Morphogenesis, Hox and ParaHox gene expression in the invasive zebra mussel

Mollusca is a highly diverse animal group that includes popular representatives such as the gastropods (snails and slugs), cephalopods (squids and octopuses), and bivalves (oysters, cockles, clams), as well as lesser-known groups such as worm-like forms (the aplacophorans) or the polyplacophorans with eight shell plates. Being one of the most diverse assemblages of animals, molluscs are ideally suited for evolutionary studies into how morphological variation may evolve. However, data on their development using modern methods such as gene expression analyses or high-end microscopy techniques are still scarce, especially for the bivalves, which, in addition to their relevance for evolutionary studies, are also of considerable economic value as an important protein resource. The present project will combine morphological analyses such as fluorescence labeling, various microscopy applications, and 3D-reconstruction software in order to reconstruct the development of important organ systems during development from the free-swimming larva to the settled, benthic juvenile in the local species Dreissena polymorpha (commonly known as zebra mussel). These data will aid in achieving the central goal of the study, which is to reveal the expression of important genes (the Hox and ParaHox genes) in the respective developmental stages of this bivalve. By comparing the obtained data to the few accounts available for gastropods and cephalopods, we will significantly contribute to the question as to whether these genes are expressed in similar body regions and/or organ systems as in their related groups (and therefore are likely to serve similar functions), or whether the Hox and ParaHox genes may have putative different functions in bivalves. Despite these evolutionary questions, this project will also be of relevance for environmental, ecological, and economical issues, because Dreissena is an important biofouling species that attaches by aid of their so-called byssus threads to, e.g., water and sewage pipelines (which may lead to clogging of these systems). In addition, these animals are highly efficient filter-feeders with a high reproductive output, which has led to severe decline of other local species in Northern America and Europe (e.g., endangered unionid bivalves). The gene sequence data generated in this project may also be used in numerous other future projects, e.g., those concerned with unraveling the protein composition of the above-mentioned delicate, yet sturdy, byssus threads, an emerging biomaterial of putative high potential for the applied physical sciences.

Hox genes are important regulators during animal development. Usually, they are expressed along the anterior-posterior axis when the embryos forms, in a way where the (relative) location of each Hox gene on the genome corresponds to the (relative) time and region of its expression. In other words, Hox genes that lie anteriorly on the genome are expressed earlier and in a more anterior region of the embryo relative to its sister genes. This phenomenon is often called colinearity. Mollusks have been shown to sometimes deviate frome this mode of expression, with Hox genes being specifically expressed during formation of well-defined morphological structures such as the shell, foot, or the nervous system. The present project was the first to in detail describe Hox gene expression in a bivalve mollusk. The data revealed, that expression of Hox (and ParaHox) genes in an invasive bivalve species, the quagga mussel (Dreissena rostriformis), neither follows a colinear manner along the bivalve longitudinal axis, nor is their expression confined to to mollusk- or bivalve-specific structures (as, e.g., is the case for cephalopods and gastropods). Two notable exceptions to this are Hox1 (which is expressed in the developing shell field) and Xlox (which is expressed in the developing hindgut as known from other bilaterians). The remaining 8 Hox genes for which we were able to produce expression pattern data were found to have largely overlapping expression domains at the early gastrula stage. Later, in the trochophore larva, these genes are largely expressed in broad mesodermal domains but again without being confined to specific morphological structures. These findings illustrate the high variability of Hox gene expression domains in members of closely related molluscan taxa and suggests a diverse suite of functions during animal development that remains to be explored.