A model for germ cell development in a fully segmented worm

INTRODUCTION

Polychaetes are segmented marine worms with body segments separated by a complete or incomplete septum. In most polychaetes the whole body cavity is filled with gametes during the breeding season. Platynereis dumerilii (Pl. dumerilii), which has an incomplete septum was shown to develop a single gonadal structure for gamete production located in the neck region. However, in Perinereis nuntia (Pe. nuntia), which has a complete septum separating each segment, the developmental feature of gametes remains unknown. To clarify this, the marker gene vasa was used to trace the development of germ cells throughout the life stages of Pe. nuntia.

RESULTS

In three-segmented juveniles, Pn-vasa was expressed in the parapodia and in the two cells localized in the pygidium. During the addition of a new segment, Pn-vasa positive cells in the pygidium increased from two to four and two new Pn-vasa positive cells were found in the newly-generated segment. In adults, Pn-vasa was expressed in a large cell cluster at the distal end of the parapodia, in smaller cell clusters (which had an elongated form in the trunk area of the parapodia), and in oocytes in the coelomic cavity. This may suggest that germ cells settle in the parapodia and later translocate into the coelomic cavity to develop into oocytes.

CONCLUSION

Our observations will help in understanding the mechanism of germ cell development in all body segments of Pe. nuntia. We hypothesize that primordial germ cells are supplied from the pygidium to every newly-generating segment which later settle in the parapodium. This will explain how polychaetes can generate gametes in each body segment, even those that are independently separated with a complete septum.

Expression of three novel cytochrome P450 (CYP) and antioxidative genes from the polychaete, Perinereis nuntia exposed to water accommodated fraction (WAF) of Iranian crude oil and benzo[a]pyrene

To report a novel CYP genes and to evaluate its potency as a biomarker for oil pollution, we cloned three CYP genes and measured their expression profiles under controlled lab conditions using real-time reverse transcription PCR (real-time RT-PCR) after exposure of the water accommodated fraction (WAF) of Iranian crude oil and benzo[α]pyrene (B[α]P) as a positive control. Of these, CYP432A1 (CYP3 clan) gene was significantly induced by B[α]P exposure, indicating that the CYP3 clan gene would play an important role in polycyclic aromatic hydrocarbon (PAH) metabolisms, particularly for B[α]P in this species. However, the Perinereis nuntia CYP431A1 mRNA, a CYP2 clan gene, was sensitively expressed to WAF exposure with other two CYP genes. As one of Phase II detoxification enzymes, the glutathione S-transferase (GST) genes also upregulated with other antioxidant genes (SOD and CAT), indicating that WAF-exposed P. nuntia was properly responding to this kind of chemical stress. Thus, three CYP genes from the polychaete, P. nuntia have a potential as a biomarker in monitoring of the marine sediment after an oil spill accident.