Are echinoderm embryos more evolutionarily derived than chordate embryos? – Let’s distinguish “conservation” and “derivedness”
by Jason Cheok Kuan Leong | Masahiro Uesaka | Echinoderm Genomes Sequencing Consortium | Naoki Irie | Research Center for Integrative Evolutionary Science, SOKENDAI, Hayama, Japan | Graduate School of Life Sciences, Tohoku University, Sendai, Japan | International Consortium | Research Center for Integrative Evolutionary Science, SOKENDAI, Hayama, Japan
Abstract ID: 131
Event: The 3rd AsiaEvo Conference
Topic: Evolvability: a common currency of evolution, ecology and development
Presenter Name: Jason Cheok Kuan Leong

Which embryo of which species evolved more from the common ancestor? For example, echinoderms achieved an exceptional five-fold symmetrical body plan despite being bilaterian animals – are their embryos more evolutionary derived than the sister group (chordates), i.e., did the way how they form their bodies accumulate much more evolutionary changes than chordates since the divergence from their common ancestor? In order to answer this, methods to properly evaluate their “evolutionary derivedness” have to be devised. Previous approaches mainly list up individual traits while comparing their embryonic phenotypes, but this can be highly arbitrary, and no consensus has been made to address this issue. Importantly, to evaluate “derivedness”, a conceptual ambiguity with the concept of “evolutionary conservation” has hindered biologists from using proper methods to tackle this question. Here we discuss the essential differences between “conservation” and “derivedness”, and highlight the necessity for the development of more derivedness-oriented molecular methods in the genomics era. In brief, “conservation” represents information retained during evolution while “derivedness” represents evolutionary changes, which additionally cover information that is not shared among the species being compared. As an application of the concept, we devised a method to estimate the degree of derivedness of embryos to evaluate whether echinoderms are more derived than chordates or not. We found consistent tendencies with the current understanding that the stages in echinoderms developing their characteristic penta-radial structures indeed tend to be more derived than the bilateral stages. However, the results surprisingly showed that the echinoderm embryos as a whole may not show higher derivedness than the chordate embryos. Through developing this method, we propose that the distinction between "conservation" and "derivedness" is a key to more properly understanding phenotypic evolution and evolvability.