- From a newly fertilized mouse egg, maternal pronucleus was removed and replaced with a second paternal pronucleus. Following observations/ statements were made:
(A) This will result in formation of an androgenetic embryo.
(B) This will result in formation of a gynogenetic embryo.
(C) The embryo will not survive beyond mid-gestation since parental genomes serve distinct complementary functions due to variable imprinting pattern.
(D) The adult originating from the embryo will be a clone of the father.
(E) The embryo will develop as adult but will die early due to rapid shortening of the telomeres.
Which one of the following represents correct combination of above statements?
(1) A, B, C (2) B, C, E
(3) A and C only (4) A and E only
The concept of replacing a maternal pronucleus with another paternal pronucleus in a fertilized mouse egg provides fascinating insights into genomic imprinting, embryonic development, and the necessity of balanced parental contributions. This experimental setup plays a pivotal role in understanding how different parental genomes complement each other during early embryogenesis. Here, we discuss in detail what happens when the maternal genetic material is replaced with paternal DNA, and why such embryos do not survive.
The Background of Pronuclear Manipulation
In mammals, a newly fertilized egg contains two pronuclei—one from the sperm (paternal) and one from the egg (maternal). These pronuclei fuse to form a diploid zygote nucleus, containing a full set of chromosomes. Each parental genome is not identical in its gene expression patterns because certain genes are “imprinted,” meaning they are selectively activated based on whether they come from the mother or the father. This parent-specific gene marking is known as genomic imprinting.
The Experimental Setup
In the described experiment, researchers start with a fertilized mouse egg:
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The maternal pronucleus is removed.
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It is then replaced with a second paternal pronucleus, resulting in a zygote that contains two paternal genomes and no maternal genome.
At first glance, one might assume that since the total genetic material remains diploid, the embryo should develop normally. However, this is not the case. The change in parental origin of the genomes disrupts the delicate balance necessary for proper development.
Key Observations and Interpretations
From this experiment, the following statements can be evaluated:
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(A) This will result in the formation of an androgenetic embryo.
This statement is correct. When both sets of chromosomes are derived from the father, the resulting zygote is called an androgenetic embryo. It contains only paternal genetic material. -
(B) This will result in the formation of a gynogenetic embryo.
This is incorrect. A gynogenetic embryo refers to one derived from two maternal genomes (no paternal contribution). Here, both genomes are paternal, so it does not fit this definition. -
(C) The embryo will not survive beyond mid-gestation since parental genomes serve distinct complementary functions due to variable imprinting pattern.
This is correct. Mammalian embryonic development requires contributions from both maternal and paternal genomes because certain imprinted genes crucial for placental and embryonic development are active only when inherited from a specific parent. The absence of maternal imprints leads to abnormal growth and early embryonic death, typically around mid-gestation. -
(D) The adult originating from the embryo will be a clone of the father.
This is incorrect. Androgenetic embryos do not develop to full term, so no adult organism is produced. Although genetic material comes solely from the father, development halts early due to impaired embryonic and placental development. -
(E) The embryo will develop as an adult but will die early due to rapid shortening of the telomeres.
This is also incorrect. Telomere length does not account for the developmental failure in such embryos; the central issue lies in genomic imprinting, not telomere dynamics.
Based on this analysis, the correct combination is (A) and (C) only.
Understanding Androgenetic vs Gynogenetic Embryos
To appreciate the significance of this experiment, it helps to compare androgenetic and gynogenetic embryos. Androgenetic embryos are bipaternal, while gynogenetic embryos are bimaternal.
| Aspect | Androgenetic Embryo | Gynogenetic Embryo |
|---|---|---|
| Origin | Two paternal genomes | Two maternal genomes |
| Pronucleus source | Both from sperm | Both from egg |
| Development outcome | Abnormal embryo with overgrown placenta and poor embryonic tissues | Poor placental development and limited embryonic growth |
| Viability | Dies in mid-gestation | Dies in mid-gestation |
| Cause of failure | Lack of maternal imprinting | Lack of paternal imprinting |
The distinct developmental fates of these embryos demonstrate that both paternal and maternal genomes carry unique imprints essential for full-term development. This dual contribution underscores the biological principle of genomic imprinting.
Role of Genomic Imprinting
Genomic imprinting ensures that certain genes are expressed in a parent-of-origin–specific manner. For example, some genes involved in placental growth are active only when inherited from the father, while others that regulate embryonic development depend on maternal alleles. This coordinated expression prevents dominance of either parental genome and maintains normal fetal development.
When this balance is disrupted, as in the androgenetic embryo, developmental abnormalities arise:
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Overgrowth of extraembryonic tissues due to excessive paternal expression.
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Poor development of embryonic tissues due to absence of maternal imprints.
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Failure of the embryo to progress beyond the mid-gestation stage.
Insights from Experimental Findings
These experiments with mice have provided critical insights into reproductive biology and disease mechanisms:
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They demonstrate that genetic material alone is not sufficient for viable offspring; epigenetic factors like DNA methylation and imprinting play major roles.
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They offer models for studying molar pregnancies, a condition in humans where an abnormal fertilization leads to development dominated by paternal genes, similar to an androgenetic conceptus.
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They contribute to our understanding of stem cell research, cloning, and assisted reproductive technologies, emphasizing the need for proper genomic reprogramming.
Broader Implications on Mammalian Embryogenesis
The necessity of both maternal and paternal genomes is unique to mammals. In some lower organisms, such as amphibians or certain fish, uniparental development can occur naturally. However, in mammals, the balance between the maternal and paternal genetic contributions is indispensable. This difference underlines the evolutionary complexity of mammalian reproduction and the importance of epigenetic regulation.
Conclusion
Replacing the maternal pronucleus with a second paternal pronucleus in a fertilized mouse egg results in the formation of an androgenetic embryo—a diploid zygote bearing two paternal genomes. Although initially viable in structure, this embryo cannot develop beyond mid-gestation due to failures induced by genomic imprinting differences between the maternal and paternal genomes. In conclusion, among the given statements, A and C are correct, reflecting that balanced parental contributions are essential for normal mammalian development.
