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Many aquatic invertebrates possess complex life cycles, which include a switch from a vegetative to a sexual mode of reproduction. While sexual reproduction is usually linked to slow rates of propagation, vegetative reproduction leads to high rates of clonal propagation and a fast increase in population size. I have attempted to identify developmental genes which are differentially expressed between sexually and vegetatively reproducing individuals. quantitative assays of the expression of those diagnostic genes could be applied to field samples to gain information on the reproductive status of the sample. Here I outline a general strategy for identifying and testing the usefulness of homeobox genes as candidates for regulatory genes whose expression patterns are indicative of a sexual or vegetative developmental phase.

Cnidarians display most of the characters considered as milestones of metazoan evolution. Whereas a tissue-level organization was probably already present in the multicellular common ancestor of all animals, the Urmetazoa, the emergence of important animal features such as bilateral symmetry, triploblasty, a polarized nervous system, sense organs (eyes, statocysts), and a (chitinous or calcium-based) continuous skeleton can be traced back before the divergence between cnidarians and bilaterians. Modularity and metamery might be also regarded as two faces of the same medal, likely involving conserved molecular mechanisms ruling animal body architectures through regional specification of iterated units. Available evidence indicates that the common ancestor of cnidarians and bilaterians, the UrEumetazoa, was a surprisingly complex animal with nerve cell differentiation. We suggest that paedomorphic events in descendants of this ancestor led to the array of diversity seen in the main extant animal phyla. The use of molecular analyses and identifying the genetic determinants of anatomical organizations can provide an integrative test of hypotheses of homologies and independent evidence of the evolutionary relationships among extant taxa.