RNA interference is now a well-recognized post-transcriptional mechanism for regulation of gene expression in both animals and plants. In this process, microRNAs (miRNAs) direct silencing complexes to complementary RNA sequences, leading to either degradation or repression of translation. Plants also contain another type of small RNA, small interfering RNAs (siRNAs), that play a role in gene silencing by directing cytosine methylation activities of complementary DNA sequences and thus, differ from miRNAs. This nuclear regulation system is referred to as RNA-directed DNA methylation (RdDM). In plant genomes, transposable elements were initially thought to be regulated by DNA methylation alone. However, several recent reports have revealed that siRNAs and RdDM also play crucial roles in silencing of transposons and endogenous repeats. It is also becoming apparent that transposons are subjected to different levels of regulation in response to developmental and environmental cues. Transposons are tightly regulated in germ cells to protect the host genome from transgenerational mutagenic activity. In plants, transposons are also activated by biotic and abiotic stress. The regulation of transposons in these different situations has been associated with both the DNA methylation and siRNA-mediated regulation systems, suggesting that plants likely evolved “multi-lock” systems for transposon regulation to ensure tight control during the developmental phase and environmental changes.
A mechanism is required to repress the expression and transposition of transposable elements (TEs) to ensure the stable inheritance of genomic information. Accumulating evidence indicates that small non-coding RNAs are important regulators of TEs. Among small non-coding RNAs, PIWI-interacting RNAs (piRNAs) serve as guide molecules for recognizing and silencing numerous TEs and work in collaboration with PIWI subfamily proteins in gonadal cells. Disruption of the piRNA pathway correlates with loss of proper genomic organization, gene expression control and fertility. Moreover, recent studies on the molecular mechanisms of piRNA biogenesis and on piRNA function have shown that piRNAs act as maternally inherited genic elements, transferring information about repressed TEs to progeny. These findings enable a molecular explanation of mysterious epigenetic phenomena, such as hybrid dysgenesis and TE adaptation with age. Here, I review our current knowledge of piRNAs derived from biochemical and genetic studies and discuss how small RNAs are utilized to maintain genome organization and to provide non-DNA genetic information. I mainly focus on Drosophila but also discuss comparisons with other species.
Short interspersed elements (SINEs) are a class of retrotransposons, which amplify their copy numbers in their host genomes by retrotransposition. More than a million copies of SINEs are present in a mammalian genome, constituting over 10% of the total genomic sequence. In contrast to the other two classes of retrotransposons, long interspersed elements (LINEs) and long terminal repeat (LTR) elements, SINEs are transcribed by RNA polymerase III. However, like LINEs and LTR elements, the SINE transcription is likely regulated by epigenetic mechanisms such as DNA methylation, at least for human Alu and mouse B1. Whereas SINEs and other transposable elements have long been thought as selfish or junk DNA, recent studies have revealed that they play functional roles at their genomic locations, for example, as distal enhancers, chromatin boundaries and binding sites of many transcription factors. These activities imply that SINE retrotransposition has shaped the regulatory network and chromatin landscape of their hosts. Whereas it is thought that the epigenetic mechanisms were originated as a host defense system against proliferation of parasitic elements, this review discusses a possibility that the same mechanisms are also used to regulate the SINE-derived functions.
Eukaryotic genomes comprise numerous retroelements that have a major impact on the structure and regulation of gene function. Retroelements are regulated by epigenetic controls, and they generate multiple miRNAs that are involved in the induction and progression of genomic instability. Elucidation of the biological roles of retroelements deserves continuous investigation to better understand their evolutionary features and implications for disease.
