Long-lasting redundant expression in GnRH neurons enabled apparent switching of paralog usage during evolution.

Expressed subtype of paralogous genes in functionally homologous cells sometimes show differences across species, the reasons for which have not been explained. The present study examined hypophysiotropic gonadotropin-releasing hormone (GnRH) neurons in vertebrates to investigate this mechanism. These neurons express either or paralogs, depending on the species, and apparent switching of the expressed paralogs in them occurred at least four times in vertebrate evolution. First, we found redundant expression of and in a single neuron in piranha and hypothesized that it may represent an ancestral GnRH system. Moreover, the enhancer of piranha induced reporter RFP/GFP co-expression in a single hypophysiotropic GnRH neuron in both zebrafish and medaka, whose GnRH neurons only express either or . Thus, we propose that redundant expression of of relatively recent common ancestors may be the key to apparent switching of the paralog usage among present-day species.