6: C elegans Flashcards
What are some characteristics of C. Elegans (worms) that make it a good model organism for studying development?
1)Culture: agar plates covered with e.coli (bacterial plates) can also be grown in liquid broth with ecoli too
2) Size: just over 1mm, small enough and thin enough that all cells are visible by lught (phase or DIC) microscopy
3) Storage of Strain: can be frozen
4) Crosses: population is 99% “selfing” hermaphrodites (meaning that they have both male and female reproductive organs, so they can reproduce by selfing which is self reproduction: can use their own sperm to fertilize their own egg)
5) Generation time: 3 days (20 degrees)
6): Lifespan: reproduce for 3-4 days but live two more weeks: allwo researchers time to study all phases
7) # of progeny: 250-1000 per hermaphrodite (lower if herm selfed, higher if male cross)
8) Markers: ample visible markers (usually things like body shape and movement):
Genome size: 97 mB; total genes 19,000
# needed for development: 1700
Simple nervous system: 302 neurons exhibit limited behavior
What does being a selfing hermaphrodites allow them to create
genetically identical offspring since they are using their own sperm to fertilize their own eggs:
hermaphrodite means that they have both male and female reproductive organs
selfing: means self fertilization where they fertilize their owb sperm
what percentage of them are hermaphrodites: 99%
the remaining 1% are males
-result from X chromosome non-disjunction
(hermaphroditic female= XX
males: XO)
Why is the number of progeny less for hermaphrodite selfing
ebcause they are limited by the production of sperm in hthe hermaphrodite
Hermaphrodite and fertilization
-in early stages of development the herm produces sperm adn stores in the spermatheca: sperm is non-flagellate crawling amoeboid like cells
-Later in development, the herm switches to egg production (oogenesis): distal tip of ovary is syncytium and oocyte nuclei ar ebound by plasma membrane and bud off from the syncytium (oocyte develop in syncitium and undergo cellularization as it moves down the ovarhy)
-Oocytes arrested in prophase 1 meiosis: moves and are fertilized as they ar epushed through theSPERMATHECA:
essentially: the oocytes are rolled down the ovary until they reach the spermatheca where the sperm is and they are fertilized
where are the oocytes of c elegans fertilized
as they are pushed through the spermatheca:
triggers completion of meiosis since before hte ooxyte was arrested in prophase.1 of meiosis
How is polyspermy thiught to be blocked
by rapid deposition of chitin over the egg
What type of cleavage is observed in c elegans blastula?
Why can the lineage of any cell in the adult be traced back to the first division
Reproducible patterns of cell divison:
they have invariant embryonic development meaning that adult male or hermaphrodites display an EXACT NUMBER of cells (959 for herm, 1031 for male)
-since its invariant embryponic development: reproducible patterns of division occur meaning that it can be traced back to the first division that occured
What is type one embryogensis
-immediate activation of zygotic genes
-small number of cells at the start of gastruka
-rapid blastomere specfiication occurs in situ
Why is cell ablation possible in c elegans
because of invariaten lineage pattern of development and becaue all cells are visible, we can use a lasar to distry ceterain cells to understand it more
RNAi in c elegans
The process leverages the natural RNAi mechanism found in most eukaryotic organisms, where double-stranded RNA (dsRNA) induces the degradation of complementary messenger RNA (mRNA), effectively “turning off” the target gene.
how it owrks: can be introduced using e coli (as food source) that express dsRNA or by injecting it
-inside the cytosplasm, the enzyme DICER splices the dsRNA into short interfering RNAS
step 2: assembly of risc:
these are RNA-induced silencing complexes, the short interfering rnas are encorporated into this complex, the complex uses it as a guide to bind to complimentary mrna
step 3: now the RISC degrades the target mrna, preventing production of the corresponding protein
this is useful for researchers BECAUSE:
it allows them to silence certain genes in order to study their importance, fjunction, and effect on development , behaviour etc
What types of specification is used
Autonomous specification (assymetric divisions, so unequal distribution of cytoplasmic determinnats) and conditional (cell- cell interactions, so regulative developemtn(
is cell fate determined by cell to cell basis
yes, it is mosaic like and invariant embryonic development is caused by cell to cell basis
Cleavage is…
rotational holoblastic
holoblastic: complete cleavagel so the entirecell divides into seperate cells
rotational: meanins that at second division, one daughter cell divides longitudinally while the other divides transversley
for C elegans:
division are assymtric so resulting cells are different in size: anterior is larger than the posterior p cells in c elegans
Anterior founder cells: produce differentiated descendants
PosteriorP lineage: stem cells that become the germ line
Outline the model for how the anterior-posterior axis is
established in the C. elegans egg
LOOK AT card 27
unfertilized c elegans had no predetermined axis (unlike drosophilia)
-it is thought that the sperm specifies the initial assymetries by directing cytoplasmic rearrangements that cause determinants to become assymteirically localized
1) elongated egg shape: defines future anterior-posterior axis: decision for anterior end relates to position of sperm pronucleus
- the sperm centriole intiates cytoplasmic movement that pushes the male pronucleus to the nearest end- that will become the posterior pole
-one stage cell becomes polarized by a contraction of the actomyosin cortex away from the site of fertilization
-the fertilizing sperm has two cues for the assymetric contraction: centrsome and protein called CYK-4
SOOOOOO the different components of the cytoskeleton(ie the microfilaments: actin and myosin and the microtubules) are responsible for the assymetry (anterior and posterior sections)
-this assymetryic contraction is needed to bring the par proteins to the proper side to establish the axis
ESSENTIALLY: microfilaments and microtubles are needed for the assymetric contraction which brings the par proteins to the proper side that then establishes the axis (defines it)
-par 3, nd 6 in anterior
1 and 2 in posterior
Partition defective genes
-Maternal effect lethal mutations: occur in c elegans
-earliest defecgs observed as disturbances in te assymetry of the first mitotic division. Rather than making an unequal division, the mutants made the first division symmetrical
-all 6 par genes are required for the assymetric division of zygote
-these par genes have other functions yet they are incredibly important for polarity establishments.
in early zygotes:
-par 3 and 6: anterior cortex (become enriched here during one cell stage)
-par 1 and 2: enriched in posterior cortex
-par 4 and 5:remain symmetrically localized thru this period, both cortical and cytoplasmic
When do they adopt assymetric localization patters:
Post fertilization
When does blebbing occur
(pseudocleavage)
-before fusion of male pronucleus to female pronucleus
-cortical cytoplasm flows away from smooth domain at posterior
What does CYK-4 affect
the actin based cytoskeleton
Does actin mysoin network move away or toward the site of fertilization
away
After the cortical flow, where is the concenration of 1) f actin, b) myosin 2 c) par 3 d)par 2
a) anterior
b) anterior
c) anterior
d) posterior
Where does par 2 accumulate if there is is a meotic arrest phenotype (failure to complete meiosis after fertilization)
near the arrested meiortic microtuble spindle (on oocyte side)
