Model organisms: C. elegans Very small--can keep 10,000 worms on a Petri dish! Short generation time: 3 days Embryo is transparent and develops outside the body in 14 hours!! - can watch with a microscope Hermaphrodites (XX)- No need for males! (but there are males for crosses) The adult has 959 (XX) or 1031 (XO) cells, and the lineage of each cell is known very cheap to keep Small, sequenced genome = 100 Mb

Life as a worm-- the nematode C. elegans

Hermaphrodites do it by themselves Hermaphrodite: XX Male: XO

The C. elegans reproductive system Figures\Chapter08\DevBio7e08420.jpg Fig. 8.42

Early Cleavage events in C. elegans Figures\Chapter08\DevBio7e08431.jpg This is one representation of a lineage Fig. 8.42

allowing one to follow the cell lineage. An entire C. elegans hermaphrodite worm consists of exactly 959 cells EVERY SINGLE TIME, allowing one to follow the cell lineage. Another representation of the cell lineage- family trees, cellular style Your Family Tree Funny Bone Neuron Family Tree Great grandma Fertilized egg Granny stem cell Mom Neuroblast Cell death RIP muscle Auntie cell Em Nose Funny bone neuron Sis Me neuron

the secret of embryonic development Within this lineage is the secret of embryonic development John Sulston

All neural synapses have been mapped

New biology career option, worm geneologist. Learn to read a lineage diagram! Branching = Increasing cell division age of worm embryo 1st stage larva 2nd stage larva Line ending = differentiated cell = Cell death

Most lineages do not consist of single tissue types but the germline and the gut both arise from single founder cells Fig. 8.42

Even cell death is programmed into the lineage C. elegans was used to identify the machinery that regulates programmed cell death in ALL animals

The Nobel Prize in Physiology or Medicine 2002 "for their discoveries concerning ’ genetic regulation of organ development and programmed cell death'" Sidney Brenner H. Robert Horvitz John Sulston

One mechanism is through asymmetric segregation of determinants How can lineage control cell fate? One mechanism is through asymmetric segregation of determinants

segregated into one cell (P4) at the 16-cell stage. A determinant within the P granules is asymmetrically segregated into one cell (P4) at the 16-cell stage. The P4 cell is the progenitor of the germline! DNA P granules Fig. 8.44

In partition (par) mutants P granules are found in ALL daughter cells wildtype par-3 mutant

New biology career option, worm geneologist. Learn to read a lineage diagram! Branching = Increasing cell division age of worm embryo 1st stage larva 2nd stage larva Line ending = differentiated cell = Cell death

Mutations can alter lineages in many ways

Changes in the pattern of cell division Example #1- lin-22 mutant Changes in the pattern of cell division

Changes in the pattern of cell division: division asymmetry mutant Example #1- lin-22 mutant Changes in the pattern of cell division: division asymmetry mutant

Changes in the pattern of cell division Example #1- lin-22 mutant Changes in the pattern of cell division Lin-22 is homologous to the Drosophila pair-rule gene Hairy

Anterior-Posterior Pattern Formation in Drosophila Maternal effect genes ie. Even-skipped, Fushi tarazu, Hairy, Sloppy-paired Figures\Chapter09\DevBio7e09081.jpg Figure 9.17

Changes in the timing of cell division Example #2- lin-14 mutant Changes in the timing of cell division L1 L1 L2 L1 L1 L1

Changes in the timing of cell division Example #2- lin-14 mutant Changes in the timing of cell division L1 L2 L1 L2 L1 L1 L1 LIN-14 is a transcription factor

Cell-cell interactions are also important! How can lineage control cell fate? Bob Goldstein Signal from P2 cell required to induce EMS cell to produce E cell which forms the gut (see p. 248) Cell-cell interactions are also important!

The vulva provides a great model for how cell interactions shape cell lineage! Hermaphrodite: XX Male: XO Vulva

What happens if worm can’t lay eggs? vulvaless mutants: “bag of worms”

Making a vulva is a complicated business. anchor cell gonad 3° cell 3° cell 2° cell 1° cell 2° cell 3° cell Normal Normal hypoderm hypoderm = skin = skin Make vulva

C. elegans Vulval Precursor Cells and Their Descendants Figures\Chapter06\DevBio7e06191.jpg Figure 6.27

Cell ablation studies helped define the key players Experiment #1

All cells are created equal (or, the road to fame is paved with dead bodies) gonad anchor cell 3° cell 3° cell 2° cell 1° cell 2° cell 3° cell Normal hypoderm Normal hypoderm = skin Normally the central 3 cells = skin make the vulva Experiment #2 However, if you kill these cells with a laser anchor cell gonad 2° cell 1° cell dead! dead! dead! 2° cell the outside 3 cells take over and make the vulva

Genetic analysis of vulva formation: vulvaless mutants multivulval mutants Experiment #3

If anchor cell signaling is disrupted, all cells adopt 3° fate. vulvaless mutants: If anchor cell signaling is disrupted, all cells adopt 3° fate. anchor cell gonad 3° cell 3° cell 3° cell 3° cell 3° cell 3° cell no vulva

multivulval mutants: the consequences of not saying no Step 1- anchor cell signal reaches 3 central cells normally gonad anchor cell 1°/2° cell 3° cell If inhibitory signal to neighbors is blocked, all 3 central cells adopt 1° fate anchor cell gonad 3° cell 3° cell 1° cell 1° cell 1° cell 3° cell Extra vulvas

It takes two steps to make the vulva Step 1- Anchor cell signal reaches 3 central cells anchor cell gonad 3° cell 3° cell 1°/2° cell 1°/2° cell 1°/2° cell 3° cell Step 2- Central cell sends inhibitory signal to neighbors anchor cell gonad 3° cell 3° cell 2° cell 1° cell 2° cell 3° cell

The vulvaless mutations helped define the RTK-Ras pathway, which is abnormally activated in about half of all human tumors (VPC cells) signal

It takes two steps to make the vulva Step 1- Anchor cell signal reaches 3 central cells anchor cell gonad 3° cell 3° cell 1°/2° cell 1°/2° cell 1°/2° cell 3° cell Step 2- Central cell sends inhibitory signal to neighbors anchor cell gonad 3° cell 3° cell 2° cell 1° cell 2° cell 3° cell

Does this remind you of anything we learned earlier?

The story of epidermal vs. neuronal fate in Drosophila If signal is missing... Some cells become neuroblasts and signal their neighbors to remain epidermis all cells eventually ingress and become neuroblasts Nervous system Extra nervous system Epidermis No epidermis!

The C. elegans vulval precursor cells and their descendants Figures\Chapter06\DevBio7e06192.jpg Figure 6.27

Generation of Different Cell Types From Equivalent Cells in C Generation of Different Cell Types From Equivalent Cells in C. elegans: Initial specification of the Anchor Cell also requires lin-12 lag-2 (delta) lin-12 (notch) Figures\Chapter06\DevBio7e06300.jpg Figure 6.28