A complex dominance hierarchy is controlled by polymorphism of small RNAs and their targets.

In diploid organisms, phenotypic traits are often biased by effects known as Mendelian dominant-recessive interactions between inherited alleles. Phenotypic expression of SP11 alleles, which encodes the male determinants of self-incompatibility in Brassica rapa, is governed by a complex dominance hierarchy1-3. Here, we show that a single polymorphic 24 nucleotide small RNA, named SP11 methylation inducer 2 (Smi2), controls the linear dominance hierarchy of the four SP11 alleles (S44 > S60 > S40 > S29). In all dominant-recessive interactions, small RNA variants derived from the linked region of dominant SP11 alleles exhibited high sequence similarity to the promoter regions of recessive SP11 alleles and acted in trans to epigenetically silence their expression. Together with our previous study4, we propose a new model: sequence similarity between polymorphic small RNAs and their target regulates mono-allelic gene expression, which explains the entire five-phased linear dominance hierarchy of the SP11 phenotypic expression in Brassica.

Dependence of the timing system regulating the onset of gastrulation on cytoplasmic, but not nuclear, activities in the Xenopus embryo.

This study examines the properties of the timer that regulates the onset of gastrulation in the Xenopus embryo. Pre-gastrulation embryos were exposed to aphidicolin, vinblastine, 6-dimethylaminopurine (6-DMAP) or urethane. Embryos exposed to aphidicolin or vinblastine for 0.5-2 h before the presumptive onset of gastrulation, began gastrulation at the same time as control embryos. However, those exposed to 6-DMAP or urethane commenced gastrulation significantly later than controls. In 6-DMAP- and urethane-treated embryos, the onset of gastrulation was retarded by approximately 25% and 120%, respectively. 6-DMAP and urethane, but not vinblastine, also lowered the rate of nuclear doubling by 30% and 120%, respectively, in late-blastula to early-gastrula embryos. 6-DMAP and urethane also lowered the rate of cleavage and cleavage-relevant cytoplasmic cycling by 30% and 80%, respectively, in cleavage-stage embryos. We propose that cytoplasmic activities that can be retarded by 6-DMAP and urethane, but not aphidicolin or vinblastine, may be responsible for regulating the onset of gastrulation in Xenopus embryos.