Antennapedia protein expresses in the thoracic segment of the fly and not in the head region. However, a dominant mutation of Antennapedia replaces antenna with leg-like structure. The following statements were made with reference to Antennapedia: A. In a dominant Antennapedia mutant, the Antennapedia gene is expressed in the head as well as in thorax. B. In the Antennapedia mutants, the Antennapedia gene is expressed in head region only and thus promotes leg-like structure in head socket C. In addition to promoting thoracic structures, Antennapedia protein binds to and represses the enhancer of homothorax and some other genes responsible for antenna specification. D. Antennapedia has no role in thoracic region specification and thus recessive Antennapedia mutants show no phenotypic effect. Which one of the following options represents correct statement(s)?
A and C
Antennapedia Gene Function and Mutation
The question describes the function of the Antennapedia gene in fruit flies (Drosophila melanogaster) and the effect of a dominant mutation in this gene. Normally, the Antennapedia gene is a HOX gene that plays a crucial role in specifying the identity of the thoracic segments, where legs develop. It is typically expressed in the thorax but not in the head.
The dominant Antennapedia mutation results in a striking change: antennae, which are head structures, are replaced by leg-like structures. This transformation suggests that the Antennapedia gene product is now active in the head region, where it normally isn't, and is instructing head cells to develop characteristics of thoracic segments (legs).
Analyzing Statements about Antennapedia
- Statement A: In a dominant Antennapedia mutant, the Antennapedia gene is expressed in the head as well as in thorax.
This statement aligns with the observed phenotype. The development of legs in the head region strongly implies that the Antennapedia gene is being expressed ectopically (in the wrong place), specifically in the head, in the dominant mutant. Since the gene's normal function is in the thorax, it is likely still expressed there as well. Therefore, expression in both head and thorax explains the mutant phenotype.
- Statement B: In the Antennapedia mutants, the Antennapedia gene is expressed in head region only and thus promotes leg-like structure in head socket.
This statement is less precise than A. While expression in the head is key to the mutant phenotype, stating it's expressed "only" in the head contradicts the gene's normal role and location of expression in the thorax. The gene is still expected to be expressed in the thorax for normal thoracic development. Therefore, saying it's expressed "only" in the head is likely incorrect.
- Statement C: In addition to promoting thoracic structures, Antennapedia protein binds to and represses the enhancer of homothorax and some other genes responsible for antenna specification.
This statement provides a potential molecular mechanism for the antenna-to-leg transformation. Homothorax is known to be involved in antenna development in flies. If Antennapedia protein represses genes necessary for antenna formation (like those regulated by homothorax enhancers) when expressed in the head, it would disrupt antenna development. Simultaneously, its inherent function of promoting thoracic/leg development would then lead to the formation of leg-like structures in that position. This explanation is consistent with how Hox proteins can act as transcription factors, repressing or activating downstream genes to specify regional identity.
- Statement D: Antennapedia has no role in thoracic region specification and thus recessive Antennapedia mutants show no phenotypic effect.
This statement directly contradicts the established role of Antennapedia as a thoracic HOX gene. HOX genes are fundamental for segment identity specification. Saying it has "no role" in the thorax is incorrect. Also, loss-of-function (recessive) mutations in HOX genes typically result in transformations, often towards a more anterior segment identity (e.g., a thoracic segment might take on characteristics of a head segment). Therefore, recessive Antennapedia mutants are expected to show phenotypic effects related to thoracic development, not "no phenotypic effect".
Conclusion on Correct Statements
Based on the analysis:
- Statement A is correct because the dominant mutation causes ectopic expression in the head leading to leg formation, while normal expression in the thorax is likely maintained.
- Statement B is incorrect because it claims expression is *only* in the head, neglecting the normal thoracic expression.
- Statement C is correct as it provides a plausible mechanism for the antenna-to-leg transformation by repressing antenna fate genes and promoting leg fate.
- Statement D is incorrect because Antennapedia is crucial for thoracic specification, and recessive mutants would likely show phenotypic effects.
Therefore, the correct statements are A and C.
Important Questions from Morphogenesis and organogenesis in animals
The individuals considered in this question are having two haploid sets of autosomes and no Y- chromosome. The X∶A ratio of the individuals, the type of organisms chosen, their primary sex and number of Barr bodies expected in their cells are shown in the table below:
X∶A ratio Organism Primary Sex Number of Barr bodies i. 0.5
ii. 2
A. Human
B. Drosophila
I. Male
II. Female
III. Metafemale
a. Zero
b. One
c. Two
d. Three
Select the option below with all correct matches:
Several marine organism release their gametes into the environment, where sperm attraction and subsequent events lead to successful fertilization. With reference to sea urchins, which one of the following statements is NOT true?
The following statements regarding the generation of dorsal/ventral axis in Drosophila was made:
A. Gurken protein moves along with the oocyte nucleus and signals follicle cells to adopt the ventral fate.
B. Maternal deficiencies of either the gurken or torpedo gene cause ventralization of the embryo.
C. Gurken is active only in the oocyte and Torpedo is active only in the somatic follicle cells.
D. The Pipe protein is made in the dorsal follicle cells.
E. The highest concentration of Dorsal is in the dorsal cell nuclei, which becomes the mesoderm.
Which one of the following combination of the above statements is true?
During wing development in chick, if Apical Ectodermal Ridge (AER) is removed, the limb development ceases, on the other hand placing leg mesenchyme directly beneath the wing AER, distal hindlimb structures develop at the end of the wing, and if limb mesenchyme is replaced by non-limb mesenchyme beneath AER, the AER regresses. This may demonstrate that:
A. the limb mesenchyme cells induce and sustain AER.
B. the mesenchyme cells specify the type: wing or limb.
C. the AER is responsible for specifying the type: wing or limb.
D. the AER is responsible for sustained outgrowth and development of the limb.
E. the AER does not specify the type: wing or limb.
Which combination of above statements is demonstrated by the experiment?
The following statements are made regarding the amphibian early-embryonic development:
A. The Nieuwkoop center cells are mesodermal in origin.
B. Chordin, Noggin and Goosecoid are secreted by the Organizer.
C. The default fate of the ectoderm is to become neural tissue.
D. BMP levels are high in the presumptive dorsal mesoderm.
Which one of the following options represents the combination of all correct statements?
