CSIR NET Life Sciences Developmental Biology Syllabus

Name the molecules mediating sperm-egg recognition in sea urchin and in mammals.

Sea urchin: bindin on the sperm binds species-specific bindin receptor (EBR1) on the egg vitelline layer. Mammals: sperm proteins (e.g. Izumo1) bind egg receptor Juno; the zona pellucida glycoprotein ZP3 acts as the primary sperm receptor and triggers the acrosome reaction.

What are the fast and slow blocks to polyspermy?

Fast block: rapid membrane depolarization of the egg (Na+ influx) within seconds prevents additional sperm fusion (in sea urchin). Slow block (cortical reaction): Ca2+ wave triggers cortical granule exocytosis, raising the fertilization envelope/hardening the zona (zona reaction, ZP2 cleavage), permanently blocking polyspermy.

Describe embryo sac (female gametophyte) development in angiosperms (Polygonum type).

A megaspore mother cell undergoes meiosis to give 4 megaspores; 3 degenerate. The functional megaspore undergoes 3 mitotic divisions producing an 8-nucleate, 7-celled embryo sac: 1 egg + 2 synergids (egg apparatus at micropylar end), 3 antipodal cells (chalazal end), and 1 central cell with 2 polar nuclei.

Explain 'double fertilization' in flowering plants and its two products.

Two sperm cells delivered by the pollen tube: one fuses with the egg to form the diploid (2n) zygote; the other fuses with the two polar nuclei of the central cell to form the triploid (3n) primary endosperm nucleus, which develops into the nutritive endosperm.

What molecular and structural events define zygote formation (egg activation) immediately after fertilization?

Sperm-egg fusion triggers a Ca2+ release wave that activates the egg: completion of meiosis, cortical reaction, increased protein synthesis and metabolism, fusion of male and female pronuclei (karyogamy/amphimixis) to restore diploidy, and initiation of DNA replication for first cleavage.

What is cleavage and how does it differ from ordinary cell division?

Cleavage is a rapid series of mitotic divisions of the zygote with NO growth between divisions (G1 and G2 phases reduced/absent). It partitions the large egg cytoplasm into many smaller cells (blastomeres) without increasing total embryo size, increasing the nucleo-cytoplasmic ratio.

How does yolk amount affect cleavage pattern? Define holoblastic and meroblastic cleavage.

Yolk impedes cleavage. Holoblastic (complete) cleavage occurs in eggs with little yolk (isolecithal—mammals, sea urchin; moderately telolecithal—amphibians). Meroblastic (incomplete) cleavage occurs in yolk-rich eggs: discoidal in heavily telolecithal eggs (birds, fish, reptiles) and superficial in centrolecithal eggs (insects).

Define radial, spiral, and rotational cleavage with an example each.

Radial: blastomeres aligned directly above/below (deuterostomes—echinoderms, amphibians). Spiral: blastomeres offset at oblique angles due to tilted spindles (protostomes—molluscs, annelids). Rotational: first division meridional, second one cell meridional and the other equatorial (mammals).

What is a blastula and what is the blastocoel?

A blastula is the hollow ball of cells (blastomeres) formed at the end of cleavage. The blastocoel is the fluid-filled cavity within it. In mammals the equivalent is the blastocyst, with an inner cell mass and an outer trophoblast surrounding the blastocoel.

What is an 'embryonic field' (morphogenetic field)?

An embryonic field is a group of cells whose position and developmental fate are coordinately specified, behaving as a self-regulating unit that can form a complete structure (e.g. limb field, eye field) even if part is removed or rearranged—exhibiting regulation.

Define gastrulation and the three germ layers it establishes.

Gastrulation is the coordinated cell movement that converts the blastula into a multilayered gastrula, establishing the three germ layers: ectoderm (outer—skin, nervous system), mesoderm (middle—muscle, skeleton, blood, kidney), and endoderm (inner—gut lining, liver, lungs).

Name the major morphogenetic movements of gastrulation.

