Autoradiographische Untersuchungen über die synthetische Aktivität neurosekretorischer Zellen im Gehirn vonPlatynereis dumerilii während der sexuellen Entwicklung und Regeneration

Discovery of methylfarnesoate as the annelid brain hormone reveals an ancient role of sesquiterpenoids in reproduction

Animals require molecular signals to determine when to divert resources from somatic functions to reproduction. This decision is vital in animals that reproduce in an all-or-nothing mode, such as bristle worms: females committed to reproduction spend roughly half their body mass for yolk and egg production; following mass spawning, the parents die. An enigmatic brain hormone activity suppresses reproduction. We now identify this hormone as the sesquiterpenoid methylfarnesoate. Methylfarnesoate suppresses transcript levels of the yolk precursor Vitellogenin both in cell culture and in vivo, directly inhibiting a central energy–costly step of reproductive maturation. We reveal that contrary to common assumptions, sesquiterpenoids are ancient animal hormones present in marine and terrestrial lophotrochozoans. In turn, insecticides targeting this pathway suppress vitellogenesis in cultured worm cells. These findings challenge current views of animal hormone evolution, and indicate that non-target species and marine ecosystems are susceptible to commonly used insect larvicides.

DOI:http://dx.doi.org/10.7554/eLife.17126.001

All organisms need energy to survive and grow. However, sources of energy are limited and so organisms need to decide how to spend the resources they have available. For instance, animals must choose whether they should continue to grow or if they should invest energy into reproduction instead. This decision becomes even more important for animals that reproduce in an “all-or-nothing” manner and invest all their available energy into reproduction and die soon after.

Bristle worms live in coastal areas around world. In mass spawning events, thousands of individuals raise from the sea floor to the surface, to release sperm and eggs. While the fertilized eggs start to develop in the water, the parents invariably die. The female worms spend roughly half their body mass in producing eggs and supplying them with yolk as a source of energy. It has been known for decades that the brains of bristle worms produce a master hormone that promotes growth and suppresses reproduction. Yet the identity of this hormone that controls the life-or-death decision was not clear.

Schenk et al. took advantage of new molecular tools to solve this puzzle. The experiments show that this hormone directly regulates how much yolk the female animals produce. This allowed Schenk et al. to design a new molecular assay that helped to identify the hormone itself. Unexpectedly, the hormone – called methylfarnesoate – belongs to a family of small molecules called sesquiterpenoids, which researchers previously thought were only found in insects and related groups. Hence, many insecticides have been developed to target sesquiterpenoid signaling and they are used in massive amounts to fight pests like the tiger mosquito (which transmits the Zika virus). Schenk et al. also found that these insecticides also cause severe problems in bristle-worms.

These findings challenge current views of how animal hormones have evolved and indicate that common insecticides may be harming bristle worms and other animals in marine environments. The next steps are to find out whether methylfarnesoate is found in other closely related animals, such as snails and mussels, and whether the insecticides are harmful to these animals too. Another future challenge will be to investigate how this hormone actually promotes animal growth.

DOI:http://dx.doi.org/10.7554/eLife.17126.002

Endocrine and environmental control of reproduction in Polychaeta

In Polychaeta, as in many invertebrates, reproduction is controlled by both environmental and endocrine factors. Although the effects of environmental factors on reproductive behaviour are briefly discussed, this review focuses on the endocrinology of reproduction. As Nereidae are the most intensively studied polychaetes, their epigamic monotelic strategy is discussed first in this review. Although a large number of physiological observations have been made, biochemical data have been greatly lacking until recent years, except, however, for the recent isolation of several pheromones. These substances, such as uric acid and L-cysteine gluthathione disulfide, occur widely and must be present at high concentrations in order to exert their physiological effects. Results obtained from iteroparous species are also considered. The stolonization strategy of Syllidae, the control of vitellogenesis in Nephtyidae, Phyllodocidae, Polynoidae, and Cirratulidae, and the regulation of gamete maturation in Arenicolidae and Pectinariidae are discussed. As with Nereidae, our knowledge of endocrine control is mainly based on experimental data, since only sperm-maturation factor in the genus Arenicola has been identified. Therefore, despite numerous interesting experimental studies in which functional roles for polychaete reproductive hormones have been described, their nature, their primary targets, and their mechanism of action are unfortunately still largely unknown.

Immunohistochemical demonstration of a CCK-like peptide in the nervous system of a marine annelid worm, Nereis diversicolor O.F. Müller

Perikarya and nerve fibers containing a substance immunologically related to CCK-8 were detected in the nervous system of Nereis, a marine annelid worm. The most noteworthy immunostaining was seen in cell bodies, localized at the periphery of the brain, within nuclei 5, 6, 7, 9, 10, 13, 14, 15, 17, 20, 23-24. Immunoreactive fibers were also found in the neuropile without any particular grouping. Numerous other "positive" perikarya occur in the medioventral portion of the ventral nerve cord, and in the ventral and dorsal parts of the suboesophageal ganglion. In addition to the cell bodies in the cerebral external layer, immunoreactive axons were abundantly observed in the connectives between the ganglia. Moreover, our results demonstrate CCK-like staining in neurons showing variations in size and shape, and in affinity for paraldehyde fuchsin. The present results support the hypothesis that this peptide may exert a role as neurotransmitter or neuromodulator in annelids.

Regeneration and endocrinology in the polychaetePlatynereis dumerilii : An experimental and structural study

1. In the polychaetePlatynereis dumerilii, the hormone-elaborating portion of the prostomium was determined by means of prostomium transection and implantation experiments. The area in question lies between the two pairs of eyes, extending longitudinally from the posterior border of the anterior eyes to about the posterior border of the posterior eyes. This corresponds approximately with the brain area delimited by the anterior and posterior dorsoventral connective tissue tubes and which is covered ventrally by the infracerebral gland epithelium. 2. The infracerebral gland-complex and neurosecretory neurons within the brain were envisaged as possible sites of hormone synthesis. 3. The infracerebral gland-complex inPl. dumerilii was investigated with light-and electron-microscopical techniques. A leaf-shaped area (measuring 120 by 95 μm at the most) of the pericapsular epithelium at the ventral side of the brain, adjacent to the main blood vessel and to its efferent branches, consists of specialized columnar epithelial cells. Numerousa-cells and scarceb-cells can be distinguished. Fibre tracts with glia fibres and axons (some being neurosecretory axons) descend from the neuropile and in part terminate with prominent end-structures at the inner face of the brain capsule in the gland region. Probably some axons penetrate the capsule and make contact with the gland cells. Neither structural nor experimental findings prove that the infracerebral gland synthesizes the brain hormone. Accessory functions are discussed. 4. Investigations in secretory brain cells ofPl. dumerilii are reported. In agreement with Müller (1973), a lack of correlation between the number of stainable neurosecretory neurons and the hormonal activity of the brain was found: in immature worms (to which high hormonal titers are ascribed) only few or even no neurosecretory brain cells at all were detectable. Prostomium transection and implantation experiments show further that not all regions of the brain which enclose neurosecretory neurons produce brain hormone. The results are discussed with reference to the hypotheses of Müller (1973) which suggest that the appearance of stainable neurosecretory brain cells indicates inactivation of neurons possibly previously involved with hormone synthesis.