Hormonal interplay regulating growth and reproduction

In nature many strategies have evolved to ensure successful reproduction. One of the most fascinating is the occurrence of semelparity, an all-or-nothing strategy of reproduction finally leading to death of the individual animal. In many cases semelparous reproduction culminates in the mass spawnings observed in many aquatic species such as the Palolo worm, corals or salmon. Concomitant with this is a massive breakdown of muscle mass and energy stores and the generation of enormous amounts of germ cells, representing up to 80% of the individuals` body mass at spawning. To secure a maximum of reproductive success, the presence of mature gametes has to be synchronized in the individual as well as in the population. However, despite the broad occurrence of semelparity in nature nothing is known about the molecular mechanisms underlying this mode of reproduction. This can be mainly attributed to the lack of an appropriate laboratory model species. The marine annelid Platynereis dumerilii has in recent years emerged as a powerful molecular model species in the field of brain- and genome evolution. Furthermore, it has a longstanding history in studies of reproductive timing and synchronization of gametogenesis, as well as the role of circalunar oscillators in semelparous reproduction. In Platynereis the onset of sexual maturation and thereby synchronized gametogenesis, has been inevitably linked to the sinking titer of a yet uncharacterized neurohormone termed nereidin, which has been paralleled with insect juvenile hormones. Nereidin has a central role in setting in motion sexual maturation and the concomitant metamorphosis of the animal from a feeding, growing, benthic being to a non-feeding, reproducing pelagic one. However, nothing is hitherto known about its molecular nature and mode of action. Eleocytes (free coelomic cells), comparable to vertebrate liver and insect fat body, represent the central metabolic organ in Platynereis and eventually take over the main metabolic load associated with maturation and are therefore essential for sexual maturation. As massive redistribution of stored energy from the soma to the germ line accompanies sexual maturation, it is hypothesized that eleocytes act as the central integration platform for neuroendocrine signals mediating this non-reproductive/reproductive switch. The central questions addressed in this proposal are: What is the molecular nature of nereidin? What is the (molecular) action of nereidin on eleocytes? How is the central process in sexual maturation (vitellogenesis) regulated by nereidin and the newly demonstrated gonadotropic factor? The work proposed will cover biochemical, cell biological and genetic aspects. To unravel the molecular nature of nereidin, mainly biochemical work will be carried out. As a read out for the biological activity of the obtained fraction(s), the known inhibitory effect of nereidin on vitellogenesis (i.e. vitellogenin synthesis) will be used. The action of nereidin on the eleocytes will be assessed by transcriptome profiling of eleocytes derived from different stages of sexual maturation, i.e. different nereidin titers. The regulation of the central trait of sexual maturation, vitellogenesis, will be investigated on the eleocyte level by transcriptome profiling to unravel differentially regulated genes, as well as by analysis of vitellogenin mRNA-levels changes in response to different treatments.