Invagination, involution, ingression, epiboly, delamination, and convergent extension. These movements bring presumptive endoderm and mesoderm inside and spread ectoderm over the surface.

What is the Spemann-Mangold organizer and what did the experiment show?

The dorsal lip of the blastopore (Spemann's organizer) in amphibians, when transplanted to the ventral side of a host gastrula, induced a complete secondary body axis (a twinned embryo). It demonstrated that the organizer is the primary inductive center patterning the dorso-ventral and antero-posterior axes.

Define 'embryogenesis' and contrast it with organogenesis.

Embryogenesis is the entire process from zygote to fully formed embryo—covering cleavage, blastula, gastrulation and early body plan formation. Organogenesis is the subsequent phase in which the germ layers interact to form specific organs and organ systems.

How is apical-basal polarity (symmetry) first established in the plant embryo?

The first (asymmetric) division of the zygote produces a small apical cell (gives rise to most of the embryo proper/shoot) and a large basal cell (gives rise to the suspensor and the root pole/hypophysis). Polar auxin transport via PIN proteins establishes the apical-basal axis and positions the root.

Outline the stages of dicot embryo development after the zygote.

Zygote → asymmetric division → proembryo → globular stage → heart stage (cotyledon primordia appear, radial symmetry) → torpedo stage (elongation) → mature embryo with cotyledons, shoot apical meristem, hypocotyl, and radicle/root apical meristem.

What events occur during seed formation and what is dormancy?

After double fertilization the ovule matures into the seed: the zygote forms the embryo, the central cell forms the endosperm (food reserve), and the integuments harden into the seed coat (testa). The seed dehydrates and often enters dormancy—a metabolically arrested state allowing survival until conditions favor germination.

Distinguish epigeal from hypogeal germination.

Epigeal germination: hypocotyl elongates and pushes the cotyledons above the soil (e.g. bean, castor). Hypogeal germination: epicotyl elongates while cotyledons remain below the soil surface (e.g. pea, maize). Germination begins with imbibition of water.

Describe cell aggregation and differentiation in Dictyostelium discoideum.

Upon starvation, amoebae secrete cyclic AMP (cAMP) as a chemoattractant; cells stream and aggregate toward cAMP pulses. The aggregate forms a motile slug (pseudoplasmodium) which culminates into a fruiting body: anterior prestalk cells form the dead stalk and posterior prespore cells form viable spores.

What signal organizes Dictyostelium aggregation and what determines prestalk vs prespore fate?

Pulsatile cAMP relayed cell-to-cell organizes chemotactic aggregation. Cell fate is influenced by DIF-1 (differentiation-inducing factor, promotes prestalk/stalk) and cAMP (promotes prespore), with position in the slug correlating to fate.

In Drosophila, name the maternal-effect genes that set up the anterior and posterior poles.

Anterior is set by bicoid (Bicoid protein gradient, a transcription factor, high anterior) and posterior by nanos (Nanos represses maternal hunchback). caudal and hunchback also contribute. These maternal gradients establish the antero-posterior axis.

List the hierarchy of zygotic segmentation genes in Drosophila in order of action.

Gap genes (e.g. hunchback, Krüppel, knirps—broad regions) → pair-rule genes (e.g. even-skipped, fushi tarazu—alternate segments/7 stripes) → segment polarity genes (e.g. engrailed, wingless, hedgehog—anterior/posterior of each segment). Then homeotic (Hox) genes specify segment identity.

What do Drosophila homeotic (Hox) genes do, and name the two complexes?

Hox genes specify the identity of each segment along the A-P axis; mutations cause homeotic transformations (e.g. Antennapedia—legs in place of antennae; bithorax—extra wings). The two complexes are the Antennapedia complex (ANT-C) and the Bithorax complex (BX-C), together the HOM-C, showing spatial colinearity.

How is dorso-ventral polarity established in the Xenopus embryo?

Sperm entry triggers ~30° cortical rotation, moving dorsal determinants (Wnt pathway components stabilizing β-catenin) to the future dorsal side opposite the sperm entry point. This creates the Nieuwkoop center, which induces the dorsal organizer; the gray crescent marks the dorsal region.

