Nematodes•early development of Caenorhabditis elegans

9e; Page 243-251

Caenorhabditis elegansmany attributes for studying developmental

biologyCompact genome (approximately same

number of genes as humans) but 3% of the size

Completely sequenced genome (first multicellular organism)

Short life cycle (3 days); good for genetics

Can freeze and thaw them with no apparent effect on mortality. So, can keep genetic

stocks easily.

Completely known cell lineage

Life cycle

3 germ layers, but no body cavity

Unsegmented, cylindrical body shape

Complete cell lineage. This allows highly precise investigation into cellular interactions

and cell fate specification.

Keep marking cells later and later in development with tracers until you only get a

single cell type

Only 959 cells in adults and 558 in larva. The number of cells is invariant!

Germline is set aside at the very first cell division

Eggs fertilized internally, but laid at the 28C stage

A simplified cell lineage, grouped by fate.

Worms have rotational holoblastic cleavage

•Site of sperm entry determines the anterior-posterior axis. Sperm usually enters the opposite side from egg pronucleus (maternal polarity)

rotational means that each daughter cell divides at a 90 degree angle relative to its sister

Par-3

Mex-5

The mother segregates cell fate determinants in response to a spatial cue (sperm entry) to establish a

specific axis of polarity

Par-3

Par-2

The Par proteins (meaning partition deficient) are important proteins for establishing and

maintaining polarity

Mex-5

+CYK protein

Par- 2 domain expands, while Par-3 contracts

Female nucleus begins to migrate toward the male pronucleus

Pseudocleavage

Egg now polarized along the A-P axis

Pronuclei meet in the middle

Par-2 and Par-3 are segregated to cortical cytoplasm of future daughter blastomeres

Cytokinesis completes the segregation of

determinants

Similar partitioning of determinants and asymmetrical division creates founder cells.

Founder cells have a particular cleavage rate and fate.

C. elegans uses both autonomous and conditional cell fate

specification: If blastomeres at first cleavage are separated, AB the fate of AB descendents are

less than their potential.

When separated from the P-lineage ABa blastomeres fail to make pharyngeal

muscle

P-granules determine germ cell fate

P granules are segregated into the lineage that will become the germ line.

At each cell division the P granules are segregated

into only one of the daughter cells.

Microfilament but not microtubule inhibitors

prevent segregation of P granules

Gastrulation occurs at the 24 cell stage

E cells move in ventrally first (2), then the P4 (1), MS cells migrate in from the anterior and

CD cells move in from the posterior. very unusual!

Compare urchins to worms

Recall, that dorsal ventral polarity determined by early cell interactions

L-R axis is also determined by cell interactions.

Flies are next!