Zoology Lesson: The early development of C. elegans Lesson Developer: Dr.Meena Yadav College/Dept: Maitreyi College University of Delhi

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The early development of C. elegans

Table of Contents

 Introduction  Cleavage in C. elegans  Anterior-posterior axis formation in C. elegans  Dorsal-ventral and left-right axes in C. elegans  Fate of blastomeres after first cleavage (experimental proof)  Autonomy of P1 cell  Gene expression in early development  Fate of cells in C. elegans embryo subject to specific conditions  Gastrulation in C. elegans  Summary  Exercise/Practice  Glossary  References/Bibliography/Further Reading

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The early development of C. elegans

INTRODUCTION Caenorhabditis elegans (C. elegans) is a free living, transparent nematode that lives in the soil of temperate regions. It is an unsegmented pseudocoelomate that lacks respiratory and circulatory system. The majority of population consists of female hermaphrodite (Figure 1). There is a specialized tail in males that is used for mating. The tail contains spicules. The major anatomy of C. elegans includes pharynx, intestine, mouth, gonad and cuticle (Figure 2).

Figure 1: C. elegans hermaphrodite Source:

http://en.wikipedia.org/wiki/Caenorhabditis_elegans

CC

There are have XX chromosomes while the rare males that occur in the population have XO five pairs of autosomes and one pair of sex chromosome in C. elegans. Hermaphrodites chromosomes. Sydney Brenner (1974) organized a research programme to identify a suitable organism in which the expression of the genes during development could be studied. The researchers identified and agreed that a small, free living nematode (1 mm long) Ceanorhabditis elegans is a suitable candidate for such studies. The reasons for its selection are: 2 Institute of Life Long Learning, University of Delhi

The early development of C. elegans i.

Rapid period of embryogenesis i.e. 16 hours

ii.

Can be cultured in a petri-dish

iii.

Its body has relatively few cell types

iv.

The adult is hermaphrodite and can produce both eggs and sperm

v.

They can either self-fertilize or can cross-fertilize with infrequently occurring males

(Figure 2).

Value Addition: Do you know? Heading Text: The morphology of C. elegans Body Text:

A) Adult hermaphrodite: thin arrow shows vulva at mid body B) Alimentary canal opens to outside through anus C) Lateral side of the body showing annuli and furrows D) An egg being expelled from vulva (arrow) E) Excretory pore at ventral midline of head 3 Institute of Life Long Learning, University of Delhi

The early development of C. elegans

Caenorhabditis

elegans

is

a

free-living

transparent

nematode

(roundworm), about 1 mm in length that lives in temperate soil environments. It is unsegmented, vermiform, and bilaterally symmetrical. It has a cuticle (a tough outer covering), four main epidermal cords, and a fluid-filled pseudocoelom (body cavity).It lacks a respiratory and a circulatory system. The basic anatomy includes a mouth, pharynx, intestine, gonad, and collagenous cuticle The four bands of muscles that run the length of the body are connected to a neural system that allows the muscles to move the animal's body only in the forward direction. They possess gut granules which emit a brilliant blue fluorescence, a wave of which is seen at death in a 'death fluorescence' The majority of these nematodes are female hermaphrodites. Males have specialized tails for mating that include spicules

.

EMBED GIF FILE Video: Movement of wild-type C. elegans

Source: https://en.wikipedia.org/wiki/Caenorhabditis_elegans http://www.wormatlas.org/ver1/handbook/anatomyintro/anatomyintro.htm

The body of the adult C. elegans contains only 959 somatic cells. The entire lineage of its cells can be traced from its transparent cuticle. The cell lineages in C. elegans are almost the same in all individuals unlike vertebrates where there is a lot of diversity from individual to individual. C. elegans has 19000 genes and its genome has been completely sequenced. It was the first multicellular organism whose genome was sequenced.

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The early development of C. elegans

Figure 2: Anatomy of C. elegans Source: http://en.wikipedia.org/wiki/Caenorhabditis_elegans CC In their early life the C. elegans are male and produce sperm which they store for later use. Once, they grow old, they develop ovaries which produce ova. The ova when released pass through the region where sperm are stored and get fertilized. They then are released from the nematode body via vulva. According to the developmental pattern seen in C. elegans, exactly 131 cells die. All the cells in this nematode are programmed for death by default. But, if they are signaled to not undergo apoptosis, they will survive longer.

