Evolution and Biomechanics of the Male Copulatory Organ in Schizorhynchia
(Platyhelminthes; Kalyptorhynchia)
Julian P.S. Smith III
The flatworm clade Schizorhynchia comprises approximately 150 species
of predators that use an anterior proboscis to capture prey.
Taxonomy
in this group has relied heavily on the light-microscopic structure of the
proboscis.
Recent molecular-phylogenetic studies do not support the
current classification.
In addition to the proboscis, schizorhynchs possess
a rather complex male organ, variously armed with spines or cirri.
However, the last comprehensive review of this organ in Schizorhynchia
was published in 1956 using only light microscopy.
Using our recent
molecular phylogeny for guidance, I have investigated the male copulatory
organ in several schizorhynch species by confocal-laser-scanning and
transmission-electron microscopy. These studies reveal common features,
including
an
outer
sheath
of
longitudinal
muscles
surrounding
the
copulatory bulb (conflicting, in at least one case, with the description
originally published), copulatory “hard parts” comprising an eversible cirrus
and/or a tubular extension of the ejaculatory duct, muscular connections
between the copulatory bulb and hard parts that appear to be capable of
cirrus protrusion during copulation (and retraction thereafter), and an
epithelial lining of the ejaculatory duct through which pass neck-like
extensions of the prostate glands (a feature that was usually missed in
earlier light-microscopic studies).
These studies have a allowed the
development of a general biomechanical hypothesis for the function of
these organs, and this hypothesis appears to allow selection of species
to be added to the molecular phylogeny, which is presently rather
weakly-supported at the base of the Schizorhynchia.
More broadly
across the Platyhelminthes, certain of these traits in the male organ may
represent functional constraints on the evolution of the organ.
instance, it appears that the
For
“conjuncta-duplex” type of male organ (as
defined by Karling), which is widely distributed in Platyhelminthes, is
simply a functional necessity for for protrusion of a
cirrus during
copulation.
Supported by funds from the Winthrop University Research Council and SC-INBRE
(National Center for Research Resources [5 P20 RR016461] and the National Institute
of General Medical Sciences [8 P20 GM103499] from the National Institutes of Health).