Paedomorphosis as an Evolutionary Driving Force: Insights from Deep-Sea Brittle Stars

Abstract

Heterochronic development has been proposed to have played an important role in the evolution of echinoderms. In the class Ophiuroidea, paedomorphosis (retention of juvenile characters into adulthood) has been documented in the families Ophiuridae and Ophiolepididae but not been investigated on a broader taxonomic scale. Historical errors, confusing juvenile stages with paedomorphic species, show the difficulties in correctly identifying the effects of heterochrony on development and evolution. This study presents a detailed analysis of 40 species with morphologies showing various degrees of juvenile appearance in late ontogeny. They are compared to a range of early ontogenetic stages from paedomorphic and non-paedomorphic species. Both quantitative and qualitative measurements are taken and analysed. The results suggest that strongly paedomorphic species are usually larger than other species at comparable developmental stage. The findings support recent notions of polyphyletic origin of the families Ophiuridae and Ophiolepididae. The importance of paedomorphosis and its correct recognition for the practice of taxonomy and phylogeny are emphasized.

Multiple paedomorphic lineages of soft-substrate burrowing invertebrates: parallels in the origin of Xenocratena and Xenoturbella

“…The morphological criterion well agrees with recent molecular data in which the Corambidae never appear as basal-most dorids but are placed inside of the group of families which possess exactly the same type of specialized buccal apparatus [36,38]. A parallel case is represented by ophiuroid echinoderms, where previously some strongly simplified groups were considered ancestral ones [39], however it was shown that such groups have specific features that are highly similar to the postlarval stages of common ophiuroid families with complex adult stages [40,41], thus fulfilling the first criterion of paedomorphosis. Most recently, a genome-scale analysis of a whole class of Ophiuroidea confirmed that previously assessed"ancestrally simple" ophiuroid taxa are nested within ophiuroid families with complex adult morphology [42].…”

“…This additional criterion should be applied especially carefully, because it is indirect, compared to the two main criteria. For example, it is well established that deep-sea environments facilitate the appearance of paedomorphic organisms, including such different groups as fishes [43] and ophiuroids [40,41]. An environment may contain non-paedomorphic groups as well, so such ecological information cannot be used solely as an indicative criterion and must be checked against the two main ones.…”

“…Another crucial consideration is that "progenetic" and "neotenic" patterns are just different sides of the same paedomorphic process [32,54]. In various organismal groups, often a taxon that demonstrates evident juvenile characters at the adult state is difficult to attribute exactly to "progenetic" or "neotenic" ones due to a strong heterochronic mosaicism of delayed and accelerated growth characters [41,52,53,55,56]. Contrasted paedomorphic pathways can also occur within same species [57] and there are many cases when a species demonstrates only partial paedomorphosis in some particular characters [4].…”

“…Therefore, the general term paedomoprhosis is preferable for current usage and more importantly avoids the pitfalls of the numerous inconsistent definitions of "progenesis" and "neoteny" [55,58]. While in biology at a general scale the importance of paedomorphosis has been recognised [51,59], the fields of systematics and phylogeny (both at theoretical and practical levels) essentially remains ontogeny-free, and only recently was a special field of research of ontogenetic systematics [4,36,40,41,60,61] outlined that targets linking such fundamental, but still very loosely connected, disciplines as evo-devo and taxonomy.…”

“Noah’s Ark” Project: Interim Results and Outlook for Classic Collection Development

“…Essentially new results were obtained for the classification of the Nudibranchia (Mollusca) order, in which three new families were described [11]. The importance of pedomorphosis in the formation of new taxa of high rank and the need to study the diversity of ontogenetic patterns for their identification has been demonstrated within the framework of the ontogenetic systematics concept [12][13][14][15]. The molecular phylogenetic analysis demonstrated the monophily of eight genera of the Acrothoracica (Copepoda) superorder [16].…”