RESUMO
Genomic imprinting-the non-equivalence of maternal and paternal genomes-is a critical process that has evolved independently in many plant and mammalian species1,2. According to kinship theory, imprinting is the inevitable consequence of conflictive selective forces acting on differentially expressed parental alleles3,4. Yet, how these epigenetic differences evolve in the first place is poorly understood3,5,6. Here we report the identification and molecular dissection of a parent-of-origin effect on gene expression that might help to clarify this fundamental question. Toxin-antidote elements (TAs) are selfish elements that spread in populations by poisoning non-carrier individuals7-9. In reciprocal crosses between two Caenorhabditis tropicalis wild isolates, we found that the slow-1/grow-1 TA is specifically inactive when paternally inherited. This parent-of-origin effect stems from transcriptional repression of the slow-1 toxin by the PIWI-interacting RNA (piRNA) host defence pathway. The repression requires PIWI Argonaute and SET-32 histone methyltransferase activities and is transgenerationally inherited via small RNAs. Remarkably, when slow-1/grow-1 is maternally inherited, slow-1 repression is halted by a translation-independent role of its maternal mRNA. That is, slow-1 transcripts loaded into eggs-but not SLOW-1 protein-are necessary and sufficient to counteract piRNA-mediated repression. Our findings show that parent-of-origin effects can evolve by co-option of the piRNA pathway and hinder the spread of selfish genes that require sex for their propagation.
Assuntos
Caenorhabditis , Impressão Genômica , RNA de Interação com Piwi , Sequências Repetitivas de Ácido Nucleico , Animais , Feminino , Masculino , Alelos , Proteínas Argonautas/genética , Proteínas Argonautas/metabolismo , Caenorhabditis/genética , Caenorhabditis/metabolismo , Cruzamentos Genéticos , Pai , Genoma/genética , Impressão Genômica/genética , Organismos Hermafroditas/genética , Histona Metiltransferases/genética , Histona Metiltransferases/metabolismo , Mães , Oócitos/metabolismo , RNA de Interação com Piwi/genética , Biossíntese de Proteínas , Sequências Repetitivas de Ácido Nucleico/genética , RNA Mensageiro/genética , Toxinas Biológicas/genética , Transcrição GênicaRESUMO
Horizontal gene transfer, the movement of genetic material between species, has been reported across all major eukaryotic lineages. However, the underlying mechanisms of transfer and their impact on genome evolution are still poorly understood. While studying the evolutionary origin of a selfish element in the nematode Caenorhabditis briggsae, we discovered that Mavericks, ancient virus-like transposons related to giant viruses and virophages, are one of the long-sought vectors of horizontal gene transfer. We found that Mavericks gained a novel herpesvirus-like fusogen in nematodes, leading to the widespread exchange of cargo genes between extremely divergent species, bypassing sexual and genetic barriers spanning hundreds of millions of years. Our results show how the union between viruses and transposons causes horizontal gene transfer and ultimately genetic incompatibilities in natural populations.