Cleavage in C. elegans The zygote of C. elegans exhibits rotational holoblastic cleavage. The early cleavage divisions are asymmetrical producing two types of cells: one type is called founder cell (which produces cell types like MS, E, C, A, B and D) and the other type is called as stem cell (that produce lineages of P cell). The founder cells produce differentiated descendents (Figure 3). The first cleavage is asymmetrical with the cleavage furrow lying closer towards the posterior axis in the anterior-posterior direction. It forms a founder cell (AB), at the future anterior side, and a stem cell (P1) on the opposite side. In the second division, the founder cell divides equatorially i.e. perpendicular to the anterior-posterior axis. The stem cell divides meridionally to again produce a founder cell (EMS) towards anterior side and a stem cell (P2) towards posterior side. The stem cells with every further division produce a founder cell towards anterior side and a stem cell towards the opposite side. The descendents 5 Institute of Life Long Learning, University of Delhi

The early development of C. elegans of the founder cells divide at specific times which are identical in all individuals. Thus, the newly hatched larva contains exactly 558 cells. The descendents of the founder cells can be observed conspicuously through the cuticle and even counted.

Figure 3: The first cleavage in the zygote of C. elegans Source: Author EMBED VIDEO 2

Video file: Early cleavage in C. elegans Source: http://en.wikipedia.org/wiki/File:Epigenetic-Regulation-of-Histone-H3Serine-10-Phosphorylation-Status-by-HCF-1-Proteins-in-C.-pone.0001213.s003.ogv CC

Anterior-posterior axis formation in C. elegans The signal of the formation of anterior-posterior axis in C. elegans is first seen at the first cleavage itself. With further divisions, the elongated body of the nematode, itself defines the anterior-posterior axis. The position of the sperm nucleus in the zygote determines the anterior-posterior axis in C. elegans. When the sperm enters the oocytes, the centriole, which it carries with itself, starts cytoplasmic movements that carry the sperm nucleus near one pole of the oocytes. This pole becomes the future posterior end of the nematode body. The formation of the anterior-posterior axis also depends on the migration of the P-granules immediately after fertilization. Strome and Wood used fluorescent labeled antibodies against a region of the P-granules, to localize their movement in the egg cytoplasm and also observed their distribution in the cytoplasm after fertilization. They observed that the Pgranules were localized at the posterior end of the zygote and entered only the blastomeres (P1) formed from posterior cytoplasm.

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The early development of C. elegans

Value Addition: Do you know? Heading Text: Radial and Spiral cleavage Body Text:

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The early development of C. elegans

Source: http://cronodon.com/images/radial_cleavage.jpg http://cronodon.com/images/spiral_cleavage.jpg

The P-granules of the P1 cell remain in the posterior end of the cell and thus are passed to the P2 cell. When the P2 cell divides, the P-granules associate themselves with the nucleus and are passed along with to the P3 cell. The P-granules when get inside the P4 cells, the division of P4 cell produces two cells which becomes eggs and sperm respectively (Figure 4). The movement of the P-granules, as shown by Strome and Woods, requires microfilaments, but they can move even in the absence of these microfilaments. Two experiments showed that microfilaments are needed for proper localization of P-granules in the oocytes cytoplasm:

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The early development of C. elegans i)

When the zygote is treated with cytochalasin D, which is a microfilament inhibitor,

the P-granules do not segregate at the posterior end. ii)

If the zygote is exposed to demecolcine, which is a colchicine like inhibitor of

microtubules, the segregation of P-granules takes place. The par genes i.e. partition defective genes, are expressed in the cortex of the embryo and their protein product interact with the actin filaments. If the mothers are deficient in any of the par genes, then the partitioning of the P-granules and orientation of the mitotic spindles are deficient in such embryos.

Value Addition: Do you know? Heading Text: Variant staining with antibodies against C. elegans Pgranules Body Text: a) C. elegans at 2-cell stage. P granules are only present in germline cells b) A. nanus: 2-cell ambryo. Strong perinuclear staining in the germ line cell P1. c) Rhabditis terricola. 8-cell stage, intranuclear staining in all blastomeres.

