Efficient mRNA splicing is a prerequisite for protein biosynthesis and the eukaryotic splicing machinery is evolutionarily conserved among species of various phyla. At its catalytic core resides the activated splicing complex Bact consisting of the three small nuclear ribonucleoprotein complexes (snRNPs) U2, U5 and U6 and the so-called NineTeen complex (NTC) which is important for spliceosomal activation. CWC15 is an integral part of the NTC in humans and it is associated with the NTC in other species. Here we show the ubiquitous expression and developmental importance of the Arabidopsis ortholog of yeast CWC15. CWC15 associates with core components of the Arabidopsis NTC and its loss leads to inefficient splicing. Consistent with the central role of CWC15 in RNA splicing, cwc15 mutants are embryo lethal and additionally display strong defects in the female haploid phase. Interestingly, the haploid male gametophyte or pollen in Arabidopsis, on the other hand, can cope without functional CWC15, suggesting that developing pollen might be more tolerant to CWC15-mediated defects in splicing than either embryo or female gametophyte. Angiosperms are the predominant group of land plants. A hallmark of this dominance in the course of evolution is the establishment of a reduced haploid phase (called gametophyte) in the life cycle of flowering plants. In free-living gametophytes of mosses, the gametophytic generation forms independently recognizable plants that can even be the dominant structure. In contrast, the gametophyte of flowering plants is reduced to a small dependent structure with an almost minimal number of cells and a short lifetime 1. In Arabidopsis, the female gametophyte is deeply embedded in sporophytic tissue, whereas the male gametophyte or pollen has to be released from the sporophytic anther tissue for pollination to occur. Upon successful pollen-stigma interaction, the pollen tube grows inside the transmitting tract towards the ovule. Recent research has revealed that several signaling molecules including peptides play a role in the guidance of the pollen tube, attraction by the female gametophyte and burst of the pollen tube tip within one of the two synergid cells. If any of the aforementioned processes is disrupted, fertilization does not take place. Several mutants with disruptions in these processes have been isolated in the past 1-3. Upon successful fertilization, the Arabidopsis zygote initiates a precise developmental program, which results in a heart-shaped embryo already comprising all major seedling organs: two primary leaves or cotyledons, a shoot meristem, a hypocotyl, and a primary root with a root meristem 4. This invariant embryo patterning and development is impaired in mutants defective for various cellular response pathways e.g. responses to phytohormones, small RNA pathways, vesicular trafficking, cytoskeletal structure, and cell cycle control 5-9. Furthermore, mutations in genes that are components of the RNA splicing machinery (spliceosome) affect gametophyte function and embryoge...