How is the anterior-posterior axis established in the chick embryo, and what is Hensen's node?

In the chick, gravity and the primitive streak set the A-P axis; the streak elongates from posterior to anterior. Hensen's node, at the anterior tip of the primitive streak, is the avian/amniote equivalent of the Spemann organizer, directing gastrulation and axis induction.

What is organogenesis and give an example of an organ formed by epithelial-mesenchymal induction.

Organogenesis is the formation of organs from the three germ layers via inductive tissue interactions, proliferation, migration and differentiation. Example: vertebrate limb (AER/ZPA interactions), kidney (ureteric bud induces metanephric mesenchyme), and lens (optic vesicle induces overlying ectoderm).

Describe the organization (zonation) of the shoot apical meristem (SAM).

The SAM has cytohistological zones: a central zone (slowly dividing stem cells, maintained by WUSCHEL-CLAVATA feedback), a peripheral zone (rapidly dividing cells giving rise to lateral organs), and a rib zone (forms central stem tissue). It also has tunica (surface layers, anticlinal divisions) and corpus (inner cells).

Explain the WUS-CLV feedback loop maintaining the shoot apical meristem.

WUSCHEL (WUS) is expressed in the organizing center and moves to the central zone to maintain stem cell identity. Stem cells express CLAVATA3 (CLV3), a peptide that signals through CLV1/CLV2 receptors to restrict WUS expression. This negative feedback keeps the stem cell pool size constant.

Contrast the organization of the root apical meristem (RAM) with the SAM, and define the quiescent center.

The RAM contains the quiescent center (QC)—mitotically inactive cells that maintain surrounding stem cells (initials) and prevent their differentiation. Unlike the SAM, the RAM produces cells in two directions: distally the root cap (protective) and proximally the root body tissues. SCR/SHR and PLT genes pattern the RAM.

Define phyllotaxy and explain how leaf primordium positioning is controlled.

Phyllotaxy is the arrangement of leaves around the stem (alternate, opposite, whorled, or spiral—often following the Fibonacci angle ~137.5°). Positioning is governed by auxin maxima created by PIN1-mediated polar auxin transport: a new primordium forms at the auxin maximum, depleting auxin nearby and spacing successive primordia.

Describe leaf development including establishment of adaxial-abaxial polarity.

A leaf primordium arises from the SAM peripheral zone and grows along proximo-distal, medio-lateral, and adaxial-abaxial axes. Adaxial (upper) identity is specified by HD-ZIPIII genes (e.g. PHABULOSA) and abaxial (lower) by KANADI and YABBY genes plus miR165/166; their juxtaposition is required for lamina (blade) outgrowth.

What is the 'transition to flowering' and which major pathways control it?

It is the switch of the SAM from vegetative growth to forming an inflorescence/floral meristem. Controlled by integrating pathways: photoperiod (CONSTANS→FT 'florigen' from leaves), vernalization (cold represses FLC), gibberellin, autonomous, and age (miR156/SPL) pathways, converging on floral integrators FT and SOC1 and meristem identity gene LEAFY.

State the ABC model of floral organ identity in Arabidopsis.

Floral organ identity is specified by combinatorial homeotic gene action across four whorls: A alone → sepals; A+B → petals; B+C → stamens; C alone → carpels. A and C are mutually antagonistic. In Arabidopsis: A = APETALA1/APETALA2; B = APETALA3/PISTILLATA; C = AGAMOUS (the ABC(E) genes are mostly MADS-box).

How does the ABC model in Antirrhinum compare with Arabidopsis, and what do classes E (and D) add?

Antirrhinum (snapdragon) uses orthologous genes: B = DEFICIENS/GLOBOSA, C = PLENA; the same A+B+C logic applies. Class E (SEPALLATA, MADS-box) proteins are required together with A/B/C to form all organ types (the ABCE 'quartet' model), and class D (e.g. SEEDSTICK) specifies ovule identity.