MICROMORPHOLOGY AND GENE EXPRESSION IN MUSCLE AND SHELL DEVELOPMENT OF THE MOLLUSCA

This work comprises detailed studies by scanning electron
microscopy (SEM), transmission electron microscopy (TEM),
fluorescence staining combined with confocal laser scanning
microscopy (CLSM), as well as serial sectioning analyses and
reconstruction techniques to elucidate the development of the
larval and adult musculature of several basal representatives of
the molluscan classes Polyplacophora, Bivalvia, Scaphopoda, and
Gastropoda. Special reference is given to the shell musculature. In
addition, aspects of the myo-anatomy of adult Solenogastres are
reconsidered. A further part of this study deals with scaphopod
shell morphogenesis and expression of the homeobox gene engrailed
(en), in order to gain insights regarding the scaphopod-bivalve
relationship. The results enable far reaching conclusions regarding
the evolution and the phylogeny of the Mollusca. Solenogastres TEM
analysis of adult Solenogastres revealed a mesenchymate body wall
musculature which consists of outer ring, intermediate diagonal,
and inner longitudinal muscles and resembles the condition of other
worm-shaped taxa. The ventrally inter-crossing dorso-ventral
musculature, which is diagnostic for the Mollusca, is arranged in
multiple serial units along the anterior-posterior body axis.
Polyplacophora During development, the chiton larva undergoes an
intermediate stage in which the dorso-ventral musculature is
serially arranged as in adult Solenogastres. The concentration into
seven (and later eight) functional shell plate muscle units is a
secondary condition which takes place after metamorphosis. Thus,
assumptions of a primarily "segmented" (i.e. annelidlike) character
of the polyplacophoran shell plate musculature are rejected. In
addition, the anterior (i.e. pre-trochal) body region of chiton
larvae shows a muscular grid which is lost at metamorphosis and
resembles the body wall musculature of adult aplacophoran
(Solenogastres + Caudofoveata) molluscs. Both, the multiple
seriality of the dorso-ventral muscles and the apical muscle grid
are regarded as ontogenetic recapitulation of the basal molluscan
condition which is fully expressed in the adult body plan of
Solenogastres. This infers a non-segmented, worm-shaped ancestor at
the base of molluscan evolution. The existence of a larval
ring-shaped muscle that underlies the prototroch cells (prototroch
muscle ring) is a shared feature of polyplacophoran, gastropod, and
bivalve larvae (see below) and suprataxic homology of this organ is
proposed. Bivalvia Besides a rather complicated set of larval
retractor muscles, the veligers of autobranchs (i.e. all Bivalvia
except the Protobranchia, the latter with a test-cell larva)
exhibit a distinct prototroch muscle ring similar to chitons and
gastropods. Both systems are entirely larval and are resorbed
during metamorphosis. Scaphopoda The general ontogeny and
especially myogenesis in the dentaliid scaphopod Antalis entalis
proceeds much more direct than in polyplacophorans or gastropods.
Accordingly, distinct larval muscle systems are lacking. However,
the paired cephalic and pedal retractors both form additional
fibers which project into the region of the prototroch and are lost
at metamorphosis. The existence of a distinct, paired cephalic
retractor system, which is also found in the basal gastropod and
cephalopod bauplan but not in the Bivalvia, suggests a clade
comprising the Scaphopoda and Gastropoda + Cephalopoda. This is
strengthened by expression data of the homeobox gene engrailed,
which plays a significant role in molluscan shell formation. While
two dorso-lateral centers of engrailed expression, which correspond
to the two centers of initial shell calcification, are found in
early bivalve veligers, engrailed is exclusively found in mantle
margin cells surrounding the single anlage of the embryonic
scaphopod shell. In contrast to bivalves, the scaphopod shell is
thus formed from a single center of calcification, and a
scaphopod-bivalve sistergroup relationship is therefore rejected.
Gastropoda Primitive gastropods, such as the patellogastropods
Patella vulgata and Patella caerulea, show one pair of
asymmetrically positioned larval retractor muscles which have
distinct insertion sites at the embryonic shell. Another strict
larval muscle system is the prototrochal muscle ring. All these
muscle are lost before, during, or shortly after metamorphosis.
Parts of the adult mantle musculature as well as the muscles of the
cephalic tentacles are formed prior to metamorphosis, while the
buccal musculature is of entire postmetamorphic origin. The process
of gastropod ontogenetic torsion is mainly caused by muscular
activity of the larval retractors, while the adult shell
musculature arises after the completion of torsion. Thus,
ontogenetic torsion is regarded as an entirely larval process
inferring that the arrangement of the adult shell musculature -
which can often be reconstructed by muscle scars on fossilized
shells - is not indicative for the question whether paleozoic
univalved molluscs were torted or not.