foxn4 Expression Pattern Suggests Its Association with Neurosensory Cells in the White Sea Hydrozoan Sarsia loveni

The foxn4 is one of the key transcription factor genes controlling retinal formation in vertebrates. However, it is not clear whether its association with light-sensitive organ formation is evolutionary conserved. To answer this question, we tested whether the expression of this gene is localized within light-sensitive organs in a representative of basal Metazoa, the hydroid Sarsia lovenii (Hydrozoa, Cnidaria). Usually, the life cycle of hydroids includes stages of a pelagic medusa and a benthic polyp. However, in many species, attached medusoids, in which many medusa structures are reduced, form instead of free-swimming medusa. The White Sea hydrozoan Sarsia lovenii is an exceptional example of the species, in which polyps of different haplotypes produce either pelagic medusae or attached medusoids. Comparison of gene expression in medusae and medusoids of S. lovenii is a promising model to study how the formation of morphological traits is regulated in hydrozoan cnidarians. We compared the spatial pattern of Foxn4 expression in medusae and medusoids of S. lovenii by in situ hybridization. In medusae, Foxn4 is expressed not in the photoreceptive ocelli, but in the ectoderm of the tentacle bulb around the ocellus. Although, unlike medusae, S. lovenii medusoids lack ocelli, we detected Foxn4 expression in their reduced tentacle bulbs. It is known that the tentacle bulb in hydrozoan medusae is a zone of localized formation of nematocytes, which are considered to be derivatives of mechanosensory cells. Thus, our results indicate that, in medusae and medusoids of S. lovenii, the foxn4 is not associated with the formation of photoreceptor organs, as in vertebrates. However, it may be associated with nematocytes, another type of neurosensory cells.