Source: http://www.wormbook.org/chapters/www_embryovariationdevelop/embryovariationdevelop.html

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The early development of C. elegans

Figure 4: Distribution of P-granules in wild type C. elegans Source: http://dev.biologists.org/content/128/21/4301/F7.expansion.html CC

Dorsal-ventral and left-right axis in C. elegans The foundation of the dorsal-ventral axis can be seen in the AB cell. When the AB cell divides, it becomes longer than the width of the eggshell. This arrangement causes the daughter cells to slide such that one AB daughter cell (ABa) lies on the anterior side and the other daughter cell (ABp) lies on the posterior side. The ABp cell lies above the EMS cell which is formed by the division of the P1 cell (Figure 5). The future dorsal side is defined by the ABp cell while the ventral side by the EMS cell. The EMS cell is the precursor of the muscle and gut cells. The left-right side of the adult body is defined at the 12 cell-stage embryo. These sides are defined when the MS blastomere, produced by the division of the EMS cells, contacts the half of the cells derived from the ABa cell.

Figure 5: Second cleavage division in C. elegans Source: Author



Fate of blastomeres after first cleavage (experimental proof)

What happens if the two blastomeres are separated after the first cleavage division? The answer was provided by Priess and Thomson (1987). When these two blastomeres were separated, the P1 cell divided autonomously in the absence of the AB cell and produced the posterior half of the body. But the AB cell, although could divide, but could produce only few cell types. For example, the ABa blastomere, which normally makes the anterior pharyngeal muscles in a normal embryo, failed to produce them. Thus, AB cell descendants 10 Institute of Life Long Learning, University of Delhi

The early development of C. elegans cannot develop autonomously and they need the presence of P1 cells and their descendents to interact with to produce a complete embryo and then adult.



AUTONOMY OF P1 CELL

The P1 cell can divide autonomously to produce its descendents. It does not need interaction with neighboring cells. However, the fates of the resulting cells are determined by the cytoplasmic factors. The speculation is that certain proteins from the cytoplasm enter the nucleus of the P1 cell descendents and activate or suppress expression of specific genes. Pgranules only act as factors determining the anterior-posterior axis but they do not enter the nucleus and their other roles, if any in the embryonic development, are not known. The P1 descendant cells are somatic founders and are named as MS, E, C and D. The fates of these cells are determined or regulated by the transcription factors, which are proteins like SKN-1, PAL-1 and PIE-1.

Value Addition: Do you know Heading Text: Phenotypes of the mutants defective in P-granule degradation in the soma Body Text: Blue: The CCCH finger proteins (PIE-1, POS-1, MEX-1). These proteins transiently localize on P granules and are also dispersed in the germ cell cytoplasm in the wild type. In zif-1(RNAi) embryos, their degradation in the soma is defective (asymmetry of PIE-1 in the one-cell embryo is unaffected). Green: PGL-1 and PGL-3, constitutive components of P granules. In the wild type, their localization is limited to P granules in the germ lineage. In autophagy mutants (for example, lgg-1), ectopic PGL granules are formed in somatic cells after about the 16-cell stage. In the sepa-1 mutants, PGL1 and PGL-3 are dispersed in the somatic cytoplasm.

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The early development of C. elegans

Source: http://f1000.com/prime/reports/b/1/49/fig-001

 1.

GENE EXPRESSION IN EARLY DEVELOPMENT Of the three proteins expressed early in the development, the SKN-1 protein has

maternal origin which controls the fate of the EMS cells that are destined to form the posterior pharynx. After the first cleavage, if the P1 cell is isolated, it can autonomously produce pharyngeal cells. Of the daughter cells of P1, only the EMS cell can produce pharyngeal cells autonomously. When the EMS cell divides, only one of the daughter cell i.e. MS has the ability to produce pharyngeal cells in isolation. These results suggest that the fate, that which of the daughter cells will form the pharyngeal cells, is determined by certain maternal factors that are inherited in one cell. Any mutation in the skn-1 gene leads to abnormal development of the pharyngeal cells as discovered by Bowerman et al. The embryos that are derived from mother cells that are homozygous for this mutation lack pharyngeal mesoderm and endoderm derivatives of EMS. Instead, these embryos make extra hypodermal and body wall tissue at the same location where the pharyngeal cells should lie. These mutations only affect the cells that are destined to form pharynx and intestine. The SKN-1 protein has a DNA binding motif similar to those seen in bZip family of transcription factors.

