The evolution of reproductive barriers is fundamental to the formation of new species and can help us understand the diversification of life on Earth. These reproductive barriers often take the form of hybrid incompatibilities, where genes derived from two different species no longer interact properly. Theory predicts that incompatibilities involving multiple genes should be common and that rapidly evolving genes will be more likely to cause incompatibilities, but empirical evidence has lagged behind these predictions. Here, we describe a mitonuclear incompatibility involving three genes within respiratory Complex I in naturally hybridizing swordtail fish. Individuals with specific mismatched protein combinations fail to complete embryonic development while those heterozygous for the incompatibility have reduced function of Complex I and unbalanced representation of parental alleles in the mitochondrial proteome. We localize the protein-protein interactions that underlie the incompatibility and document accelerated evolution and introgression in the genes involved. This work thus provides a precise characterization of the genetic architecture, physiological impacts, and evolutionary origin of a multi-gene incompatibility impacting naturally hybridizing species.

Early childhood experiences have long-lasting effects on subsequent mental and physical health, education, and employment. Measurement of these effects relies on insensitive behavioral signs, subjective assessments by adult observers, neuroimaging and neurophysiological studies, or remote epidemiologic outcomes. Despite intensive search, no biomarkers for developmental changes in the brain have been identified. We analyzed scalp hair from healthy children and their mothers using an unbiased proteomics platform to reveal 1368 hair proteins commonly observed in children, 1438 proteins commonly observed in mothers, and 1288 proteins observed sporadically in individual subjects. Mothers showed higher numbers of peptide spectral matches and hair proteins compared to children, with important age-related differences between mothers and children. Age-related differences were also observed in children, with differential protein expression patterns between younger (2 years and below) and older children (3-5 years). Boolean analyses showed greater conservation of hair protein patterns between mothers and their biological children as compared to mothers and unrelated children. The top 5% proteins driving population variability represent biological pathways associated with brain development, immune signaling, and stress response regulation. Non-structural proteins observed in scalp hair may include promising biomarkers to investigate the developmental changes associated with early childhood experiences.One Sentence SummaryThe non-structural proteins observed in scalp hair from preschool children show evidence for heritability, reflect biological functions such as brain development, or immune function and regulation of stress responses, and exhibit age- and sex-related differences across periods of early childhood development.