Clonal animals do not sequester a germ line during embryogenesis. Instead, they have adult stem cells that contribute to somatic tissues or gametes. How germ fate is induced in these animals, and whether this process is related to bilaterian embryonic germline induction, is unknown. We show that transcription factor AP2 (Tfap2), a regulator of mammalian germ lines, acts to commit adult stem cells, known as i-cells, to the germ cell fate in the clonal cnidarian Hydractinia symbiolongicarpus. Tfap2 mutants lacked germ cells and gonads. Transplanted wild-type cells rescued gonad development but not germ cell induction in Tfap2 mutants. Forced expression of Tfap2 in i-cells converted them to germ cells. Therefore, Tfap2 is a regulator of germ cell commitment across germ line–sequestering and germ line–nonsequestering animals.

Organelle gain through endosymbiosis has been integral to the origin and diversification of eukaryotes, and, once gained, plastids and mitochondria seem seldom lost. Indeed, discovery of nonphotosynthetic plastids in many eukaryotes-notably, the apicoplast in apicomplexan parasites such as the malaria pathogen Plasmodiumhighlights the essential metabolic functions performed by plastids beyond photosynthesis. Once a cell becomes reliant on these ancillary functions, organelle dependence is apparently difficult to overcome. Previous examples of endosymbiotic organelle loss (either mitochondria or plastids), which have been invoked to explain the origin of eukaryotic diversity, have subsequently been recognized as organelle reduction to cryptic forms, such as mitosomes and apicoplasts. Integration of these ancient symbionts with their hosts has been too well developed to reverse. Here, we provide evidence that the dinoflagellate Hematodinium sp., a marine parasite of crustaceans, represents a rare case of endosymbiotic organelle loss by the elimination of the plastid. Extensive RNA and genomic sequencing data provide no evidence for a plastid organelle, but, rather, reveal a metabolic decoupling from known plastid functions that typically impede organelle loss. This independence has been achieved through retention of ancestral anabolic pathways, enzyme relocation from the plastid to the cytosol, and metabolic scavenging from the parasite's host. Hematodinium sp. thus represents a further dimension of endosymbiosis-life after the organelle.organelle loss | plastid loss | endosymbiosis | plastid metabolism | diaminopimelate aminotransferase

No abstract

The ability to regenerate lost body parts is irregularly distributed among animals, with substantial differences in regenerative potential between and within metazoan phyla. It is widely believed that regenerative animal clades inherited some aspects of their capacity to regenerate from their common ancestors but have also evolved new mechanisms that are not shared with other regenerative animals. Therefore, to gain a broad understanding of animal regenerative mechanisms and evolution, a broad sampling approach is necessary. Unfortunately, only few regenerative animals have been established as laboratory models with protocols for functional gene studies. Here, we describe the methods to establish transgenic individuals of the marine cnidarian Hydractinia. We also provide methods for transient gene expression manipulation without modifying the genome of the animals.

N6-methyldeoxyadenosine (6mA) is a chemical alteration of DNA, observed across all realms of life. The functions of 6mA are well understood in bacteria but its roles in animal genomes have been controversial. We show that 6mA randomly accumulates in early embryos of the cnidarian Hydractinia symbiolongicarpus, with a peak at the 16-cell stage followed by clearance to background levels two cell cycles later, at the 64-cell stage - the embryonic stage at which zygotic genome activation occurs in this animal. Knocking down Alkbh1, a putative initiator of animal 6mA clearance, resulted in higher levels of 6mA at the 64-cell stage and a delay in the commencement of zygotic transcription. Our data are consistent with 6mA originating from recycled nucleotides of degraded m6A-marked maternal RNA post-fertilization. Therefore, while 6mA does not function as an epigenetic mark in Hydractinia, its random incorporation into the early embryonic genome inhibits transcription. Alkbh1 functions as a genomic 6mA 'cleaner', facilitating timely zygotic genome activation. Given the random nature of genomic 6mA accumulation and its ability to interfere with gene expression, defects in 6mA clearance may represent a hitherto unknown cause of various pathologies.