Endosperm-based incompatibilities in hybrid monkeyflowers

The sexual endosperm is an angiosperm innovation central to flowering
plant reproduction. Genomic interactions between parental alleles control
its development and help determine seed viability. These interactions are
characterized by genomic imprinting, where expression from certain genes
is parent-specific. Unsuccessful imprinting has been linked to failed
hybridization between plants of different species or ploidies. Here, we
describe an endosperm-based barrier between Mimulus guttatus, a diploid,
and M. luteus, an allotetraploid. Hybrid seeds suffer from underdeveloped
endosperm, reducing viability, when M. guttatus is the seed parent, and
from arrested endosperm and abortion when M. luteus is the seed parent.
The two parental species differ in patterns of endosperm DNA methylation,
expression dynamics, and their sets of imprinted genes; and transgressive
patterns of methylation and expression emerge in their hybrids. The two
inherited M. luteus subgenomes, which are genetically distinct but
epigenetically similar, are expressionally dominant over the M. guttatus
genome in the hybrid embryo and especially endosperm, where paternal
imprints, in particular M. luteus', are perturbed. We suggest that
diverged epigenetic landscapes between parental genomes result in
epigenetic repatterning in hybrids that drive global shifts in expression
patterns and can lead to endosperm-based hybridization barriers.