World Congress of Malacology Antwerp ... - Unitas Malacologica

Photoreception and the evolution of photoreceptors (with special reference to Mollusca)
Salvini-Plawen, Luitfried
University of Vienna, Faculty of Life Sciences, Center of Zoology, Department of Evolutionary
Biology, EF: Molecular Phylogenetics, Althanstraße 14, A-1090 Vienna, Austria,
Email: luitfried.salvini-plawen@univie.ac.at
Due to the general expression of the transcription factor gene Pax-6 in eyes of Triploblastica, as well
as due to the overall presence of the opsin as photopigment, the polyphyletic evolution of
photoreceptors (rather than of photoceception) has recently been put into question. It is proposed that
the developmental pathway of photoreceptor evolution reflects two different, subsequent processes:
(a) the (monophyletic?) differentiation of the photoreception as such, mediated by a certain
transcription factor gene (such as Pax-6 or sine oculis), and (b) the genetic induction by that
regulative factor (normative unit) to differentiate the stimulated (polyphyletic) morphogeneses of
photoreceptors proper by multiple convergent co-option with variable network-modifications.
The structural significance of the epidermal photosensitive receptors with ciliary versus rhabdomeric
surface enlargements is reconsidered: the differentiation of both receptor types in identical cells
during development (of veliger larvae) evidences them as interchangeable structures (mere morphs),
apparently dependent on functional requirement. Consequently, on the one side, the structural type of
the receptive organelle has no direct bearing upon the homology identification of the photoreceptors.
On the other side, there is no need anymore to propose separate (ciliary and rhabdomeric) precursor
cells in metazoans.
Photoreceptive organs in Mollusca show a rich morphological diversity. All photoreceptors, ocelli
and eyes (including the larval organs), however, appear to represent ingroup differentiations. Their
polyphyletic lines are surveyed and the fine structure of eyes of three additional Bivalvia species,
among them of the pteriomorph Lima, is reported.
Identification of several members of Hox and paraHox genes in heteroconch bivalves
Samadi, Leyli; Steiner, Gerhard
Moleculare Phylogenetic, Department of Evolutionary Biology, Faculty of Life Sciences, University
of Vienna, Althanstraße 1090 Vienna, Austria,
Email: leili.samadi@univie.ac.at, gerhard.steiner@univie.ac.at
Homeobox genes encode transcription factors characterized by the homeodomain, a 60-amino acid
DNA binding motif. Hox and ParaHox genes are particular subgroups of homeobox genes, which are
found in special gene clusters and function in the patterning of body axis. Up to know Hox and
ParaHox genes have been studied in several molluscs but the number of the genes identified in the
clusters ranges severely in different classes. We carried out a polymerase chain reaction survey using
degenerate primers to amplify Hox and paraHox genes in the heteroconch bivalves Abra nitida,
Thyasira sp., Hiatella sp., and Astarte elliptica. Our results show that at least one anterior, five
central, and one posterior Hox genes exist in these species. Two to four antennapedia orthologs are
present in each species. Gene duplication of labial, deformed, Lox2 or Lox4 Hox cognates is apparent
in some species. These results suggest that some of the anterior or posterior orthologs may have been
lost in some species, while the medial cognates are either lost and/or duplicated. Mollusc-specific
signature peptides were recognized in the central part of the homeodomain (20-46 amino acid
position) of labial, deformed, Lox4, and posterior genes. We were not able to amplify paralog 3 group
(zerknult) gene in any of the species. The entire paraHox cluster genes -including Cdx, Xlox, and
Gbx- occur in all species studied here. At least one member of the cluster undergoes duplication in
each species. Hiatella sp., e.g., has a duplicated paraHox cluster. These results suggest that the
number of genes in both Hox and paraHox clusters differs from species to species in the
Heteroconchia, and that loss/duplication events probably occurred independently in different clades.
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