The cnidarian freshwater polyp Hydra sp. exhibits an unparalleled regeneration capacity in the animal kingdom. Using an integrative transcriptomic and stable isotope labeling by amino acids in cell culture proteomic/phosphoproteomic approach, we studied stem cell-based regeneration in Hydra polyps. As major contributors to head regeneration, we identified diverse signaling pathways adopted for the regeneration response as well as enriched novel genes. Our global analysis reveals two distinct molecular cascades: an early injury response and a subsequent, signaling driven patterning of the regenerating tissue. A key factor of the initial injury response is a general stabilization of proteins and a net upregulation of transcripts, which is followed by a subsequent activation cascade of signaling molecules including Wnts and transforming growth factor (TGF) beta-related factors. We observed moderate overlap between the factors contributing to proteomic and transcriptomic responses suggesting a decoupled regulation between the transcriptional and translational levels. Our data also indicate that interstitial stem cells and their derivatives (e.g., neurons) have no major role in Hydra head regeneration. Remarkably, we found an enrichment of evolutionarily more recent genes in the early regeneration response, whereas conserved genes are more enriched in the late phase. In addition, genes specific to the early injury response were enriched in transposon insertions. Genetic dynamicity and taxon-specific factors might therefore play a hitherto underestimated role in Hydra regeneration.

BackgroundWhen a large number of alleles are lost from a population, increases in individual homozygosity may reduce individual fitness through inbreeding depression. Modest losses of allelic diversity may also negatively impact long-term population viability by reducing the capacity of populations to adapt to altered environments. However, it is not clear how much genetic diversity within populations may be lost before populations are put at significant risk. Development of tools to evaluate this relationship would be a valuable contribution to conservation biology. To address these issues, we have created an experimental system that uses laboratory populations of an estuarine crustacean, Americamysis bahia with experimentally manipulated levels of genetic diversity. We created replicate cultures with five distinct levels of genetic diversity and monitored them for 16 weeks in both permissive (ambient seawater) and stressful conditions (diluted seawater). The relationship between molecular genetic diversity at presumptive neutral loci and population vulnerability was assessed by AFLP analysis.ResultsPopulations with very low genetic diversity demonstrated reduced fitness relative to high diversity populations even under permissive conditions. Population performance decreased in the stressful environment for all levels of genetic diversity relative to performance in the permissive environment. Twenty percent of the lowest diversity populations went extinct before the end of the study in permissive conditions, whereas 73% of the low diversity lines went extinct in the stressful environment. All high genetic diversity populations persisted for the duration of the study, although population sizes and reproduction were reduced under stressful environmental conditions. Levels of fitness varied more among replicate low diversity populations than among replicate populations with high genetic diversity. There was a significant correlation between AFLP diversity and population fitness overall; however, AFLP markers performed poorly at detecting modest but consequential losses of genetic diversity. High diversity lines in the stressful environment showed some evidence of relative improvement as the experiment progressed while the low diversity lines did not.ConclusionsThe combined effects of reduced average fitness and increased variability contributed to increased extinction rates for very low diversity populations. More modest losses of genetic diversity resulted in measurable decreases in population fitness; AFLP markers did not always detect these losses. However when AFLP markers indicated lost genetic diversity, these losses were associated with reduced population fitness.

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Chemical transmitters are either low molecular weight molecules or neuropeptides. As a general rule, neuropeptides activate only slow metabotropic receptors. To date, only one exception to this rule is known, the FMRFamide-activated Na ؉ channel (FaNaC) from snails. Until now FaNaC has been regarded as a curiosity, and it was not known whether peptidegated ionotropic receptors are also present in other animal groups. Nervous systems first evolved in cnidarians, which extensively use neuropeptides. Here we report cloning from the freshwater cnidarian Hydra of a novel ion channel (Hydra sodium channel, HyNaC) that is directly gated by the neuropeptides Hydra-RFamides I and II and is related to FaNaC. The cells expressing HyNaC localize to the base of the tentacles, adjacent to the neurons producing the Hydra-RFamides, suggesting that the peptides are the natural ligands for this channel. Our results suggest that neuropeptides were already used for fast transmission in ancient nervous systems.The DEG/ENaC gene family comprises ion channels with various functions and diverse gating mechanisms (1, 2): the epithelial sodium channel (ENaC) 2 is a constitutively open channel, acid-sensing ion channels (ASICs) are gated by extracellular H ϩ , and MEC/degenerin (DEG) channels are mechanically activated channels. The function of many other family members, like the intestinal sodium channel (INaC) (3), is unknown. Arguably the most curious member of this gene family is FaNaC from snails, the only known peptide-gated ion channel, gated by the peptide neurotransmitter Phe-Met-ArgPhe-NH 2 (FMRFamide) (4, 5). Our study was driven by an interest in defining the original properties and gating mechanism of the primitive ancestor of this gene family. BLAST homology searches revealed that the sequenced genomes of bacteria, yeast, and unicellular eukaryotes do not contain genes for DEG/ENaC channels. Thus, it seemed that this gene family evolved later in evolution, perhaps first in multicellular animals, suggesting that the primitive ancestor had a role in intercellular communication.Hydra belongs to the phylum Cnidaria that is characterized by a radial symmetry and a primitive nervous system that extensively uses neuropeptides for transmission (6). Because all other DEG/ENaC family members characterized to date are from animals with a bilateral symmetry, features common to channels from Hydra and other family members are likely to be primitive features of an ancestral channel that was present in metazoan animals that lived 600 -700 million years ago and that were at the base of the Radiata-Bilateria dichotomy. Here we report cloning and characterization of members of the DEG/ENaC gene family from Hydra magnipapillata, revealing a novel peptide-gated ion channel. The presence of related peptide-gated ion channels in Bilateria and Radiata shows that such channels have been present in organisms in which nervous systems first evolved.