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The early development of C. elegans 2.

Another transcription factor required for differentiation of P1 lineage is PAL-1. Its

activity is required for development of P2 blastomeres. P2 blastomeres produce somatic cells. Any mutation in the PAL-1 gene or abnormality in the function of PAL-1 leads to embryos without somatic cells that are derived from the C and D cells. A protein, MEX-3, regulates the activity of PAL-1. MEX-3 is an RNA binding protein that inhibits the translation of pal-1 mRNA. In the mutants that lack mex-3, PAL-1 is present in all the blastomeres. SKN-1 also inhibits PAL-1, so that EMS cells do not express PAL-1. 3.

The third transcription factor active in early development is PIE-1.

PIE-1 inhibits

both SKN-1 and PAL-1 in the P2 and its descendants that will give rise to germ cells. Any mutation in the pie-1 gene leads to formation of somatic cells even from the germ cell lineages. In such mutants, the P2 cell starts behaving like an EMS cell. PIE-1 is thought to establish somatic cell fate and maintains the totipotent nature of the germ cell lineages.

Value Addition: Do you know? Heading Text: Par genes identify a conserved signaling pathway for cell polarity Body Text: Par-1 encodes a protein kinase that is asymmetrically distributed in the posterior cortex of the fertilized egg. Par-1 has homology to human MARK1 and MARK2, shown to phosphorylate microtubule associated proteins and trigger microtubule disruption (Drewes et al.., 1997). Strong par-1 alleles have defects in kinase domain, indicating that kinase activity is essential for wild type Par-1 function. Defect in par-1 causes equal first cleavage, uniform distribution of P granules, and uniform distribution of transcription factor SKN-1. Par-2 is the only Par protein with no homologues outside the nematodes. Par-2 is also localized to the posterior cortex, and recruits Par-1 to the posterior cortex. Par-3 and Par-6 form

a

complex

with aPKC (atypical

protein

kinase

C)

and CDC42 (small GTPase), localized to the anterior cortex.

Source: http://www.devbio.biology.gatech.edu/?page_id=41

 Fate of the cells in the C. elegans embryo subject to specific conditions

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The early development of C. elegans At the 4-cell stage, the EMS cell is destined to become ancestor of germ cells by interacting with the neighboring P2 cell. The EMS cell gives rise to MS cell and E cell that produce mesodermal muscles and intestinal endoderm respectively. If the neighboring P2 cell is removed at the 4-cell stage, the EMS cell will produce two MS cells only and thus the intestinal endoderm will not be formed. If the P2 cell is isolated and then again put in the embryo, then both MS and E cells will be produced. The conditional differentiation of the EMS cell is limited to P2 cell only, if EMS cell is placed with any of AB cells, it will not produce E cell as seen with P2 cell.

Figure 6: Interaction of EMS cell and P2 cell Source: Author The behavior of EMS cell in association with P2 cell indicates that P2 cell secretes some factors that interact with EMS cell and lead to production of E cell. This message is transmitted by Wnt signaling pathway (Figure 8). The P2 cell produces MOM-2 peptide, which is a homologue of Wnt. The receptor for MOM-2 on the EMS cell is called MOM-5 protein, which is a homologue of Wnt receptor protein Frizzled. The binding of MOM-2 to MOM-5, downregulates the pop-1 gene in the EMS daughter cell, that will produce the E cell. So, mutant embryos, which are pop-1 deficient, produce only E cells after division of EMS cell (Figures 6,7).

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The early development of C. elegans The ABa and ABp cells are also influenced by P2 cell. ABa is a precursor of neurons, hypodermis and the anterior pharynx cell while ABp is a precursor of only neurons and hypodermal cells. If the position of the ABa and ABp cells is exchanged, then also a normal embryo is formed as their fate is also exchanged. This suggests that ABa and ABp are equivalent cells whose fate is fixed by the neighboring cells. Although, both ABa and ABp cells contact EMS cell, but it is only ABp cell that contacts P2 cell. Hence, the fate of both the cells is different. If P2 cell is removed at the 4-cell stage, then ABp cell will not produce its normal descendants. Alternatively, if the ABa cell is placed in contact with P2 cell, it produces the lineage similar to ABp cell.

