Parent-of-origin specific allelic expression in outbreeding Arabidopsis arenosa identifies antagonistic parental enrichment in protein degradation pathways.

In plants, the epigenetic phenomenon of parent-of-origin allele-specific expression occurs mainly in the triploid endosperm. Although well studied in inbreeding Arabidopsis thaliana, genomic imprinting has been less investigated in outcrossers. In order to investigate a wider role of parental-specific allelic expression, we have analyzed imprinting in whole seeds of the obligate outbreeder Arabidopsis arenosa. High-throughput analysis of imprinting in outbreeding species is hampered by the lack of reference genomes and available sequenced accessions. High degree of allelic variation in outbreeding species may also limit the analysis to loci with less variation. We developed a reference-independent pipeline to detect parental-specific reads. Using different accessions in reciprocal crosses, we detected more than 70 paternally biased imprinted genes and > 500 maternally biased genes. Paternally biased genes showed major enrichment for proteins with ubiquitin protein transferase and ligase activity. Maternally biased genes were enriched for protein pathways directly counteracting paternally enriched genes. Here, we demonstrate an alignment-free protocol to identify imprinted genes that may be successfully applied for imprinting studies in other highly heterozygous outcrossing species. Our results suggest a unique role of genomic imprinting affecting post-transcriptional gene regulation in outbreeding A. arenosa.