Relationship between transmission ratio distortion and genetic divergence in intraspecific rice crosses.

The strength of reproductive isolation often correlates positively with parental divergence in both animals and plants. Here, we assess the relationship between transmission ratio distortion (TRD) of marker loci and parental divergence in intraspecific rice (Oryza sativa L.) crosses. We produced 10 diverse F(2) populations by crossing a temperate japonica reference accession with each of 10 donor accessions belonging to subpopulations different from the reference accession, and then genotyped the F(2) populations using molecular markers distributed across the entire genome. Significant TRDs (α = 0.05) were detected in 9 of the 10 F(2) populations. TRD regions on chromosomes 3 and 6 were common to several populations; in contrast, other TRD regions were unique to a single population, indicating the diversification of genomic location of TRDs among the populations. The level of TRD (estimated from the overall number and magnitude of TRDs) was significantly correlated with the genetic distance between the donor accessions and the reference accession. Our results suggest that in intraspecific rice crosses, parental divergence may result in diversification of the TRD pattern, followed by an increase in the level of TRD.

The role of casein kinase II in flowering time regulation has diversified during evolution.

Casein kinase II (CK2) is a protein kinase with an evolutionarily conserved function as a circadian clock component in several organisms, including the long-day plant Arabidopsis (Arabidopsis thaliana). The circadian clock component CIRCADIAN CLOCK ASSOCIATED1 (CCA1) is a CK2 target in Arabidopsis, where it influences photoperiodic flowering. In rice (Oryza sativa), a short-day plant, Heading date6 (Hd6) encodes a CK2alpha subunit that delays flowering time under long-day conditions. Here, we demonstrate that control of flowering time in rice by the Hd6 CK2alpha subunit requires a functional Hd1 gene (an Arabidopsis CONSTANS ortholog) and is independent of the circadian clock mechanism. Our findings from overexpressing the dominant-negative CK2 allele in rice support the independence of CK2 function from the circadian clock. This lack of control of the circadian clock by Hd6 CK2alpha might be due to the presence of glutamate in OsLHY (a CCA1 ortholog in rice) instead of the serine at the corresponding CK2 target site in CCA1. However, this glutamate is critical for the control of the OsPRR1 gene (a rice ortholog of the Arabidopsis TOC1/PRR1 gene) by OsLHY for regulation of the circadian clock. We also demonstrated that the other conserved CK2 target sites in OsLHY conferred robust rhythmic expression of OsLHY-LUC under diurnal conditions. These findings imply that the role of CK2 in flowering-time regulation in higher plants has diversified during evolution.