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Lecture 16

Biology 1002B Lecture Notes - Lecture 16: Xist (Gene), Somatic Cell, Microtubule

Course Code
BIOL 1002B
Tom Haffie

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Lecture 16: Evolution of Multi-cellularity
What to see on the DNA picture
o DNA binding sites
o DNA methylation
Clicker question:
o Which of the following regulation mechanism is not “epigenetic
Cannot change the DNA sequence, not mutations
o Not mutations
Persistent beyond the original conditions where the regulation is started
o Last for the whole life
Lac binding is not epigenetics because lac varies,
Binding of Xist RNA is epigenetics, persistent, whole life will be inactive
Regulation of gene expression affords the possibility for specialization…
o Genetic vs. epigenetics
Clicker question
o “Multicellular life is monophyletic”
Multicellularity evolved several times
o Some groups have some unicellular organism and some multicellular organisms
o Multicelluarity has evolved multiple times
Volvocine algae provide a productive model system…
o Chlamy unicellular
o Organisms begin to live together
o Then organisms are living together in a “cell wall”
o Volvocine, lots of somatic cells, still have flagellum
o Volvocine and chlamy common ancestor is 50MYA
o Some cells are big and some are small
o ECM, extra cellular matrix
Volvox genus is not monophyletic
o It has evolved 4 or 5 times
Volvox embryology
o 1 cell divides in 2, 4, 8, 16 undifferentiated
o A symmetric division is very important
o The large cells will become reproductive cells
o The small cells will become somatic cells
o In a development stage, the cell is inside out, the flagellum is inside and reproductive
cells outside
o But then it flip itself, and the flagellum is outside and reproductive cells are inside, and
respond to light
o Reproductive cells will reproduce, and the somatic cells of parent cell will die
What kind of study would help to identify the genes that are important for multicellularity?
o Maybe Use restriction endonuclease enzymes
Selectively cut out genes, or mutate them
o Mutate volvox and see how it effects the cell, where is broken and etc.
o Using southern blots, compare genome of chlamy and volvox
o How do they make somatic cell and the reproductive cells, different kind of tissues
Clicker question
o In what way would you predict that the volvox genome is different from
Genome size are not dramatically different
PCG have about the same numbers
Comparative genomics
o Our comparisons of Volvox and Chlamydomonas indicate that, with the exceptions of
ECM and cyclins (regulate cell cycle), the developmental innovations in the Volvox
lineage did not involve major changes in the ancestral protein repertoire.
Mutational studies identify important genes
o GLS responsible for the asymmetric cell division
If mutated, we do not get the reproductive cells
If cell division is normal:
The large cells
o Lag turns on, represses the flagellum and eye spots, and
anything that it doesn’t need to reproduce
Small cells
o Repress reproduction, most cells need to give up reproduction
Mutational studies identify important genes in Volvox
Function: shift plane of mitotic division; asymmetric division
Chlmay orthologue can rescue Volvox mutants, it can fix the mutation in volvox
Gene from chlamy works in volvox
o Called orthologue
Orthologue means they derived from the same
common ancestor, has a different function
o Same gene got a new function
o regA
transcription repressor of nuclear chloroplast, keep the somatic cells from
can’t make chloroplast, not enough energy to reproduce, thus they
don’t reproduce
chlamy paralogue induced by low light stress
o response in low light, it shuts down photosynthesis in chlamy
o related gene in volvox, shuts down photosynthesis shuts down
paralogue: a pair of genes derived from the same ancestral gene,
different regulation doing same job
o “inversionless”, invA
Kinesin (motor) working on microtubules
Chlamy orthologue can rescue volvox mutants
A gene found in clamy (orthologue) has a different function in volvox
o Not making chlmay divide asymmetrically
A related gene (paralogue) in Chlmay has a different regulation in volvox
o Different regulation but have the same function
Division of labour and mortality…
o After the reproductive cells matures, the somatic cells will die, somatic don’t get their
genetic information to the next generation
o They die In a programmatic way, programmed cell death