Mechanisms driving endosperm-based hybrid incompatibilities: insights from hybrid monkeyflowers

Angiosperm endosperm requires genomic and epigenomic interactions between maternal and paternal genomes for proper seed development. Genomic imprinting, an epigenetic phenomenon where the expression of certain genes is predominantly contributed by one parent, is an essential part of this process and unique to endosperm. Perturbation of imprinting can be fatal to developing seeds, and can be caused by interspecific or interploidy hybridization. However, underlying mechanisms driving these endosperm-based hybridization barriers are not well understood or described. Here we investigate the consequences of genomic imprinting in a naturally occurring interploidy and interspecies hybrid between the diploid, Mimulus guttatus , and the allotetraploid (with two subgenomes), M. luteus (Phrymaceae). We find that the two parental species differ in patterns of DNA methylation, gene expression, and imprinting. Hybrid crosses in both directions, which suffer from endosperm abnormalities and decreased germination rates, display altered methylation patterns compared to parent endosperm. Furthermore, imprinting and expression patterns appear perturbed in hybrid endosperm, where we observe global expression dominance of each of the two M. luteus subgenomes, which share similar expression patterns, over the M. guttatus genome, regardless of crossing direction. We suggest that epigenetic repatterning within the hybrid may drive global shifts in expression patterns and be the result of diverged epigenetic/regulatory landscapes between parental genomes. This may either establish or exacerbate dosage-based epistatic incompatibilities between the specific imprinting patterns that have diverged between parental species, thus driving potentially rapid endosperm-based hybridization barriers.