sox17 Mutagenesis Reveals Compounding Effects ofMaternal and Zygotic Contribution on Microglia Abundance

Embryonic development is shaped by maternal and zygotic factors but how the interplay between these factors contributes to phenotypic variability and impacts specific populations of brain cells, and subsequently neurodevelopment, remains largely unknown. Here, we reveal a double hit phenomenon that impacts microglia and microglia precursors during development. Using zebrafish, we identified a mutation in sry‐related HMG box gene‐17 (sox17) that exhibits a maternal effect phenotype presenting as a reduction of microglia in the brain and precursors in the yolk sac. We demonstrate that microglia deficits are exacerbated in homozygous offspring from homozygous parents compared to homozygous offspring from heterozygous parents, suggesting a compounding effect of maternal and zygotic sox17 mutation on microglia abundance. We observe that sox17 is expressed in microglia and yolk sac precursors and is maternally contributed. Injections of sox17 RNA during embryogenesis are sufficient to rescue microglia and yolk sac cell abundance. To identify additional genes interacting with sox17 , we perform a genetic screen using CRISPR mutagenesis and identify f11r.1, gas6 , and mpp1 as microglia regulators, which we demonstrate are expressed in zebrafish microglia. Transcriptional and mutant analyses with these genes suggest sox17 positively regulates mpp1 transcription. These results demonstrate that microglia are susceptible to additive effects of maternal and zygotic sox17 mutation, and that offspring greatly differ in their susceptibility to microglia deficits depending on the mother's genotype. Our findings highlight that both the maternal and offspring genotype should be considered when examining neurodevelopmental disorders that could arise from microglia dysfunction.