Low parental conflict, no endosperm hybrid barriers, and maternal bias in genomic imprinting in selfing Draba species
In flowering plants, a distinct post-zygotic hybridization barrier between closely related species can arise during seed maturation, resulting in embryo lethality due to abnormal endosperm development. The endosperm initially works as a nutrient sink, acquiring nutrients from adjacent tissues, but later undergoes cellularization, switching to serve as a nutrient source. In hybrid seeds, this cellularization switch can be hampered if the endosperm genomic ratio is imbalanced. Disruption in the genomic ratio can be caused when species of different ploidy are crossed, but also by crosses between species with identical ploidy, if the effective ploidy differs. One factor proposed to influence effective ploidy is the epigenetic phenomenon genomic imprinting, the parent-of-origin specific expression of alleles inherited either maternally or paternally. It has been proposed that outbreeding species exhibit higher effective ploidy compared to selfing species, as a consequence of parental conflict in resource allocation to the developing progenies. This suggests a low anticipation of endosperm-based post-zygotic hybridization barriers between selfing species of similar ploidy. Here, we show that in crosses between the diploid selfing arctic species Draba fladnizensis, D. nivalis and D. subcapitata, the endosperm-based post-zygotic hybridization barrier is absent, supporting low parental conflict. To investigate parent-of-origin allele specific expression, we conducted a genomic imprinting study in D. nivalis and compared to previous studies in other Brassicaceae species. We report a high number of maternally expressed genes (MEGs) and concomitantly low numbers of paternally expressed genes (PEGs). Our results suggest rapid evolution of MEGs and loss of PEGs in a mating system with low parental conflict, proposing that selfing arctic species may exhibit a generally stronger maternal expression bias as an adaptive mechanism to efficiently cope with an extreme environment.