In C. elegans, DCR-1 is required for the maturation of both short interfering RNAs (siRNAs) and microRNAs (miRNAs), which are subsequently loaded into different Argonaute proteins to mediate silencing via distinct mechanisms. We used in vivo analyses to show that precursors of small RNAs contain structural features that direct the small RNAs into the RNA interference (RNAi) pathway or the miRNA-processing pathway. Nucleotide changes in the pre-let-7 miRNA precursor that make its stem fully complementary cause the resulting small RNA to be recognized as siRNA and induce binding to RDE-1, which leads to RNAi. Mismatches of 1 to 3 nucleotides at various positions in the stem of the precursor restore direction into the miRNA pathway, as the largest portion of such small RNA variants is associated with ALG-1. The Argonaute proteins to which the small RNAs are bound determine the silencing mode, and no functional overlap between RDE-1 and ALG-1 was detected.

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Summary Pollen germination may occur via the so‐called germination pores or directly through the pollen wall at the site of contact with the stigma. In this study, we addressed what processes take place during pollen hydration (i.e. before tube emergence), in a species with extra‐poral pollen germination, Arabidopsis thaliana. A T‐DNA mutant population was screened by segregation distortion analysis. Histological and electron microscopy techniques were applied to examine the wild‐type and mutant phenotypes. Within 1 h of the start of pollen hydration, an intine‐like structure consisting of cellulose, callose and at least partly de‐esterified pectin was formed at the pollen wall. Subsequently, this ‘germination plaque’ gradually extended and opened up to provide passage for the cytoplasm into the emerging pollen tube. BURSTING POLLEN (BUP) was identified as a gene essential for the correct organization of this plaque and the tip of the pollen tube. BUP encodes a novel Golgi‐located glycosyltransferase related to the glycosyltransferase 4 (GT4) subfamily which is conserved throughout the plant kingdom. Extra‐poral pollen germination involves the development of a germination plaque and BUP defines the correct plastic‐elastic properties of this plaque and the pollen tube tip by affecting pectin synthesis or delivery.

The discovery of small RNA silencing pathways has greatly extended our knowledge of gene regulation. Small RNAs have been presumed to play a role in every field of biology because they affect many biological processes via regulation of gene expression and chromatin remodeling. Most well-known examples of affected processes are development, fertility, and maintenance of genome stability. Here we review the role of the three main endogenous small RNA silencing pathways in Caenorhabditis elegans: microRNAs, endogenous small interfering RNAs, and PIWI-interacting RNAs. After providing an entry-level overview on how these pathways function, we discuss research on other nematode species providing insight into the evolution of these small RNA pathways. In understanding the differences between the endogenous small RNA pathways and their evolution, a more comprehensive picture is formed of the functions and effects of small RNAs.

Transposable elements (TEs) are mobile genetic elements that can have many deleterious effects on the fitness of their host. The germline-specific PIWI pathway guards the genome against TEs, deriving its specificity from sequence complementarity between PIWI-bound small RNAs (piRNAs) and the TEs. The piRNAs are derived from so-called piRNA clusters. Recent studies have demonstrated that the piRNA repertoire can be adjusted to accommodate recent TE invasions by capturing invading TEs in piRNA loci. Thus far, no information concerning piRNA divergence is available from vertebrates. We present piRNA analyses of two relatively divergent zebrafish strains. We find that significant differences in the piRNA populations have accumulated, most notably among active class I TEs. This divergence can be split into differences in piRNA abundance per element and differences in sense/antisense polarity ratios. In crosses between animals of the different strains, many of these differences are resolved in the progeny. However, some differences remain, often leaning to the maternally contributed piRNA population. These differences can be detected at least two generations later. Our data illustrate, for the first time, the fluidity of piRNA populations in vertebrates and how the established diversity is transmitted to future generations.