Stöhr S and Martynov A (2016), Paedomorphosis as an Evolutionary Driving Force...

ECB-ART-44995

PLoS One 2016 Jan 01;1111:e0164562. doi: 10.1371/journal.pone.0164562.

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

Stöhr S , Martynov A .

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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.

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Genes referenced: ass1 LOC100887844 LOC100893907 LOC105444561 LOC579609

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	Fig 1. Skeletal morphology of Ophiuroidea postlarvae, Ophiura sarsii.(A) dorsal aspect, (B) ventral aspect. AS, adoral shield, ASS, adoral shield spine, BS, buccal scale, CPP, central primary plate, D, dental plate, DAP, dorsal arm plate, IR, interradial secondary disc scale, J, jaw = oral plate, LAP, lateral arm plate, M, madreporite, OS, oral shield, RPP, radial primary shield, S, arm spine, TPo2, second oral tentacle pore, T, tooth, VAP, ventral arm plate. Scale bars 0. 1 mm.
	Fig 2. SEM images of a proximal arm vertebra of Ophiozonella falklandica, dorsal view.(A) Selecting a place to measure the length (horizontal line) and width (vertical lines) is difficult due to the complicated shape of the ossicle. (B) For a more standardized approach, in the software ImageJ a polygonal line is drawn around the shape of the ossicle. (C) Using a built in function of ImageJ an ellipse is then fitted to the ossicle; major (here length) and minor (here width) axes are measured automatically. Scale bar 0.1 mm.
	Fig 3. Examples of external morphology of postlarvae and paedomorphic adults.SEM images (except H-G). (A) Dorsal views of postlarvae of Ophiura sarsii, (B) Amphipholis squamata, (C) Ophiomusium lymani, and (D) Perlophiura profundissima, (E) adult P. profundissima, (F) juvenile Ophiozonella stellamaris, (G) adult O. stellamaris, (H) postlarva syntype of Ophiomastus bulufonica, (I) adult syntype of O. bulufonica, (J) postlarva syntype of Ophiomastus tegulitius, (K) adult syntype O. tegulitius, (L) adults of Ophiomastus tumidus, (M) Ophiopyrgus saccharatus, (N) Ophiomisidium irene, (O) Ophiotypa simplex. Scale bars in mm.
	Fig 4. Examples of internal morphology of postlarvae and paedomorphic adults, oral plates and dental plates.SEM images. (A-H) Oral plates, (A) postlarvae of Ophiura sarsii at dd 560 μm, (B) same at dd 860 μm, (C) Amphipholis squamata at dd 640 μm, (D) Ophiomusium lymani at dd 2.4 mm, and (E) Perlophiura profundissima at dd 960 μm, (F) adults of P. profundissima, (G) Ophiozonella stellamaris, and (H) Ophiotypa simplex. Dental plate external (convex) or internal (concave) aspect of (I, J) O. sarsii at dd 560 μm, (K) A. squamata at dd 640 μm, (L, M) O. lymani at dd 2.4 mm, (N) adults of P. profundissima, (O) O. stellamaris, (P) O. tumidus, (Q) O. simplex, and (R) Ophiozonella sp. 7. dd, disc diameter. Scale bars in mm.
	Fig 5. Examples of internal morphology of postlarvae and paedomorphic adults, arm vertebrae.SEM images. Postlarvae of (A) Ophiura sarsii at dd 560 μm, (B) same at dd 860 μm, (C) Amphipholis squamata at dd 640 μm, (D) Ophiomusium lymani at dd 2.4 mm, and (E) Perlophiura profundissima at dd 960 μm, (F) adults of P. profundissima, (G) Ophiozonella stellamaris, (H) Ophiotypa simplex, (I) Ophiomastus tumidus, (J) Ophiozonella sp. 7, (K) Ophiomastus bulufonica, and (L) Ophiopyrgus saccharatus. Scale bars in mm.
	Fig 6. Total paedomorphosis scores for all species and ontogenetic stages.Sorted by external character score. 21 datasets do not include any internal characters (see Table 4).
	Fig 7. Total paedomorphosis scores for species and ontogenetic stages with both external and internal characters.Sorted by external character score. The general trend is the same as in Fig 6.
	Fig 8. Degree of paedomorphosis by measurements of arm vertebrae.Only species for which vertebrae were available are included (see Table 4). X-axis values are length/width, sorted from low to high.

References [+] :

Bonett, Heterochrony repolarized: a phylogenetic analysis of developmental timing in plethodontid salamanders. 2014, Pubmed