentalization by
membranes and organelles.
– The chloroplast and mitochondria are two organelles now believed to have
developed by endosymbiosis.
Overview
– Consider this passage from textbook and ask yourself
is this inference from observation to fit an a priori
paradigm or derived from testable and falsifiable
experimentation?
“Paradoxically, we know more about the origin of the mitochondrial and
chloroplast organelles of eukaryotic cells than we do about the origins of
the organisms that contain them. Ancient anaerobic eukaryotic cells
engulfed prokaryotic organisms in a process known as endosymbiosis,
established a symbiotic relationship with the prokaryote. Subsequently,
the prokaryotes were retained as cellular organelles — mitochondria and
chloroplasts — providing eukaryotes with additional source of DNA.”
Evolution of Eukaryotic Organization
• As early as 1.5 Bya “possible” eukaryotic cells appear as fossils.
• Dr. Strohmeyer’s observations: First cell(s) arise presumably
almost instantaneously (2-500 my) but it takes another 2 BILLION
years to get from prokaryote to eukaryote. Another 1 BILLION to
Cambrain explosion. Then just 550 million years for all that to
diversify into all known extinct and living creatures (more on this in
Ch 11).
Microfossils of probable
eukaryotic cells
Reproduced from Schopf, J.W., Scientific American 239 (1978): 111-138. Courtesy of J. William
Schopf, Professor of Paleobiology & Director of IGPP CSEOL
• A Tree of Life was
established using
nucleotide sequences from
5S rRNA of over 30 species
of prokaryotes and
eukaryotes—does this
seem like a large data set
for a tree of life?
1.2 Bya
• What about HGT?
• Diversification of animals,
plants, and fungi 1.2 Bya?
• Eukaryotic multicellularity
maybe as long as 1 Bya?
• This TOL is OLD… let’s
check out the current
version…
Adapted from Hori, H. and S. Osawa, Proc. Natl Acad. Sci. USA 76
(1979): 381-385.
Single-Celled Eukaryotes: Protistans
• Early eukaryotes were single-celled organisms or simple filaments.
• Today, most eukaryotes are multicellular.
• All unicellular eukaryotes classified in the kingdom Protista.
– Protistans are the essential link between early unicellular organisms and all
multicellular eukaryotes—certain ancestors evolved into multicellular
organisms (more in Ch10-11).
• Protists share many features with eukaryotes also—nuclear
membrane, cilia, flagella, microtubules, etc.
• Endosymbiotic events provided protists with mitochondria,
chloroplasts.
Single-Celled Eukaryotes: Protistans
• Microtubules reflect the nuclear chromosomal division (mitosis)
that replaces binary fission of prokaryotes.
• It seems likely for 2 reasons that once eukaryotic mitosis evolved,
sex cell division (meiosis) would have quickly followed:
– (1) Meiosis and sexuality almost universally appear among the major
eukaryotic taxonomic groups.
– (2) All asexual multicellular eukaryotes are derived from sexual forms with
meiosis.
• Review genetic concept of eukaryotic introns/exons (split genes) on
your own.
Origin and Evolution of Mitochondria and
Chloroplasts
• Ancient anaerobic eukaryotic cells evolved the ability to engulf
(endocytose/phagocytose) molecules, supramolecular assemblies,
and prokaryotes—anaerobes capable of oxidative metabolism and
cyanobacteria capable of photosynthesis.
• Supporting evidence for endosymbiotic theory
– Uniparental inheritance (maternal common)
– Physical structure more like that of a prokaryote.
Mitochondria and plastids are similar in size to
bacteria.
– New mitochondria and plastids are formed only
through a process similar to binary fission.
– Surrounded by two or more membranes, and the
innermost of these shows differences in composition
from the other membranes of the cell. They are
composed of a peptidoglycan cell wall characteristic of
a bacterial cell.
• Supporting evidence for endosymbiotic theory
– Both mitochondria and plastids contain DNA that is
different from that of the cell nucleus and that is
similar to that of bacteria (in being circular in shape
and in its size).
