29. Dosage compensation in drosophila is mediated by
(1) Inactivation of one X chromosome in female
(2) Hyper activation of one X chromosome in male
(3) Hypo activation of both X chromosome in female
(4) Methylation of both X chromosomes and autosomes

Dosage compensation in Drosophila is mediated by hyper activation of the single X chromosome in males to balance gene expression between XY males and XX females.

Explanation of the options:

(1) Inactivation of one X chromosome in female: This mechanism is typical of mammals (like humans and mice), where one of the two X chromosomes in females is transcriptionally inactivated to equalize gene dosage with males. However, this is not the mechanism in Drosophila.

(2) Hyper activation of one X chromosome in male: In Drosophila, males have a single X chromosome, and dosage compensation is achieved by doubling (hypertranscribing) gene expression from that single X chromosome, so its output matches that of the two X chromosomes in females. This process involves the male-specific lethal (MSL) complex that acetylates histone H4 at lysine 16 to increase transcriptional activity.

(3) Hypo activation of both X chromosome in female: This mechanism occurs in Caenorhabditis elegans, where hermaphrodites reduce transcription from both X chromosomes to achieve dosage compensation; it does not apply to Drosophila.

(4) Methylation of both X chromosomes and autosomes: DNA methylation as a dosage compensation mechanism is not prominent in Drosophila’s X chromosome regulation and is not the known mode of dosage compensation in this species.

Introduction

Dosage compensation in Drosophila is a crucial biological process that equalizes gene expression from the single X chromosome in males with that of the two X chromosomes in females. Unlike mammals, where one female X chromosome is inactivated, Drosophila accomplishes this by hyper activating the male X chromosome. This ensures males and females have balanced expression of X-linked genes despite different sex chromosome numbers.

Explanation of Dosage Compensation Mechanism in Drosophila

In Drosophila males (XY), dosage compensation involves upregulating transcription from the single X chromosome approximately twofold. This hypertranscription is mediated by the male-specific lethal (MSL) complex, which modifies chromatin by acetylating histone H4 at lysine 16, increasing gene activity on the male X chromosome. This compensates for dosage imbalance and restores parity with females (XX), who do not undergo X chromosome inactivation.

Why Other Options Are Incorrect

• Inactivation of one X chromosome in females (Option 1) is a mechanism seen in mammals but not in Drosophila.

• Hypoactivation of both female X chromosomes (Option 3) happens in C. elegans but not flies.

• Methylation of X chromosomes and autosomes (Option 4) is not a recognized compensatory process in Drosophila.

Thus, the correct answer is (2) Hyper activation of one X chromosome in male, which accurately describes how dosage compensation is achieved in Drosophila.

This explanation aligns with scientific consensus and studies on chromatin modification and dosage compensation complexes in Drosophila males.