Y chromosome shredding in Anopheles gambiae: insight into the cellular dynamics of a novel synthetic sex ratio distorter.

Abstract

Despite efforts to explore the genome of the malaria vector Anopheles gambiae, the Y chromosome of this species remains enigmatic. The large number of repetitive and heterochromatic DNA sequences makes the Y chromosome exceptionally difficult to fully assemble, hampering the progress of gene editing techniques and functional studies for this chromosome. In this study, we made use of a bioinformatic platform to identify Y-specific repetitive DNA sequences that served as a target site for a CRISPR/Cas9 system. The activity of Cas9 in the reproductive organs of males caused damage to Y-bearing sperm without affecting their fertility, leading to a strong female bias in the progeny. Cytological investigation allowed us to identify meiotic defects and investigate sperm selection in this new synthetic sex ratio distorter system. In addition, alternative promoters enable us to target the Y chromosome in specific tissues and developmental stages of male mosquitoes, enabling studies that shed light on the role of this chromosome in male gametogenesis. This work paves the way for further insight into the poorly characterised Y chromosome of Anopheles gambiae. Moreover, the sex distorter strain we have generated promises to be a valuable tool for the advancement of studies in the field of developmental biology, with the potential to support the progress of genetic strategies aimed at controlling malaria mosquitoes and other pest species. Author summaryGenetic elements known as sex ratio meiotic drive can manipulate the sex ratio of offspring, favouring the male or female sex. This fascinating phenomenon has inspired the development of synthetic sex ratio distorter systems in several organisms. In species where females and males harbour XX and XY sex chromosomes respectively, the X-chromosome can be shredded during male gametogenesis, leading to the production of non-functional X-bearing sperm, while Y-bearing sperm are left intact and able to fertilise the eggs. These systems can produce offspring that are extremely biased towards males, which can be used as genetic tools to control harmful insect populations. In our study, we applied this molecular strategy to target the Y chromosome of Anopheles gambiae. Our aim was to investigate the cellular consequences of the shredding of this chromosome, the impact on meiosis and sperm selection, and the potential to achieve strong female bias in the offspring. The outcome of this study enhances our understanding of the molecular and biological mechanisms behind synthetic sex-ratio distorters in Anopheles mosquitoes, which could inform the development of vector control strategies that target sex ratio. Additionally, we present a genetic sexing strain able to produce mostly females, providing a valuable genetic tool for fundamental studies on this deadly vector.