Fig 1.
Different hypotheses on the evolutionary transformation of annelid and capitellid segmental chaetae (A) into echiuran ventral (B) and caudal anal chaetae (C).
Annelid chaetae are formed on the ventral edge of the notopodial chaetal sac and on the dorsal edge of the neuropodial chaetal sac. Developing chaetae are red, completed chaetae are black, chaetal sac is grey. All structures that are reduced according to the different hypothesis are paler; presumably reduced chaetal sacs are marked by a dotted line. Arrows mark expansion or shifting of chaetal sac. H1 hypothesis one: notopodial chaetae have been reduced and the neuropodial chaetal sac shifted ventrally. H2 hypothesis two: notopodial chaetae plus one neuropodial group of chaetae have been reduced and the remaining neuropodial sac shifted ventrally. H3 hypothesis three: all chaetal sacs expand dorsally and ventrally, respectively. H4 hypothesis four: neuropodia are reduced and the notopodial chaetae expand dorsally and ventrally.
Fig 2.
Ventral chaetae of Echiurus echiurus (A, B) and Thalassema thalassemum (C-F).
A. Extended focus light micrograph of a critical point dried part of a specimen showing the apical hooks of the ventral chaetae. B. Extended focus light micrograph, frontal view of a pair of dissected ventral chaetae (Ch) showing the chaetal radial muscles (rm) and interconnecting muscle (icm). Chaetal sacs are delicate and transparent, arrow marks a developing chaeta. C. Maximum projection of a phalloidin (cyan) stained and chitin autofluorescent (yellow to light blue) dissected pair of ventral chaetae, frontal view, CLSM. Apical microvilli of the chaetoblast marked by arrow heads. Large arrows mark developing chaetae, small arrows mark the collar of the ventral chaetae. Inset: early chaetogenesis at higher magnification. D.-F. Light micrographs of semi-thin (0.5 μm) toluidine blue stained parasagittal sections. D. Apical part of ventral chaeta (Ch), chaetal sac with follicles (bold arrows, cf) and developing chaeta (dCh). Large arrows mark the uppermost follicle cells. Note muscles running between follicles. E. Basal part of a completed ventral chaeta, large arrows mark follicle cells with their strong bundles of intermediate filaments, small arrows mark microvilli brush border of chaetoblast. F. Developing chaeta (dCh) with chaetoblast (cb) plus microvilli brush border (small arrows) and follicle cells (fc). Note that intermediate filaments are absent at this stage. Coelothel (ct) surrounds the follicle except for the apical section. bwm body wall muscles, coel coelom, Cu cuticle, ep epidermis, ecm extracellular matrix, en enamel, icm interconnecting muscle, lm longitudinal muscle, rm radial muscle.
Fig 3.
Chaeta and chaetogenesis in Thalassema thalassemum reconstructed from a stack of semi-thin sections (Direct link: www.morphdbase.de/?E_Tilic_20140929-M-21.1).
A. Chaetal sac with four developing chaetal follicles (CF1–CF4) and one completed follicle (CF5). Developing chaetae (dCH) in CF3 and CF4 and complete chaeta (Ch) of CF5 are dark grey. Chaetal muscles are not shown. B. Maximum projection of a CLSM stack of the basal section of CF5 with propidium iodide staining of nuclei (cyan) and autofluorescent chaeta (yellow). Note large nucleus (nc) of chaetoblast. bwm body wall muscles, cb chaetoblast, Cu cuticle, ecm extracellular matrix, ep epidermis, ns ventral nerve cord, pe peritoneum.
Fig 4.
Ultrastructure of ventral chaetae of Echiurus echiurus (A, D-F) and Thalassema thalassemum (B, C, G-I), TEM.
A-G. late chaetogenesis, H-I. complete chaeta. A. Hemidesmosomes (hd) connect chaeta (Ch) to intermediate filaments (if) of follicle cells (fc), chaeta is surrounded by an enamel (en). B, C. Intermediate filaments (if) inside branched and unbranched microvilli (mv) adhere chaeta to follicle cells (fc). Apical adherens junctions (arrows) interconnect follicle cells, note apical pair of centrioles (ce) in follicle cells in B. D. Semi-transverse section of microvilli (mv) brush border of a chaetoblast (cb) during chaetogenesis. E, F. Parasagittal section of chaetoblast with apical microvilli, arrows mark chitin polymerizing between the microvilli. Note densely packed f-actin in microvilli (mv). E. center of chaetoblast. F. lateral part of chaetoblast. Note its tight interdigitation with follicle cells (fc), white arrows mark adherens junctions. G. Semi-transverse section of apical microvilli of the chaetoblast, white arrows mark chitin polymerizing between the microvilli (mv). H. Base of old chaeta, parasagittal section. The apical microvilli have been replaced by protrusions of the chaetoblast that contain intermediate filaments (if) which adhere to the chaeta (arrows) by hemidesmosomes (hd). I. Fully differented chaeta with ridges. Hemidesmosomes (hd) firmly connect intermediate filaments (if) of follicle cells (fc) to chaeta. mi mitochondria.
Fig 5.
Echiurus echiurus, anal chaetae.
A. Extended focus view of the caudal face of the animal, B. Extended focus semi-lateral view into the dissected caudal end; Hindgut is removed, nerve cord (nc) marks ventral side. Note complex muscular system, interwoven interconnecting muscles (icm), and twisted orientation of anal chaetae. C. Reconstruction of completed (orange) and developing (bluish) anal chaetae based on a series, D-G. Azan stained histological sections from a series of transverse sections from anterior to posterior. The chaetal sacs extend deeply into the coelom (coel), the anal chaeta are numbered (outer hemi-circle 1–8, inner hemi-circle 9–14). Distances: D-E = 600 μm, E-F = 200μm, F-G = 600 μm. D. Section of the base of the anal chaetae dorsal to hindgut. Note connection of chaetal sacs by interconnecting muscles (icm). E. Section of the chaetae at the level of the origin of the radial muscles (rm). Anal chaetae are grouped in two hemi-circles that are next to each other and partly surround the hindgut (hg). F. Section close to the posterior end of the trunk body cavity (coel). Hemi-circles of anal chaetae are clearly separated and surround three quarters of the hindgut. Note developing chaeta (arrow) in the sacs of chaetae 7, 8, and 10. G. Section of the anal chaeta through the body wall muscles. Hemi-circles of anal chaetae are widely separated. Note tips of developing chaetae (arrow) in sacs of chaetae 3, 4, 7, 8, 10, and 11. a anus, as anal sac.
Fig 6.
Rejection, support and new hypotheses on the evolutionary transformation of the segmental chaetal pattern characteristic for Capitellidae and most other Annelida (A) into that of Echiura (B, C).
The number of the ventral chaetal sacs and position of the growth zone supports the first hypothesis by suggesting that the ventral chaetae evolved from neuropodia by reduction of the number of chaetae and loss of the neuropodia. All other hypotheses are rejected by the results of this paper. A new hypothesis (hypothesis 5) is developed for the evolution of the anal chaetae in Urechidae and Echiurinae, according to which the anal chaetae evolved by multiplication of ventral chaetae.