Notch signaling is an evolutionarily conserved mechanism that controls many cell-fate specifications through local cell-cell interactions. The core mechanisms of Notch activation and its subsequent intracellular signaling are well understood. Various cellular functions are required for the activation and regulation of Notch signaling. Among them, the endocytosis of Notch and its ligands is important for the activation and suppression of Notch signaling. The endosomal sorting complex required for transport (ESCRT) proteins are required to sort ubiquitinated membrane proteins, such as Notch, into early endosomes. A loss-of-function allele of vacuolar protein sorting 2 (vps2), which encodes a component of ESCRT-III, has been reported. However, this vps2 mutant still produces the N-terminal half of the protein, and its phenotypes were studied in only a few organs. Here, we generated the first null mutant allele of Drosophila vps2, designated vps22, to better understand the function of this gene. In Drosophila wing imaginal discs homozygous for the vps22 allele, early endosomes and multivesicular bodies (MVBs) were enlarged, and Notch and Delta accumulated inside them. As reported for the previous vps2 mutant, the epithelium grew excessively under this condition. We further studied the roles of vps2 by RNA interference-knockdown. These experiments revealed that a partial reduction of vps2 attenuated Notch signaling; in contrast, the loss-of-function vps2 mutant is reported to up-regulate the Notch signaling in eye imaginal disc cells. These results suggest that Notch signaling can be up- or down-regulated, depending on the level of vps2 expression. Finally, we found that vps2 overexpression also resulted in early-endosome enlargement and the accumulation of Notch and Delta. In these cells, a portion of the Vps2 protein was detected in MVBs and colocalized with Notch. These data indicate that the expression of vps2 must be precisely regulated to maintain the normal structure of early endosomes.
Genus Babina is a member of Ranidae, a large family of frogs, currently comprising 10 species. Three of them are listed as endangered species. To identify mitochondrial (mt) genes suitable for future population genetic analyses for endangered species, we determined the complete nucleotide sequences of the mt genomes of 3 endangered Japanese Babina frogs, B. holsti, B. okinavana, and B. subaspera and 1 ranid frog Lithobates catesbeianus. The genes of NADH dehydrogenase subunit 5 (nad5) and the control region (CR) were found to have high sequence divergences and to be usable for population genetics studies. At present, no consensus on the phylogenetic position of genus Babina has been reached. To resolve this problem, we performed molecular phylogenetic analyses with the largest dataset used to date (11,345 bp from 2 ribosomal RNA- and 13 protein-encoding genes) in studies dealing with Babina phylogeny. These analyses revealed monophyly of Babina and Odorrana. It is well known that mt gene rearrangements of animals can provide usable phylogenetic information. Thus, we also compared the mt gene arrangements among Babina species and other related genera. Of the surveyed species, only L. catesbeianus manifested typical neobatrachian-type mt gene organization. In the B. okinavana, an additional pseudogene of tRNA-His (trnH) was observed in the CR downstream region. Furthermore, in the B. holsti and B. subaspera, the trnH/nad5 block was translocated from its typical position to the CR downstream region, and the translocated trnH became a pseudogene. The position of the trnH pseudogene is consistent with the translocated trnH position reported in Odorrana. Consequently, the trnH rearrangement seems to be a common ancestry characteristic (synapomorphy) of Babina and Odorrana. Based on the “duplication and deletion” gene rearrangement model, a single genomic duplication event can explain the order of derived mt genes found in Babina and Odorrana.
The raccoon dog (Nyctereutes procyonoides) is distributed from southeastern Siberia to northern Vietnam, including Korea and Japan, as well as Europe. In Korea, most of its predators and competitors are extinct, which has resulted in rapid growth of the raccoon dog population. This population increase has raised concerns about its role in the ecosystem and the zoonotic transfer of various contagious diseases, and thus an effective method of raccoon dog population control in Korea is required. To investigate the genetic diversity and structure of raccoon dog populations, 12 polymorphic microsatellite loci were identified and characterized. These novel microsatellite markers were employed to obtain basic population genetic parameters for 104 N. procyonoides specimens from five locations in South Korea. The mean allele number of 12 loci across samples was 8.7, and the number of alleles per locus ranged 2–13. Mean expected and observed heterozygosities were 0.723 and 0.619, respectively. Genetic differentiation, estimated by pairwise FST, was significant for all population pairs excepting Seoul/Gyeonggi and Gangwon pair, with a moderate level of genetic differentiation for all the population pairs (mean FST = 0.054), but little differentiation between Seoul/Gyeonggi and Gangwon (FST = 0.024). Bayesian-based clustering analysis predicted that Korean raccoon dog population is composed of four distinct genetic subpopulations. These genetic information and structure of raccoon dog will be very useful to prevent spreading infectious diseases.