Figure 7: P2-EMS signaling Source: http://www.wormbook.org/chapters/www_wntsignaling/wntsignaling.html CC 15 Institute of Life Long Learning, University of Delhi

The early development of C. elegans

Figure 8: Wnt signaling pathway Source: http://commons.wikimedia.org/wiki/File:Wnt_signaling_in_biological_signal_trans duction.svg CC Experimental evidence suggests that the interaction between ABp and P2 cell is mediated by GLP-1 protein on ABp cell and APX-1 (anterior pharynx excess) protein on P2 blastomere. In the embryos, whose mothers have mutant glp-1 gene, ABp is transformed into ABa cell. The GLP-1 protein is a member of Notch family proteins, which act as receptors during cell-cell interaction and are highly conserved. This protein is seen in both ABa and ABp cells. The common ligand for GLP-1 and other notch proteins is another cell surface protein called Delta. In C. elegans, the homologue of Delta is APX-1 and is found on P2 cell. The interaction between GLP-1 and APX-1 distinguishes ABp from ABa. Thus, the P2 cell can initiate the dorsal-ventral axis (Figure 9).

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The early development of C. elegans

Figure 9: Interaction of ABp cell and P2 cell Source: Author

Gastrulation in C. elegans In C. elegans, gastrulation starts when the embryo has 24-cells i.e. just after the generation of the P4 cell. The two daughters of the E cell i.e. Ea and Ep, migrate from the ventral side to the centre of the embryo. They are destined to form gut carrying only 20 cells. There is a very small blastocoel at the centre just before the Ea and Ep cells migrate. After their migration and further division, they form a tiny blastopore. After this, the P4 cell migrates through the blasotopore, which is a precursor of the germ cells. The P4 cell settles at a position beneath the gut primordium. The descendents of the MS cell migrate inside from the anterior side of the blastopore while the C and D-derived muscle precursor cells enter from the posterior side. These groups of cells align themselves on the left and right side of the gut (Figure 10). When the Ea and Ep cells migrate inside the embryo, they are surrounded by the six neighboring cells: three MS granddaughters (MSap, MSpa & MSpp), two AB progeny (ABplpa & ABplpp) and P4. In the figure one MS granddaughter (MSxx) and P4 can be seen moving towards each other over the surface of Ea and Ep. Of the 26 cells at the start of the gastrulation, 16 are descendants of AB cell (Figure 11).

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The early development of C. elegans

Figure 10: Gastrulation in C. elegans: A & C: 4-cell embryo (PAR -3 staining in A and PAR-2 staining in C;PAR-3 i.e. red arrows, localizes in entire cortex of somatic cell while PAR-2 localizes in the basolateral surface of somatic cell) B & D: 7-8 cell embryos (PAR-3 in B is more at apical surfaces of somatic cells while PAR-2 is localized at basolateral surfaces) The PAR proteins determine the anterior-posterior axis in the embryo Source: http://www.wormbook.org/chapters/www_gastrulation/gastrulation.html CC After 6 hours of fertilization, the AB descendants that will form the pharynx are brought inside, whereas hypodermal precursor cells i.e. hypoblast, move ventrally by epibolly, finally they thus close the blastopore. After this, in the next 6 hours, the cells organize into organs and the circular embryo stretches to form the worm. The hermaphrodite worm normally has 558 somatic cells. 115 more cells are formed, but they undergo apoptosis. The sexually mature worm is formed after four molts and contains 959 somatic cells along with several sperm and eggs.

