Genetics News Plants

Sea anemone is genetically part animal, part plant

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File:Nematostella vectensis.JPG
star sea anemone/Cymothoa exigua

Or something. From ScienceDaily:

Simple organism with complex gene content

In the last decades the sequencing of the human and many animal genomes showed that anatomically simple organisms such as sea anemones depict a surprisingly complex gene repertoire like higher model organisms. This implies, that the difference in morphological complexity cannot be easily explained by the presence or absence of individual genes. Some researchers hypothesized that not the individual genes code for more complex body plans, but how they are wired and linked between each other. Accordingly, researchers expected that these gene networks are less complex in simple organisms than in human or “higher” animals.

Gene regulation comparable to higher animal model systems

“Since the sea anemone shows a complex landscape of gene regulatory elements similar to the fruit fly or other model animals, we believe that this principle of complex gene regulation was already present in the common ancestor of human, fly and sea anemone some 600 million years ago” , Michaela Schwaiger states.

MicroRNAs were also discovered in plants, but it has been assumed that they arose independently from animal microRNAs, since they (1) don’t show any sequence similarity to them, (2) have a different biogenesis pathway and (3) have a substantially different mode of action: … Yehu Moran, David Fredman and Daniela Praher from the Technau team were able to show that the microRNAs of the sea anemone depict all the hallmarks of plant microRNAs …

In summary, while the sea anemone’s genome, gene repertoire and gene regulation on the DNA level is surprisingly similar to vertebrates, its post-transcriptional regulation is plant-like and probably dates back to the common ancestor of animals and plants.

Abstract Despite considerable differences in morphology and complexity of body plans among animals, a great part of the gene set is shared among Bilateria and their basally branching sister group, the Cnidaria. This suggests that the common ancestor of eumetazoans already had a highly complex gene repertoire. At present it is therefore unclear how morphological diversification is encoded in the genome. Here we address the possibility that differences in gene regulation could contribute to the large morphological divergence between cnidarians and bilaterians. To this end, we generated the first genome-wide map of gene regulatory elements in a nonbilaterian animal, the sea anemone Nematostella vectensis. Using chromatin immunoprecipitation followed by deep sequencing of five chromatin modifications and a transcriptional cofactor, we identified over 5000 enhancers in the Nematostella genome and could validate 75% of the tested enhancers in vivo. We found that in Nematostella, but not in yeast, enhancers are characterized by the same combination of histone modifications as in bilaterians, and these enhancers preferentially target developmental regulatory genes. Surprisingly, the distribution and abundance of gene regulatory elements relative to these genes are shared between Nematostella and bilaterian model organisms. Our results suggest that complex gene regulation originated at least 600 million yr ago, predating the common ancestor of eumetazoans.


Abstract In bilaterians, which comprise most of extant animals, microRNAs (miRNAs) regulate the majority of messenger RNAs (mRNAs) via base-pairing of a short sequence (the miRNA “seed”) to the target, subsequently promoting translational inhibition and transcript instability. In plants, many miRNAs guide endonucleolytic cleavage of highly complementary targets. Because little is known about miRNA function in nonbilaterian animals, we investigated the repertoire and biological activity of miRNAs in the sea anemone Nematostella vectensis, a representative of Cnidaria, the sister phylum of Bilateria. Our work uncovers scores of novel miRNAs in Nematostella, increasing the total miRNA gene count to 87. Yet only a handful are conserved in corals and hydras, suggesting that microRNA gene turnover in Cnidaria greatly exceeds that of other metazoan groups. We further show that Nematostella miRNAs frequently direct the cleavage of their mRNA targets via nearly perfect complementarity. This mode of action resembles that of small interfering RNAs (siRNAs) and plant miRNAs. It appears to be common in Cnidaria, as several of the miRNA target sites are conserved among distantly related anemone species, and we also detected miRNA-directed cleavage in Hydra. Unlike in bilaterians, Nematostella miRNAs are commonly coexpressed with their target transcripts. In light of these findings, we propose that post-transcriptional regulation by miRNAs functions differently in Cnidaria and Bilateria. The similar, siRNA-like mode of action of miRNAs in Cnidaria and plants suggests that this may be an ancestral state.

They’re suggesting the system dates from 600 million years ago, but one doesn’t get the impression they really know. This isn’t a tree of life, it’s a grab bag.

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