Sex chromosome dosage compensation in non-model insects
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Abstract
Sexual reproduction is common among animals. The sex of the animal is often
determined by sex chromosomes. Differentiated sex chromosomes evolve from autosomal
progenitors, when one chromosome from the pair gains a sex-determining region and loses its
genetic activity over time. Degeneration of the Y chromosome effectively leads to aneuploidy
of the X chromosome in males. The resulting gene dosage imbalance can be detrimental and
is frequently corrected by a mechanism named dosage compensation (DC). Understanding
dosage compensation and other molecular differences between sexes is highly relevant. For
example, only the female mosquitos bite and transmit the malaria parasite.
The mechanism regulating DC in Anopheles gambiae, the major vector of malaria,
was unknown when I began my PhD work. To identify putative DC factors, I generated a
sex-specific RNA expression atlas along embryogenesis. I observed that DC initiates shortly
after zygotic genome activation. I then discovered a previously uncharacterized gene with
consistently male-biased expression: AGAP005748. Based on its suspected function, I named
this gene SOA (sex chromosome activation). I discovered that SOA produces two sex-specific,
alternatively spliced isoforms. Males express a canonical transcript, while in females the
second intron is retained. Intron inclusion results in a premature stop codon and production of
a truncated protein in females.
I discovered that SOA binds the promoters of expressed X-linked genes in mosquito
males. Next, I aimed to assess the effect of SOA on gene expression. I observed that SOA
expressed ectopically in a female cell line binds gene promoters of active X-linked genes
resulting in their upregulation. To comprehend its physiological function, our collaborator dr.
Eric Marois generated transgenic mosquitoes devoid of SOA through a CRISPR-mediated
knock-in of a gene trap upstream of the coding sequence (SOA-KI). In these mosquitos, I
detected a loss of signal at SOA binding sites. Additionally, RNA-seq revealed
downregulation of the X chromosome in SOA mutant males, confirming SOA indeed
mediates DC in vivo. Surprisingly, SOA-KI mosquitos of both sexes are viable. Male SOA-KI
pupae exhibited a developmental delay compared to wild-type, while females were
unaffected. We also generated a transgenic line called SOA-R that expresses the full-length
SOA cDNA. In female mosquitoes carrying this allele SOA also binds and upregulates
X-linked genes.
The insights from this work, especially the discovery of non-essentiality of DC in
Anopheles have made me even more interested in the evolution of this process. Because of
this, I surveyed the published works investigating the DC status and mechanism in different
species. I wrote a review article summarizing the knowledge about sex chromosome evolution
and DC in insects. In this article, I also used the insights from my previous work to provide
guidance to researchers who aim to identify DC mechanisms in other insect species.
In my PhD work, I demonstrated that SOA is the master regulator of X chromosome
dosage compensation in A. gambiae. I also summarized the knowledge about the DC status
across insects and proposed a framework to uncover the DC factors in non-model insect
species.