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Plastid DNA fragments are often found in the plant nuclear genome, and DNA transfer from plastids to the nucleus is ongoing. However, successful gene transfer is rare. What happens to compensate for this? To address this question, we analyzed nuclear-localized plastid DNA (nupDNA) fragments throughout the rice (Oryza sativa ssp japonica) genome, with respect to their age, size, structure, and integration sites on chromosomes. The divergence of nupDNA sequences from the sequence of the present plastid genome strongly suggests that plastid DNA has been transferred repeatedly to the nucleus in rice. Age distribution profiles of the nupDNA population, together with the size and structural characteristics of each fragment, revealed that once plastid DNAs are integrated into the nuclear genome, they are rapidly fragmented and vigorously shuffled, and surprisingly, 80% of them are eliminated from the nuclear genome within a million years. Large nupDNA fragments preferentially localize to the pericentromeric region of the chromosomes, where integration and elimination frequencies are markedly higher. These data indicate that the plant nuclear genome is in equilibrium between frequent integration and rapid elimination of the chloroplast genome and that the pericentromeric regions play a significant role in facilitating the chloroplast-nuclear DNA flux.

The abundance, diversity, activity, and composition of microbial communities in sulfide structures both of active and inactive vents were investigated by culture-independent methods. These sulfide structures were collected at four hydrothermal fields, both on-and off-axis of the back-arc spreading center of the Southern Mariana Trough. The microbial abundance and activity in the samples were determined by analyzing total organic content, enzymatic activity, and copy number of the 16S rRNA gene. To assess the diversity and composition of the microbial communities, 16S rRNA gene clone libraries including bacterial and archaeal phylotypes were constructed from the sulfide structures. Despite the differences in the geological settings among the sampling points, phylotypes related to the Epsilonproteobacteria and cultured hyperthermophilic archaea were abundant in the libraries from the samples of active vents. In contrast, the relative abundance of these phylotypes was extremely low in the libraries from the samples of inactive vents. These results suggest that the composition of microbial communities within sulfide structures dramatically changes depending on the degree of hydrothermal activity, which was supported by statistical analyses. Comparative analyses suggest that the abundance, activity and diversity of microbial communities within sulfide structures of inactive vents are likely to be comparable to or higher than those in active vent structures, even though the microbial community composition is different between these two types of vents. The microbial community compositions in the sulfide structures of inactive vents were similar to those in seafloor basaltic rocks rather than those in marine sediments or the sulfide structures of active vents, suggesting that the microbial community compositions on the seafloor may be constrained by the available energy sources. Our findings provide helpful information for understanding the biogeography, biodiversity and microbial ecosystems in marine environments.

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