– DNA sequence analysis and phylogenetic estimates
suggest that nuclear DNA contains genes that probably
came from plastids.
– DNA sequencing reveals similarities in genes and
architecture
– Ribosomes more prokaryote like in structure,
biochemistry, RNA, and antibiotic sensitivity
– Proteins of organelle origin, like those of bacteria, use
N-formylmethionine as the initiating amino acid.
• Supporting evidence for endosymbiotic theory
continued
– Mitochondria have several enzymes and transport
systems similar to those of bacteria.
– Much of the internal structure and biochemistry of
plastids, for instance the presence of thylakoids and
particular chlorophylls, is very similar to that of
cyanobacteria.
– Phylogenetic estimates constructed with bacteria,
plastids, and eukaryotic genomes also suggest that
plastids are most closely related to cyanobacteria.
• Supporting evidence for endosymbiotic theory
continued
– Some proteins encoded in the nucleus are transported
to the organelle, and both mitochondria and plastids
have small genomes compared to bacteria. This is
consistent with an increased dependence on the
eukaryotic host after forming an endosymbiosis.
– Most genes on the organellar genomes have been lost
or moved to the nucleus.
• Supporting evidence for endosymbiotic theory
continued
– Most genes needed for mitochondrial and plastid
function are located in the nucleus. Many originate from
the bacterial endosymbiont.
– Plastids are present in very different groups of protists,
some of which are closely related to forms lacking
plastids. This suggests that if chloroplasts originated de
novo, they did so multiple times, in which case their
close similarity to each other is difficult to explain.
• What are some issues that contradict the endosymbiotic
theory? (Dr. Strohmeyer)
• How much Lynn Margulis got wrong and how
Darwinists, particularly Richard Dawkins, rejected her
theory for a long time!
• Linear DNA and telomeres in many cases—not all
circular DNA
• Division/replication is not true binary fission as in
prokaryotes
• Genetic data not quite as convincing as implied in biased
accounts
• No testable mechanistic explanations for how you turn a
prokaryote into an organelle.
• What are some issues that contradict the endosymbiotic
theory? (Dr. Strohmeyer)
• Genetic mutations that allow survival of internalized
prokaryote more likely to be lethal.
• Oxygen a problem for aerobic endosymbiont.
• Endosymbiont would compete for ATP.
• Gene transfer (however it could occur) would be
disruptive to nuclear genes and how would expression
work anyway. Mechanism to target proteins back must
also be incorporated into transferred genes
simultaneously.
• Genetic code differences exist in organellar DNA.
• According to what has come to be
known as primary endosymbiosis,
the first symbiotic relationship was
established with mitochondrion-like
aerobic bacteria (thought to be a
Rickettsiales-like a-proteobacteria)
and later one or more of these
repeated this with a photosynthetic
cyanobacteria-like organism to form
chloroplasts.
• Most gave rise to protists, fungi, and animals or alga and plants
respectively.
• Debate whether single or multiple primary symbiotic events occurred in
early evolution.
• HGT then took place between “organelles” and nucleus of host.
• Secondary and tertiary
endosymbiosis, later evolutionary
events in which eukaryotic algae
were engulfed by other eukaryotes.
• Indeed, some eukaryotic algae are
thought to have acquired
chloroplasts through secondary
endosymbiosis from a eukaryote
rather than from a prokaryote,
expanding even further the role of
endosymbiosis in evolution.
• Particulary important to Green
(Chlorophyta) and Red Alga
(Rhodophyta)
Mitochondrial DNA
• Mitochondria and chloroplasts both contain their own DNA/ genes.
• Such organelle DNA is separate from the nuclear DNA spatially,
organizationally, functionally and in mode of inheritance, all of
which provide evidence for the separate origins of organelle and
nuclear DNA and genes.
– This section reviews extranuclear inheritance of mtDNA—covered in genetics
but read on your own to review concept.
