The Worm Caenorhabditis elegans

1
The WormCaenorhabditis elegans
2008-09 DB3002

• Jonathan Pettitt

2
C. elegans as a model organism

• Advantages

• Disadvantages

• Simple anatomy
• Comprehensive description of development at
  single cell resolution
• all cells visible throughout lifecycle
• Ease of genetic analysis
• Mutant screening is rapid and can be automated in
  some cases
• RNA interference is highly effective

• Simple anatomy
• Small size and egg shell make embryo difficult to
  study by microsurgery
• But it can be done!
• No cell lines for cell culture analysis

3
Adult anatomy
4
Him and Her

• Males arise via X-chromosome non-disjunction
  (they are XO hermaphrodites are XX)
• This occurs naturally at a low rate (0.1-0.2),
  but can be increased by certain muations (high
  incidence of males him mutants)

5
Lifecycle
6
Embryonic Development
7
Embryonic Development
8
The Cell Lineage and The Nobel Prize
9
1 cell
959 cells
10
A cell lineage of the early embryo
fertilised egg
t i m e
P1
AB
EMS
P2
ABp
ABa
11
The Cell Lineage
12
Early embryogenesis of C. elegans the founder
cells
13
Polarity in the early C. elegans embryo

• sperm entry determines A-P axis, by providing
  centrosomes
• the maternal and paternal pronuclei meet and
  migrate to midpoint of A-P axis
• p-granules become localised to posterior
• mitotic spindle becomes displaced posteriorly
• embryo divides unequally to produce AB and P1

14
Polarity in the early C. elegans embryo

• sperm entry determines A-P axis, by providing
  centrosomes
• the maternal and paternal pronuclei meet and
  migrate to midpoint of A-P axis
• p-granules become localised to posterior
• mitotic spindle becomes displaced posteriorly
• embryo divides unequally to produce AB and P1

15
Polarity in the early C. elegans embryo

• sperm entry determines A-P axis, by providing
  centrosomes
• the maternal and paternal pronuclei meet and
  migrate to midpoint of A-P axis
• p-granules become localised to posterior
• mitotic spindle becomes displaced posteriorly
• embryo divides unequally to produce AB and P1

16
Polarity in the early C. elegans embryo

• sperm entry determines A-P axis, by providing
  centrosomes
• the maternal and paternal pronuclei meet and
  migrate to midpoint of A-P axis
• p-granules become localised to posterior
• mitotic spindle becomes displaced posteriorly
• embryo divides unequally to produce AB and P1

17

• Anterior-Posterior Axis
• Sperm entry point determines future
  anterior-posterior axis
• P granule localisation correlates with
• posterior of the embryo
• germline cell fate
• Germline, P cells express PIE-1 TF

18
Polarity in the early C. elegans embryo

• sperm entry determines A-P axis, by providing
  centrosomes
• the maternal and paternal pronuclei meet and
  migrate to midpoint of A-P axis
• p-granules become localised to posterior
• mitotic spindle becomes displaced posteriorly
• embryo divides unequally to produce AB and P1

19
Germline cell fate determination during early
embryogenesis

• At each division of Pn cells, one daughter
  becomes a somatic cell, the other, posterior
  daughter inherits P-granules

20
Maternal-effect

• Most early gene products are provided entirely by
  the mother
• Mutations in these genes show a strict
  maternal-effect
• homozygous mutant progeny of heterozygote mothers
  are viable
• progeny of homozygous mutant mothers all arrest
  development

21
Maternal effect mutations
m
m

m
All progeny viable
All progeny arrested as embryos
22
The Par mutants

• Kemphues et al screened for maternal effect
  embryonic lethal mutations
• Identified par (partioning defective) mutants
  which show defects in AB and P1 cell fates

F2
F3
23
par-3, par-6, pkc-3 mutants

• Mutant Phenotype
• AB spindle same orientation as P1
• In absence of PAR-3, PAR-6 or PKC-3 both cells
  behave like P1
• All three proteins localise to AB cell

wild-type
par-3 mutant
24
par-1 mutants
par-2 mutants

• Normal AB and P1 cells
• P granules fail to segregate to P1

• both cells same size
• P1 spindle same orientation as AB
• Both cells behave like AB cells

WT
par-2
par-1
25
Polarity in the early embryo is controlled by the
PAR proteins

• PAR-3,-6 and PKC-3 define the anterior region of
  the cell
• PAR-2 defines the posterior region of the cell
• PAR-3 localisation is dependent upon PAR-2
  function, and vice-versa

PAR-3
PAR-2
26
MEX-5/-6 regulate cell fate in the early embryo

• mex-5 and mex-6 encode almost identical proteins
• mex-5/-6 double mutants misexpress posterior
  proteins in anterior cells

27
PAR-3, PAR-2 and PIE-1
PAR-3
PAR-2
28
MEX-5 expression

• MEX-5 expression is highest in the somatic
  daughter of Pn at each cell division, i.e. the
  reciprocal of germline proteins

29
Summary Model

• In response to sperm-entry point PAR proteins
  ensure the proper segregation of MEX-5/6 proteins
  and thereby ensure proper partitioning of cell
  fate