This paper focuses on how interstitial annelids evolved to live in interstitial spaces. Three hypotheses on how this evolution occurred include progenesis, miniaturization, and that the interstitial space was originally an ancestral home to Bilateria. Progenesis is the retention of juvenile characteristics in the adult form, and miniaturization is the evolution of a small body size. Progenesis is the commonly accepted hypothesis for interstitial annelid evolution, though it has not been fully explored, which is a reason this study was conducted. The authors of the study applied a phylogenomic approach by generating transcriptome sequencing data. They found relationships and similarities between clades to figure out how different annelids evolved in interstitial spaces. Progenetic evolution was found in Orbiniida, Eunicida, and Hesionidae. Evolution through miniaturization was found in Protodriliformia, Pisionidae, and other interstitial species. One of the conclusions they found was that the simple body organization and interstitial habitat of annelids are not ancestral traits of annelida. This result led them to reject the “Archiannelida” hypothesis. The authors also found that progenesis and miniaturization were equal in terms of significance as factors that caused annelids to evolve in interstitial spaces.
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