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The early development of C. elegans

Figure 11: Gastrulation in C. elegans The Ea and Ep cells are green in color while the neighboring cells are blue in color. The arrows indicate the movement of neighboring cells. The left panel shows lateral view showing P4 and MSX cells. The right panel is the ventral view with Ea and Ep cells sinking in the embryo Source: http://www.wormbook.org/chapters/www_gastrulation/gastrulation.html CC The events of gastrulation happening in an intact embryo can also be studied in a devitellinized embryo, indicating that the vitelline envelope does not have much role in the gastrulation (Figure 12). 19 Institute of Life Long Learning, University of Delhi

The early development of C. elegans

Figure 12: Time-lapse pictures of events during gastrulation A-D: Stages in an intact embryo E-H: Stages of gastrulation in a devitellinized embryo I-P: The events of gastrulation in P1 isolates Asterisks indicate Ea and Ep cells, and arrowheads indicate MSxx and P4. 0 minutes means start of gastrulation movements. Source: http://www.wormbook.org/chapters/www_gastrulation/gastrulation.html Value addition: Videos Heading text: Gastrulation in C. elegans Body text: Watch the movie clips showing the gastrulation in C. elegans Video 1: Time lapse confocal images of live embryo labeled with SynaptoRed to visualize cell membranes. Asterisks label Ea and Ep. Ea and Ep ingress as their neighbors surround them. 20 Institute of Life Long Learning, University of Delhi

The early development of C. elegans Anterior is to the left, posterior to the right. Video 2: Ventral view of of a gastrulating C. elegans embryo, using DIC imaging. As Ea and Ep sink in towards the center of the embryo, their neighbors surround them. Video 3: DIC time-lapse images of, from left to right, an intact embryo, a devitellinized embryo, and the progeny of an isolated P1 cell. Anterior is the the left, posterior to the right. Source: Jen-Yi Lee and Bob Goldstein. (2003) Mechanisms of cell positioning during C. elegans gastrulation Development 130: 307-320 CC

Value addition: Videos Heading

text: A

visual

introduction

to

the

model

roundworm

Caenorhabditis elegans. Body text: C. elegans develops from a single cell, the fertilized egg, to a 558-celled worm in about 14 hours.

The worm that crawls out of its eggshell has a functioning feeding

apparatus, gut, nervous system and muscles.

Watch these videos showing some of the

events that occur along the way. Click the link: http://labs.bio.unc.edu/Goldstein/movies.html Source: http://labs.bio.unc.edu/Goldstein/movies.html ml

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The early development of C. elegans

SUMMARY 1. C. elegans is a small nematode (~1 mm long) and can be easily cultured in petri-dish. 2. The adult is hermaphrodite and produces both sperm and egg 3. The body of adult contains only 959 somatic cells 4. The zygote of C. elegans undergoes rotational holoblastic cleavage. The first cleavage produces a founder cell (AB) that will produce somatic cells only and a stem cell (P1), that will maintain a lineage of stem cell and also produce germ cells. 5. The AB cell further divides to produce ABa cell (anterior) and ABp (posterior) cell. The P1 cell also divides to produce EMS cell (founder cell) and a P2 cell.\ 6. The formation of the anterior-posterior axis depends on the distribution of the P granules in the cells. The P granules normally position themselves in the posterior region of the cytoplasm and thus are inherited in the P1 cell and other such cells. 7. The movement of the P granules in the cytoplasm is assisted by the microtubules and also depends on the expression of the par genes. 8. the dorsal side is determined by the ABp cell and ventral side by the EMS cell. 9. The left-right sides are fixed at the 12-cell stage embryo. 10. When the AB cell and P1 cell were separated after first cleavage, the P1 cell could divide autonomously and produce the posterior half of the body. But the AB cell could only produce few cell types. 11. The P1 descendant cells are somatic cell founders and are called as E, MS, C and D. 12. The three important genes expressed in the early development in C. elegans are – skn-1, pal-1 and pie-1. 13. The interaction of the EMS cell with P2 cell produces E and MS cells. The lack of such interaction will cause division of the EMS cell producing two MS cells only. 14. The P2 cell secretes MOM-2 protein which binds to MOM-5 protein on the surface of EMS cell and lead to production of MS and E cell. 15. The GLP-1 protein on the ABp cell interacts with APX-1 protein on the surface of P2 cell. This interaction determines the dorsal-ventral axis formation 16. The Gastrulation in C. elegans starts at 24-cell embryo. 17. Ea and Ep, daughter cells of E, migrate from ventral side to the center of the embryo. After these cells have migrated inside the embryo, there remains a tiny blastopore. After this, p4 cell migrates through the blastopore, which are precursors of germ cells. 18. The descendants of MS cell migrate from anterior side of the blastopore while the descendants of C and D cells migrate from posterior side of the blastopore. These group of cells align themselves on the right and left side of the gut. 19. After 12 hours of fertilization the cells arrange themselves into the respective organs. 22 Institute of Life Long Learning, University of Delhi

