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Comment
. 2019 Dec 2;29(23):R1229-R1231.
doi: 10.1016/j.cub.2019.09.065.

Dosage Compensation: How to Be Compensated…Or Not?

Jingyue Duan  1 , Erica N Larschan  2
Affiliations
Comment

Dosage Compensation: How to Be Compensated…Or Not?

Jingyue Duan et al. Curr Biol. .

Abstract

Diverse dosage compensation mechanisms have evolved across species to equalize gene expression between sexes and between the sex chromosomes and autosomes. New results show that two opposite modes of dosage compensation can occur within one species, the monarch butterfly.

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Figures

Figure 1.
Figure 1.. Evolutionary strategies of dosage compensation across species.
Sex chromosomes originated from autosome pairs: proto-X and proto-Y. During sex chromosome evolution, formation of sex-specific regions stopped recombination between proto-X and proto-Y, which drove Y degeneration. Ohno hypothesized that the X-linked expression level must be restored relative to autosomes in males to avoid the ‘peril’ of having only one X-chromosome. Diverse mechanisms evolved to perform dosage compensation across species. In mode I, Drosophila doubles the X-linked genes in males only, and no further step is necessary. The X:A ratio in Drosophila is 1 for complete dosage compensation. In mode II, X-linked gene expression is doubled in both sexes and results in hyper-transcription in females. Therefore, the secondary steps evolved to inactivate one of two X-chromosomes in mammalian females or halve expression from both X-chromosomes in C. elegans females. The X:A ratio in mammals and C. elegans is less than 1, causing incomplete dosage compensation. The new study by Gu et al. shows that monarch butterfly, a female heterogametic species (female ZW, male ZZ), uses mode I on its neo-Z and mode II for anc-Z. This is the first-time that two modes of dosage compensation have been found to occur on one single sex chromosome. The neo-Z:A ratio in monarch butterfly is 1 for complete dosage compensation, while the anc-Z:A ratio is close to 0.5, meaning that there is a lack of dosage compensation relative to autosomes.
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