11. Gradients of morphogens determine the future dorsal- ventral and anterior-posterior axes of the developing embryo in many organisms. How is the anterior- posterior axis developed in C. elegans?
(1) Sperm entry leads to reorganization of the cytoskeleton and redistribution of naturally packed PAR proteins, which in turn determine the anterior- posterior axis.
(2) After fertilization β-catenin gets localized into the nucleus of the future anterior cells.
(3) The P-granules are localized in a way consistent with a role as a morphogenetic determinant and they act through translational regulation to initiate anterior-posterior axis.
(4) The localization of the maternally expressed polypeptide SKN-1 is responsible for the

The anterior-posterior axis in C. elegans is established when sperm entry leads to reorganization of the cytoskeleton and redistribution of partitioning-defective (PAR) proteins, which determine the future axis of the developing embryo. This process is a classic example of how external fertilization events can initiate deep cytoplasmic and protein polarity changes, setting up cell fate and body axes in early embryogenesis.

Therefore, the correct answer is:

(1) Sperm entry leads to reorganization of the cytoskeleton and redistribution of naturally packed PAR proteins, which in turn determine the anterior-posterior axis.

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Introduction

The process of axis formation in animal embryos is essential for generating the body plan and organizing subsequent development. In the nematode Caenorhabditis elegans, the establishment of the anterior-posterior axis provides a model for studying molecular and cellular events guiding early embryogenesis.

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Axis Formation Initiated by Sperm Entry

• In C. elegans, fertilization is a trigger for asymmetry; the sperm always enters the egg at a specific site, which will become the posterior pole.

• Entry of the sperm introduces centrosomes and sets up flows of cytoplasm as well as cytoskeletal rearrangements within the zygote.

• These cues direct the positioning of specific protein complexes (particularly PAR proteins) and initiate localization of critical determinants.

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Role of PAR Proteins in Axis Establishment

• PAR (partitioning-defective) proteins are maternally deposited and interact with the cytoskeleton following sperm entry.

• The cytoplasmic flows initiated by the sperm reorganize PAR proteins and other determinants, resulting in clear anterior and posterior domains in the egg.

• This redistribution is crucial for subsequent asymmetric cell divisions that segregate cell fates and developmental potential.

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Biological Significance and Conservation

• Correct anterior-posterior axis formation is necessary for the proper arrangement of tissues and organs in the developing worm.

• PAR proteins and centrosome-related cues are found in other model organisms, underlining the evolutionary importance of these mechanisms.

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Why Other Mechanisms Are Less Central

• P-granule localization plays a role in germ cell fate but does not initiate anterior-posterior polarity.

• β-catenin nuclear localization and SKN-1 protein activity are important for later fate decisions, not primary axis formation.

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Conclusion

In C. elegans, the foundation of the anterior-posterior axis depends on sperm entry and the resulting cytoskeletal shifts and PAR protein redistribution. This elegant system exemplifies how fertilization cues can shape organismal body plans starting from the very first moments of life.

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Final Answer:
(1) Sperm entry leads to reorganization of the cytoskeleton and redistribution of naturally packed PAR proteins, which in turn determine the anterior-posterior axis.