#embryogenesis

C-Elegans Development-l

Which of the following determines the dorsal-ventral axis in C. elegans?

Dorsal-ventral axis in C. elegans originates from early cell contacts rather than diffusible gradients like BMP in vertebrates. At four-cell embryo, AB and P1 touching geometry differentially positions ABp contacting P2 receiving GLP-1 Notch signal involving APX-1, while ABa does not contact P2. This differential contact plus SKN-1, PAR polarity cues orients mitotic spindles via GPR-1/2 G-alpha regulation, positioning descendants dorsally versus ventrally. Physical interaction between AB derivatives and P1 lineage mediated by cadherin HMR-1 and catenin HMP-1 translates contact asymmetry into fate segregation essential for nervous system positioning.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 8: AB-P1 interaction establishes dorsal-ventral axis via Notch.

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C-Elegans Development-l

What happens if the P2 blastomere is removed at the 4-cell stage?

P2 blastomere provides inductive Wnt and Src signals to EMS at four-cell stage essential for endoderm. MOM-2 Wnt ligand from P2 activates MOM-5 Frizzled, WRM-1 beta-catenin asymmetry, LIT-1 Nemo kinase cascade in EMS, polarizing division into anterior MS and posterior E daughters. POP-1 TCF nuclear levels reduced posteriorly, permitting MED/END expression. If P2 is ablated surgically or genetically, extrinsic signal disappears, EMS defaults to MS-like fate, both daughters become muscle-pharynx precursors, endoderm marker ges-1 and elt-2 fail. Thus no intestine forms, demonstrating conditional specification.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 8: P2-EMS interaction - Wnt signaling induces endoderm in C. elegans.

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Elegans -ll

Which of the following statements correctly describes SKN-1 function?

SKN-1, maternally provided bZIP transcription factor related to Nrf, accumulates in P1 and later EMS blastomere due to PAR polarity and SKR-mediated regulation. It directly binds promoters of med-1, med-2 GATA factors, initiating mesendoderm program that subsequently activates end-1, end-3 and tbx-35 for E versus MS divergence. skn-1 mutants lack pharynx and intestine derived from EMS, with EMS transforming to C-like muscle fate. It does not regulate LIN-39 or vulval signaling, but serves as earliest zygotic activator connecting maternal polarity to zygotic endomesoderm specification, upstream of Wnt-modified POP-1 decision mechanism.

Ref: Bowerman et al. 1992; Gilbert Chapter 4: SKN-1 regulation of EMS fate via MED factors.

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Drosophila -l

Drosophila early development involves formation of:

Drosophila exhibits superficial cleavage where nuclear division occurs without cytoplasmic division. Zygote nucleus undergoes rapid synchronous mitoses within central cytoplasm, producing multinucleate syncytium. Around nuclear cycle 10, nuclei migrate peripherally into cortical cytoplasm, arranging beneath plasma membrane. Actin caps form above each nucleus, but no cell membranes separate nuclei, defining syncytial blastoderm. Only at cycle 14 does membrane invagination occur simultaneously around every cortical nucleus, converting syncytium into cellular blastoderm of epithelial monolayer surrounding central yolk. This syncytial stage allows rapid diffusion of morphogens like Bicoid essential for patterning.

Ref: NCBI, Molecular Biology of the Cell, Chapter 22: Syncytial blastoderm in Drosophila.

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Drosophila -ll

The dorsal gene product in Drosophila primarily functions as:

Dorsal gene encodes transcription factor containing Rel homology domain homologous to mammalian NF-kB p50 and p65. Dorsal resides cytoplasmically bound to Cactus but translocates ventrally upon Toll activation, where concentration gradient specifies fates: high activates twist and snail for mesoderm, intermediate specifies neuroectoderm, low permits dorsal ectoderm. Its function encompasses DNA binding, activation and repression, not enzymatic catalysis or scaffolding. Receptor function belongs to Toll. Dorsal represents morphogen whose nuclear concentration interpreted through high and low affinity enhancers generates discrete DV thresholds during patterning.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 9: Dorsal as NF-kB transcription factor - DV morphogen gradient action.

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Drosophila -ll

Dorsal-ventral polarity in Drosophila is initially set by:

Dorsal-ventral axis in Drosophila originates during oogenesis when Gurken mRNA localized to oocyte antero-dorsal cortex translates into TGF-alpha-like ligand activating Torpedo/EGFR in adjacent follicular epithelium. EGFR signaling represses transcription of sulfotransferase Pipe to ventral side, creating ventral domain where Pipe modifies vitelline membrane proteins. This modification permits serine protease cascade involving Nudel, Gastrulation defective, Snake, Easter that cleaves Spatzle ventrally. Processed Spatzle activates Toll uniformly present in embryonic membrane, generating ventral nuclear gradient of Dorsal transcription factor. Therefore initial polarity derives from Gurken-Torpedo communication between germline and somatic follicle cells rather than downstream Toll or Easter alone.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 9: Gurken-Torpedo EGFR signaling setting follicular DV polarity.

