As germ plasm evolved independently in many animal lineages — which suggests convergent evolution — it was thought to confer a selective advantage. One of the corresponding authors, Andrew D. Johnson (University of Nottingham), previously proposed that the selective advantage that drives the emergence of germ plasm in vertebrates is the disengagement of germline specification from somatic influences, which would free the evolution of gene regulatory networks that govern somatic development and thereby enhance species evolvability.
In this study, the researchers combined various bioinformatic techniques into a comprehensive pipeline to analyse sequence change across almost all known protein-coding sequences in vertebrates. Simple four-taxon trees were constructed on the basis of high-quality sequence alignments to investigate the relationships between species using epigenesis and those using preformation. Moreover, the relative-rate test was used to determine whether sequences in species that use preformation were changing at different rates. These results showed that species using preformation have a faster rate of sequence evolution than those using epigenesis. In other words, genes evolve more rapidly in species with germ plasm. “The sequences that are evolving at different rates have a tendency to be expressed during early development,” says Johnson. “We have previously shown dramatic differences between axolotls and Xenopus with sequences expressed during early development, but we were not prepared for the scale of the differences we observed here.”
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