The early development of C. elegans

Exercise/Practice Q.1. Multiple Choice questions a) The size of the adult C. elegans is: i) 2 mm

ii) 5 mm

iii) 1 mm

iv) 3 mm

b) The total number of somatic cells in an adult C. elegans is : i) 876

ii) 959

iii) 654

iv) 1018

c) The zygote of the C. elegans undergoes: i) Spiral cleavage

ii) Radial cleavage

iii) Rotational holoblastic cleavage

iv) Holoblastic cleavage

d) In C. elegans, gastrulation occurs at: i) 12-cell stage

ii) 24-cell

iii) 48-cell stage

iv) 6-cell stage

e) The homologue of protein delta in C. elegans is: i) Wnt

ii) Frizzle

iii) GLP-1

iv) APX-1

Q.2. Fill in the blanks: i)

There are _______________ pairs of autosomes and __________ pairs of sex chromosomes in C. elegans.

ii)

The

XX

worm

in

C.

elegans

is

_______________

while

XO

worm

is

_____________. iii)

The

first

germ

cell

produced

in

C.

elegans

is

_______________________________. iv)

MEX-3 regulates the activity of ___________________ gene.

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The early development of C. elegans v)

__________ and _____________ showed that AB cell needs the presence of P1 to produce a complete embryo.

Q.3. Define the following terms: i) Hermaphrodite

ii) Parthenogenesis

iii) Hypoblast

iv) Blastomere

Q.4. Discuss the anterior-posterior axis formation in C. elegans. Q.5. Give reasons as to why C. elegans is a suitable model organism for studying gene expression. Q.6.

a) Discuss the gastrulation in C. elegans. b) Briefly explain the distribution of P granules in the early development in C. elegans

Q.7. Briefly discuss the fates of cells after 24-cell embryo.

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The early development of C. elegans

Glossary AB cell: It is the first founder cell formed. Cytochalasin D: It is a microfilament inhibitor Demecolcine: It is a colchicine like inhibitor of microtubules EMS cell: It is one of the daughter cells of the P1 cell, which acts as the founder cell. Founder cell: It is one of the cells formed after division of the zygote and is precursor of the differentiated cells in the adult worm. GLP-1 protein: This protein is present on the surface of ABp cell. It interacts with APX-1 on P2 cell and determines anterior-posterior axis. Hermaphrodite: An adult that is capable of producing sperm and eggs in its body. MOM proteins: MOM-2 protein is secreted by P2 cell that interacts with the MOM-5 protein on the EMS cell which acts as the receptor. This interaction leads to division of the EMS cell to produce E and MS cells. P granules: They are ribonucleoprotein particles that have a role in the determination of the anterior-posterior axis of C. elegans. PAL-1: It is a transcription factor and is required for development of P2 blastomeres, which produce somatic cells. MEX-3 protein regulates the activity of pal-1 gene. Par genes: The product of these genes interacts with the actin filaments and helps in the distribution of microfilaments and thus the P granules. They are known as partition defective genes. SKN-1: It is a transcription factor of maternal origin and controls the fate of EMS cells which are destined to produce posterior pharynx. Stem cell: The cells that retain the ability to divide indefinitely Transcription factors: The proteins that regulate the expression of genes by controlling the transcription of the genes.

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The early development of C. elegans

References 1. Gilbert S.F. (2006). Developmental biology. 2. http://www.wormbook.org/chapters/www_gastrulation/gastrulation.html 3. http://en.wikipedia.org/wiki/Caenorhabditis_elegans 4. http://worms.zoology.wisc.edu/pdfs/nance_2002_gast.pdf

Suggested readings 1. Balinsky, B.I. (2008). An introduction to embryology. International Thomson Computer Press. 2. Kalthoff (2000). Analysis of Biological Development, II Edition, McGraw-Hill Professional 3. Gilbert S.F. Developmental Biology. Xth Edition.

Useful Web Links 1. http://www.wormbook.org/chapters/www_wntsignaling/wntsignaling.html 2. http://labs.bio.unc.edu/Goldstein/movies.html

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