• mtDNA thought to mutate (evolve ) 10X faster than nuclear DNA.
Used as a molecular clock to determine time frames of organismal
evolution and maternal lineage and evolutionary relationships.
Transfer of Genes between Organelles and Nucleus
• Many genes were transferred to the eukaryotic nucleus; conversely,
some nuclear genes were transferred to organelle genomes.
– Ask yourself by what mechanism? Is this believed because of a known
mechanism or from an inference based on gene sequence similarity data?
• Especially susceptible to transfer were genes in organelles acquired
by endosymbiosis that already were present in the nucleus.
– Two examples are genes for anaerobic glycolysis and genes for amino acid
synthesis.
– Such transfers to the nucleus and deletions from mitochondria had the
advantage of maintaining and replicating only two copies of a symbiotic gene
in a diploid host nucleus instead of sustaining a separate gene copy within
each of many cellular organelles.
– Because some cells carry enormous numbers of organelle genomes — more
than 8,000 copies of the mitochondrial genome in some human cells and even
more copies of chloroplast genomes in some plant cells—reductions in
organelle gene number by deletion or nuclear incorporation must have been
highly selected for.
Transfer of Genes between Organelles and Nucleus
• Chloroplasts synthesize only a small portion of the proteins they use.
– Many of the genes introduced by the original cyanobacterial endosymbionts
were transferred to the nuclei of what are now plant cells—again, HOW?.
– The protein products of these genes are transported into the chloroplast
where they carry out their function(s).
– Now ask yourself how the genes would be transferred and then in order to
maintain function and viability, the organism SIMULTANEOUSLY evolves the
complex transport machinery and various signals to get the proteins back to
the chloroplasts or mitochondria.
– Gene transfer also occurred in the opposite direction. Transfer to the
appropriate organelle of nuclear genes coding for symbiotic organelle proteins
has been widely identified in eukaryotes.
– Ask yourself, why would a cell do this? Why not just put things in the nucleus?
Why move nuclear genes to organelles? Efficiency? Then why are proteins
being imported back to these organelles from the nucleus? And again, how
would you then SIMULTANEOUSLY evolve the needed changes to make this
work in the new location?
Box 9.1 and 9.2
• Box 9.1—Supergroups in the Eukaryotes Tree of Life
– This box reviews current taxonomic hierarchy made necessary by
regrouping protista, plants, fungi, and animals into supergroups
to better reflect the hypothesized phylogenies of these
organisms. This material is covered in General Biology now but
not this way for most of you. You should update your
understanding by reading this.
• Box 9.2—Ancient DNA
– This particular example (Moas and Kiwis phylogeny—extinct and
living) is given in more detail in the General Biology textbook in
chapter 26. Read on your own to review this study and the
usefulness of PCR for forensic science.
Key Concepts
• Most eukaryotic cells are multicellular and all contain a nuclear
mem-brane and organelles.
• Single- celled eukaryotes ( protistans) arose 1.6 to 1.8 Bya and link
ancestral prokaryotic cells with all multicellular eukaryotes.
• Five supergroups of eukaryotes are now recognized, replacing old
familiar groups such as animals and plants.
• The five supergroups are Archaeplastida ( Plantae), Excavata,
Chromalveolata, Rhizaria and Unikonta.
• Eukaryotic cells usually have many chromosomes and genes
containing introns.
Key Concepts
• Organelles, including the nuclear membrane arose when
prokaryotes were engulfed ( endosymbiosis) and transformed into
organelles.
• Endosymbiosis occurred in several waves: the formation of
mitochondria and chloroplasts ( primary endosymbiosis); and
secondary and tertiary endosymbiosis in which eukaryotic algae
were engulfed by other eukaryotes. Organelles such as
mitochondria in animals and chloroplasts in plants retained their
DNA and so provided eukaryotic cells with additional genomes.
• Some genes were transferred from the organelles to the nucleus,
others from the nucleus to the organelles.