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Haldane’s rule in the developing brain: X-linked and imprinted genes contribute to male-biased transgressive expression in hybrid mice

ABSTRACT: The placenta and developing brains of eutherian mammals are intimately connected and enriched for imprinted gene (IG) expression. In hybrids, the placenta is a hotspot for genetic and epigenetic incompatibilities that preferentially affect males. Whether the hybrid brain is similarly affected is an open question. We evaluated the effects of hybridization and placental dysregulation on neural gene expression (embryonic, juvenile, adult) and behavior (juvenile, adult) in a cross between the house mouse (Mus m. domesticus) and Algerian mouse (M. spretus), in which transgressive expression of imprinted and X-linked genes is a prominent feature of undersized F1 male placentas. In hybrid hypothalamus, transgressive expression was mainly embryonic and highly male-biased. Although fewer genes were dysregulated in hybrid male embryonic hypothalamus relative to placenta, we infer a similar genetic architecture involving X-IG incompatibilities, and incompatibilities between IGs and autosomal interaction partners. Hybrids had heightened anxiety-like behaviors and reduced activity relative to parental species, with a tendency towards larger behavioral differences in adult males. As the first study to characterize the effects of interspecific hybridization on neural gene expression and behavior in a mammal, this work extends the scope of phenotypes impacted by intrinsic incompatibilities and demonstrates that the brain obeys Haldane’s rule.

ORGANISM(S): Mus spretus Mus musculus domesticus x Mus spretus Mus musculus domesticus

PROVIDER: GSE311072 | GEO | 2026/02/02

REPOSITORIES: GEO

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Project description:Hybrid phenotypes that contribute to postzygotic reproductive isolation often exhibit pronounced asymmetry, both between reciprocal crosses and between the sexes in accordance with Haldane's rule. Inviability in mammalian hybrids is associated with parent-of-origin placental growth abnormalities for which misregulation of imprinted genes is the leading candidate mechanism. However, direct evidence for the involvement of imprinted genes in hybrid growth dysplasia is limited. We used transcriptome and reduced representation bisulfite sequencing to conduct the first genome-scale assessment of the contribution of imprinted genes to parent-of-origin placental growth dysplasia in the cross between the house mouse (Mus musculus domesticus) and the Algerian mouse (Mus spretus). Imprinted genes with transgressive expression and methylation were concentrated in the Kcnq1 cluster, which contains causal genes for prenatal growth abnormalities in mice and humans. Hypermethylation of the cluster’s imprinting control region, and consequent misexpression of the genes Phlda2 and Ascl2, is a strong candidate mechanism for transgressive placental undergrowth. Transgressive placental and gene regulatory phenotypes, including expression and methylation in the Kcnq1 cluster, were more extreme in hybrid males. While consistent with Haldane’s rule, male-biased defects are unexpected in rodent placenta because the X-chromosome is effectively hemizygous in both sexes. In search of an explanation we found evidence of leaky imprinted (paternal) X-chromosome inactivation in hybrid female placenta, an epigenetic disturbance that may buffer females from the effects of X-linked incompatibilities to which males are fully exposed. Sex differences in chromatin structure on the X and sex-biased maternal effects are non-mutually exclusive alternative explanations for adherence to Haldane’s rule in hybrid placenta. The results of this study contribute to understanding the genetic basis of hybrid inviability in mammals, and the role of imprinted genes in speciation.

Project description:The formation of new species is often a consequence of genetic incompatibilities accumulated between populations during allopatric divergence. When divergent taxa interbreed, these incompatibilities impact physiology and have a direct cost resulting in reduced hybrid fitness. Recent surveys of gene regulation in interspecific hybrids have revealed anomalous expression across large proportions of the genome, with 30-70% of all genes apparently misexpressed, mostly in the direction of down-regulation. However, since most of these studies have focused on pairs of species exhibiting high degrees of reproductive isolation, the association between regulatory disruption and reduced hybrid fitness prior to species formation remains unclear. Within the copepod species Tigriopus californicus, interpopulation hybrids show reduced fitness associated with mitochondrial dysfunction. Here we show that in contrast to studies of interspecific hybrids, only 1.2% of the transcriptome was misexpressed in interpopulation hybrids of T. californicus, and nearly 80% of misexpressed genes were overexpressed rather than underexpressed. Moreover, many of the misexpressed genes were components of functional pathways impacted by mitonuclear incompatibilities in hybrid T. californicus (e.g., oxidative phosphorylation and antioxidant response). We also show that the magnitude of hybrid misregulation is not dependent on levels of protein sequence divergence, even though the latter is correlated with expression divergence between parental populations. Our results suggest that hybrid breakdown at early stages of speciation may result from initial incompatibilities amplified by the cost of compensatory physiological responses. Our experiment included nine RNA-seq samples: 3 San Diego, 2 Santa Cruz, and 4 hybrid samples. For each sample, 400-500 copepods across all developmental stages were collected from their stock cultures. They were transferred to fresh filtered seawater in a 50-mL Falcon tubes and immersed in a 20°C water bath for two hours. Water was then quickly removed, 4 mL of Tri-Reagent (Sigma) added, and tissue immediately disrupted using a tissue homogenizer. RNA was isolated following the manufacturer’s protocol. Re-suspended RNA pellets were further purified with RNeasy Mini columns (Qiagen), and final sample integrity and quantity were assessed with an Agilent 2100 BioAnalyzer. Please note that two samples (GSM1531288, GSM1531290) have been accessioned under BioProject PRJNA168170, SRA study SRP013608, while the remaining seven samples under BioProject PRJNA263967, SRA Study SRP048974. The current records including all 9 samples (PRJNA264820/SRP049247) were re-created for the convenient retrieval of the complete raw data from SRA

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Haldane’s rule in the developing brain: X-linked and imprinted genes contribute to male-biased transgressive expression in hybrid mice

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