Big roles for small RNAs in polyploidy, hybrid vigor, and hybrid incompatibility

Small RNAs, including microRNAs (miRNAs), small interfering RNAs (siRNAs), and trans-acting siRNAs (ta-siRNAs), mediate gene expression and epigenetic regulation. While siRNAs are highly diverged, miRNAs and ta-siRNAs are generally conserved but many are differentially expressed between related species and in interspecific hybrids and allopolyploids. On one hand, combination of diverged maternal and paternal siRNAs in the same nucleus may exert cis-acting and trans-acting effects on transposable elements (TEs) and TE-associated genes, leading to genomic instability and endosperm and embryo failures, constituting a bottleneck for the evolution of hybrids and polyploids. On the other hand, cis and trans-acting small RNAs induce quantitative and qualitative changes in epigenetic regulation, leading to morphological variation and hybrid vigor in F1 hybrids and stable allopolyploids as well as transgressive phenotypes in the progeny, increasing a potential for adaptive evolution.

► Several mechanisms regulate small RNAs in hybrids and allopolyploids.
► Quantitative and qualitative sRNA changes affect species variation and hybrid vigor.
► Maternal inheritance of siRNAs alters activities of TEs and TE-associated genes.
► Incompatible parental siNRAs lead to genome instability and hybrid incompatibility.
► The knowledge is valuable for improving many crops grown as hybrids and polyploids.