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plant Development - 1

Cotyledons formed during:

Cotyledons represent first embryonic leaves formed during heart stage when auxin maxima at apical margins trigger localized outgrowth from globular mass converting to bilateral symmetry. During this transition globular embryo converts to triangular heart morphology with two bulging primordia containing emerging provascular strands and accumulating storage products. Cotyledon formation involves coordinated action of PIN1-dependent auxin transport creating convergence maxima, CUC boundary genes establishing separation, and LEC regulators promoting maturation. Their emergence during heart stage establishes photosynthetic and nutritional support organs essential for seedling survival and reflects successful bilateral patterning and lateral organogenesis program.

Ref: Goldberg et al., Science 1989: cotyledons formed during heart stage transition to bilateral symmetry in Arabidopsis embryogenesis.

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plant Development - 1

Heart-shaped embryo indicates:

Heart-shaped embryo stage in Arabidopsis marks crucial transition from radial to bilateral symmetry during intermediate embryogenesis. Globular embryo with uniform cell divisions develops localized auxin maxima at apical flanks mediated by PIN1 convergence points directing periclinal divisions forming two cotyledon primordia protruding outward producing characteristic indented apex enclosing shoot meristem. Morphology indicates successful cotyledon initiation mediated by MP and NPH4 auxin response factors and boundary specification by CUC1/CUC2/STM genes, signifying onset of organogenesis and separation of functional domains in embryonic apex region essential for patterning.

Ref: Jürgens & Mayer, 1994; Capron et al.: heart-shaped embryo indicates initiation of cotyledon primordia via auxin maxima.

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plant Development - 1

First stage of Arabidopsis embryogenesis:

Arabidopsis embryogenesis initiates with zygotic stage immediately following double fertilization distinguished by zygote elongation along apical-basal axis, polarization of vacuole and mitochondria, and asymmetric division into apical and basal daughters. This initial stage precedes quadrant stage four-celled embryo, octant, dermatogen, globular, heart, torpedo, mature embryo stages. Zygotic stage encompasses activation of embryonic transcriptome after extensive chromatin reprogramming involving WOX family, auxin signaling, and epigenetic regulators. Therefore it represents earliest morphologically recognizable stage establishing apical-basal polarity foundation for all subsequent patterning and tissue differentiation events during seed formation in planta.

Ref: Jürgens et al., Annu Rev Plant Biol 1994: first stage of Arabidopsis embryogenesis is zygotic stage with asymmetric division.

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plant Development - 1

WOX2 and WOX8 regulate:

WOX2 and WOX8 are WUSCHEL-related homeobox transcription factors establishing apical and basal daughter lineages immediately after first asymmetric zygotic division. WOX2 marks apical cell promoting embryo proper development, protoderm specification, shoot identity, while WOX8 and WOX9 drive basal lineage suspensor development, auxin transport asymmetry via PIN7 polarity, and hypophysis specification. Complementary expression controls zygote elongation, division orientation, and downstream patterning cascades. Double mutants exhibit defective PIN1 distribution, misoriented divisions, abnormal apical-basal patterning, indicating these factors integrate transcriptional asymmetry with auxin transport to consolidate zygote polarity and early patterning events.

Ref: Breuninger et al., Dev Cell 2008; Haecker et al.: WOX2/WOX8 regulate zygote asymmetry and apical-basal polarity.

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plant Development -ll

Primary function of cotyledons:

Cotyledons are embryonic leaves formed during heart stage transition from radial to bilateral symmetry via localized auxin maxima at apical margins mediated by PIN1 convergence points. They accumulate storage compounds including triacylglycerols, cruciferin proteins, starch, and late embryogenesis abundant proteins providing essential nutrition during germination before true leaves become photosynthetically competent. Enclosing shoot meristem they protect SAM during seed desiccation and dormancy. Number and fusion status sensitive to auxin patterning governed by GNOM, PINOID, and GURKE lipid pathways, making cotyledon morphology key readout of embryonic patterning fidelity.

Ref: Goldberg et al., Science 1989; Gilbert Plant Development, Chap 20: cotyledons provide nutrient support for germination.

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Plant tissue culture 1

Embryogenesis in tissue culture leads to formation of:

Somatic embryogenesis produces embryo-like structures from vegetative somatic cells, mimicking zygotic embryogeny yet arising without fertilization. Unlike organogenesis which is monopolar and vascularly connected, somatic embryos are bipolar possessing simultaneous shoot and root apical meristems and remain physiologically isolated enclosed by protoderm. Process begins with induction of embryogenic competence using synthetic auxin 2,4-D which triggers global chromatin decondensation, DNA hypomethylation, and upregulation of master regulators SOMATIC EMBRYOGENESIS RECEPTOR KINASE1 SERK1, LEAFY COTYLEDON1/2, BABY BOOM and WOX2. Removal of auxin permits polar auxin transport establishment via PIN1 localization generating apical basal axis. Embryos progress through globular spherical stage, heart stage with bilateral symmetry and cotyledon initiation, torpedo elongation, and cotyledonary maturation accumulating storage reserves and LEA proteins for desiccation tolerance. Single-cell origin prevents chimerism, ideal for transformation, artificial seeds, and large scale bioreactor propagation, providing uniform embryos capable of converting into plantlets upon germination medium without hormone or low ABA, ensuring high fidelity clonal propagation.

Ref: Zimmerman, Plant Cell 1993, Somatic Embryogenesis review; NCBI Bookshelf NBK26830.

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