New insights into the evolution, hormonal regulation, and spatiotemporal expression profiles of genes involved in the Gfra1/Gdnf and Kit/Kitlg regulatory pathways in rainbow trout testis
The present study aimed to investigate whether the Gfra1/Gdnf and/or Kit/Kitlg regulatory pathways could be involved in the regulation of spermatogonial cell proliferation and/or differentiation in fish. Homologs of the mammalian gfra1, gdnf, kitr, and kitlg genes were identified in gnathostomes and reliable orthologous relationships were established using phylogenetic reconstructions and analyses of syntenic chromosomal fragments. Gene duplications and losses occurred specifically in teleost fish and members of the Salmoninae family including rainbow trout (Oncorhynchus mykiss) and Atlantic salmon (Salmo salar). Some duplicated genes exhibited distinct spatiotemporal expression profiles and were differently regulated by hormones in rainbow trout. Undifferentiated type A spermatogonia expressed higher levels of kitrb and kitra2 making them possible target cells for the gonadal kitlgb and somatic kitlga before the onset of spermatogenesis. Interestingly, gdnfa and gdnfb ohnologous genes were poorly expressed before the onset of spermatogenesis. The expression level of gdnfb was correlated with that of the vasa gene suggesting that the late increased abundance of gdnfb during spermatogenesis onset was due to the increased number of spermatogonial cells. gfra1a2 was expressed in undifferentiated type A spermatogonia whereas gfra1a1 was mainly detected in somatic cells. These observations indicate that the germinal gdnfb ligand could exert autocrine and paracrine functions on spermatogonia and on testicular somatic cells, respectively. Fsh and androgens inhibited gfra1a2 and gdnfb whereas gfra1a1 was stimulated by Fsh, androgens, and 17α, 20β progesterone. Finally, our data provide evidences that the molecular identity of the male germ stem cells changes during ontogenesis prior to spermatogenesis onset.
KeywordsOncorhynchus mykiss Spermatogenesis Germ stem cell Gfra1 Gdnf Kit Kit ligand Stem cell factor
The authors thank the animal care facility of the LPGP research department, especially Frédéric Borel, Amélie Patinote, and Cécile Melin. The authors thank Laurent Labbé and Lionel Goardon from the INRA PEIMA experimental fish farm for providing the monosex males and Drs. Alexis Fostier and Yann Guiguen for the gift of the rainbow trout YY sperm.
The research leading to these results has received funding from the European Community’s Horizon 2020 research infrastructure project (INFRAIA-1-2014/2015) under grant agreement no. 652831 (project AQUAEXCEL2020) and from the French National Research Agency under grand agreement no. 11-INBS-0003 (CRB anim project). Ahmed Maouche gratefully acknowledges the Britany province (Région Bretagne) and the INRA PHASE department for the funding received towards his PhD.
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