Horizontal gene transfer (HGT), or the transfer of genes between species, has been recognized recently as more pervasive than previously suspected. Here, we report evidence for an unprecedented degree of HGT into an animal genome, based on a draft genome of a tardigrade, Hypsibius dujardini. Tardigrades are microscopic eight-legged animals that are famous for their ability to survive extreme conditions. Genome sequencing, direct confirmation of physical linkage, and phylogenetic analysis revealed that a large fraction of the H. dujardini genome is derived from diverse bacteria as well as plants, fungi, and Archaea. We estimate that approximately one-sixth of tardigrade genes entered by HGT, nearly double the fraction found in the most extreme cases of HGT into animals known to date. Foreign genes have supplemented, expanded, and even replaced some metazoan gene families within the tardigrade genome. Our results demonstrate that an unexpectedly large fraction of an animal genome can be derived from foreign sources. We speculate that animals that can survive extremes may be particularly prone to acquiring foreign genes.
Anteroposterior polarity in early C. elegans embryos is required for the specification of somatic and germline lineages, and is initiated by a sperm-induced reorganization of the cortical cytoskeleton and PAR polarity proteins. Through mechanisms that are not understood, the kinases PAR-1 and PAR-4, and other PAR proteins cause the cytoplasmic zinc finger protein MEX-5 to accumulate asymmetrically in the anterior half of the one-cell embryo. We show that MEX-5 asymmetry requires neither vectorial transport to the anterior, nor protein degradation in the posterior. MEX-5 has a restricted mobility before fertilization and in the anterior of one-cell embryos. However, MEX-5 mobility in the posterior increases as asymmetry develops, presumably allowing accumulation in the anterior. The MEX-5 zinc fingers and a small, C-terminal domain are essential for asymmetry; the zinc fingers restrict MEX-5 mobility, and the C-terminal domain is required for the increase in posterior mobility. We show that a crucial residue in the C-terminus, Ser 458, is phosphorylated in vivo. PAR-1 and PAR-4 kinase activities are required for the phosphorylation of S458, providing a link between PAR polarity proteins and the cytoplasmic asymmetry of MEX-5.
During organogenesis of the C. elegans digestive system, epithelial cells within a cyst-like primordium develop diverse shapes through largely unknown mechanisms. We here analyze two adjacent, dorsal epithelial cells, called pm8 and vpi1, that remodel their shapes and apical junctions to become donut-shaped, or toroidal, single-cell tubes. pm8 and vpi1 delaminate from the dorsal cyst epithelium and migrate ventrally, across the midline of the cyst, on a transient tract of laminin. pm8 appears to encircle the midline by wrapping around finger-like projections from neighboring cells. Finally, pm8 and vpi1 self-fuse to become toroids by expressing AFF-1 and EFF-1, two fusogens that are each sufficient to promote crossfusion between other cell types. Notch signaling in pm8 induces AFF-1 expression, while simultaneously repressing EFF-1 expression; vpi1 expresses EFF-1 independent of Notch. Thus, the adjacent pm8 and vpi1 cells express different fusogens, allowing them to self-fuse into separate, single-cell tubes while avoiding crossfusion.
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