[Initial quantitative data on endocrine activity of the nereid brain during the sexual cycle]

The endocrine control of reproduction in Nereidae: a new multi-hormonal model with implications for their functional role in a changing environment

Nereidae are vital to the functioning of estuarine ecosystems and are major components in the diets of over-wintering birds and commercial fish. They use environmental cues to synchronize reproduction. Photoperiod is the proximate cue, initiating vitellogenesis in a temperature-compensated process. The prevailing paradigm in Nereidae is of a single 'juvenile' hormone controlling growth and reproduction. However, a new multi-hormone model is presented here that integrates the environmental and endocrine control of reproduction. This is supported by evidence from in vitro bioassays. The juvenile hormone is shown to be heat stable and cross reactive between species. In addition, a second neuro-hormone, identified here as a gonadotrophic hormone, is shown to be present in mature females and is found to promote oocyte growth. Furthermore, dopamine and melatonin appear to switch off the juvenile hormone while serotonin and oxytocin promote oocyte growth. Global warming is likely to uncouple the phase relationship between temperature and photoperiod, with significant consequences for Nereidae that use photoperiod to cue reproduction during the winter in northern latitudes. Genotypic adaptation of the photoperiodic response may be possible, but significant impacts on fecundity, spawning success and recruitment are likely in response to short-term extreme events. Endocrine-disrupting chemicals may also impact on putative steroid hormone pathways in Nereidae with similar consequences. These impacts may have significant implications for the functional role of Nereidae and highlight the importance of comparative endocrinology studies in these and other invertebrates.

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.

Changes in the protein components of vitelline envelope during oogenesis of a tubiculous polychaete, Schizobranchia insignis

In sabellid polychaetes the vitelline envelopes, in which microvilli with glycocalyx structures at the tips are invested, change in structure during oogenesis. Vitelline envelopes isolated from Schizobranchia oocytes 25-100 micron and 160-185 micron in diameter, were analyzed in protein components by iodination, electrophoresis, Western blotting and radioactive labeling technique. The observations demonstrate that the membrane proteins of the vitelline envelopes are not consistent but variable in components during oogenesis. Most of these proteins, particularly the high molecular weight proteins, are PAS-positive glycoproteins, which may have specific carbohydrate residues binding to wheat germ agglutinin. The proteins could be labeled with [3H]valine within 36 h by incubating the whole oocytes in sea water to a high level, indicating that the proteins are actively synthesized by the growing oocytes. Synthetic rates of the proteins differ from each other at one stage and are higher in the small than in the large oocytes in general, suggesting that the membrane proteins are involved in the function of the vitelline envelopes during oogenesis.

Purification and characterization of oocyte vitellin from Perinereis cultrifera (polychaete annelid)

Vitellin was identified and purified from submature oocytes of Perinereis cultrifera by concanavalin-A - Sepharose and Ultrogel AcA 34 column chromatography. The yolk protein was defined as a glycolipoprotein with a relative molecular mass of 380000. Upon dissociation by sodium dodecyl sulphate, vitellin was shown to release five polypeptide subunits which ranged in relative molecular mass from 98000-16000. The purified vitellin was further characterized by amino acid analysis and its lipid and carbohydrate contents. The molecule contained about 5% carbohydrate and 16% lipid. Antiserum prepared against vitellin was shown to react with a component from the coelomic fluid of maturing females. Thus, it was suggested that in nereid annelids, considered phylogenetically and morphologically primitive, oocyte differentiation might depend upon the incorporation of a vitellogenin as in insects and many oviparous vertebrates.

Endocrine programmed development and reproduction in Nereis

The pattern of cerebral endocrine activity in Nereis diversicolor is characterized by the maintenance of a high plateau of activity prior to, and during most of, the time occupied by gametogenesis. During the closing stages of the life cycle, the rate of secretion is progressively reduced, resulting in the production of a homogeneous population of gametocytes and subsequently in their final maturation. Secretion has been assayed in terms of the regeneration-promoting, maturation-inhibiting, and gametotrophic activities of the hormone. Assays involved transplantation of living brains, in some cases between animals at different stages of maturity, but evidence is presented that the activity of such brains reliably reflects their rate of secretion in situ. Cerebral control of growth and maturation is probably mediated by a single hormone, an ordered sequence of events occurring as different thresholds for hormone action are reached in turn.

Endocrine regulation of spermatogenesis in Nereis diversicolor (annelida polychaeta): experimental study of the control of meiotic differentiation

In Nereidae, spermatogenesis is regulated by a hormone secreted by the supraesophageal ganglion (brain hormone). The hormonal concentration decreases with worm age. Spermatogonial proliferation in young worms proceeds under a high hormonal level whereas differentiation of spermatozoa in aged worms occurs in the absence of brain hormone. Spermatogonia were removed from the endocrine influence of the brain in vitro by the organ culture method. Isolated parapodia produced spermatozoa after 13 days. Control cultures were obtained by associating a parapodium with the prostomium of a young worm. It seems that the brain hormone inhibits the spermatocyte differentiation since removal of the brain results in increased RNA synthesis, followed by DNA replication, meiosis, and spermiogenesis (Bertout, '83). Modalities of the hormonal control of meiosis, especially at the level of the RNA syntheses related to meiotic differentiation, were investigated. Effects of reintroducing the hormonal influence during meiotic differentiation were studied through parapodium-prostomium associations at various time after the brain inhibition has been lifted. Effects or RNA inhibitors (actinomycin D, alpha-amanitin, cordycepin) were also tested. The results lead us to propose a hypothesis according to which brain hormone would interfere with molecular events related to the transition from the stage of spermatogonial proliferation (mitotic behavior) to the stage of meiotic differentiation (meiotic behavior).

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.

[Control of determination and the differentiation of the sexual somatic characters of Nereids (Annelida Polychaeta)]

The modalities of determination and differentiation of sexual dimorphism of Nereids (Annelida Polychaeta) were investigated inNereis pelagica L. andPerinereis cultrifera G. with experiments involving decerebration, castration (by X irradiation), grafts or injection of genital products.The sexual somatic characters of the parapodial cirri (swelling and crenellations) always develop according to the genetic sex, whatever the genital state of the Nereis and the nature of the intervention. Therefore the determination of the sexual somatic characters of the parapodial cirri appear to be determined in a very precocious and stable way (at least 3 months before the genital differentiation).The pygidial papillae, a male sexual character in normal conditions, can be formed by very young females in which heteronereidation is realized prematurely by decerebration, and by females of any age in which heteronereid transformations are realized in a male environment (e.g. by injection of genital products, or grafting onto a male). Therefore the pygidial papillae may be determined precociously in either sex; however their differentiation is normally inhibited in females in the course of genital maturation.