Abstract
Genome sequencing has revealed examples of horizontally transferred genes, but we still know little about how such genes are incorporated into their host genomes. We have previously reported the identification of a gene (flp) that appears to have entered the Hydra genome through horizontal transfer. Here we provide additional evidence in support of our original hypothesis that the transfer was from a unicellular organism, and we show that the transfer occurred in an ancestor of two medusozoan cnidarian species. In addition we show that the gene is part of a bicistronic operon in the Hydra genome. These findings identify a new animal phylum in which trans-spliced leader addition has led to the formation of operons, and define the requirements for evolution of an operon in Hydra. The identification of operons in Hydra also provides a tool that can be exploited in the construction of transgenic Hydra strains.
Highlights
Horizontal transfer of genes is widely accepted as a significant feature of genome evolution in prokaryotes [1], and likely in unicellular eukaryotes as well [2]
One makes the argument for horizontal gene transfer (HGT) based on parsimony – with a single horizontal transfer event being considered more parsimonious than multiple secondary losses of a gene
How frequently horizontal transfer of genes into the genomes of metazoans occurs, what taxa the genes come from, and the routes they take to get into the target genome are questions about which we have very little information
Summary
Horizontal transfer of genes is widely accepted as a significant feature of genome evolution in prokaryotes [1], and likely in unicellular eukaryotes as well [2]. It has been much more difficult to build convincing cases for horizontal gene transfer (HGT) into animal genomes. The amount of HGT in animals is expected to be much less than in unicellular organisms This expectation is due to the absence in animals of the facile routes for DNA uptake seen in prokaryotes and the necessity of targeting the germ line, which is segregated in most metazoans, in order for a horizontally transferred gene to be propagated [3]. The most commonly used evidence for HGT is anomalous phylogenetic distribution of the gene being considered. A relatively small number of animal genomes have been sequenced, making models invoking gene loss [4] viable contenders for explaining anomalous phylogenetic distributions in animals. Without information on how difficult it is for a gene to be lost versus the difficulty of being horizontally transferred into a given animal genome, it is not known whether this assumption is correct
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