夜光藻有性繁殖研究进展

Progress on studies of sexual reproduction in Noctiluca scintillans

夜光藻是全球最主要的赤潮生物之一,也是我国近海常见的浮游甲藻。根据营养方式分为异养的红色夜光藻和混合营养的绿色夜光藻,前者广泛分布于温带和亚热带近岸水域,后者仅分布于热带西太平洋、阿拉伯海、阿曼湾和红海。夜光藻的生活史包括无性繁殖和有性繁殖过程。少部分营养细胞自发转变为配子母细胞,启动了有性繁殖。每个配子母细胞可形成大量配子,具有横沟、纵沟和2根鞭毛,形态与裸甲藻接近。配子两两融合形成合子,合子不经过休眠孢囊阶段直接发育成新的营养细胞。目前,对配子母细胞形成的调控机制、合子发育的影响因素的认识还存在分歧。研究发现,营养细胞经过一定次数的二分裂后都会转变为配子母细胞,而配子的存在能够中止此过程,使营养细胞继续进行二分裂。因此,有性繁殖可能通过产生新个体对种群增长做出贡献,还可能通过释放配子维持无性繁殖,进而促进种群增长。配子在相模湾水域全年都有分布,其丰度峰值与营养细胞丰度峰值同步或提前出现,配子的大量出现可能是赤潮形成的必要条件。对有性繁殖的研究佐证了夜光藻在甲藻的系统进化中处于较为古老的地位。此外,还简单介绍了研究夜光藻有性繁殖的主要方法,回顾了国内的夜光藻研究,并对相关研究进行了展望。

Current knowledge about sexual reproduction in Noctiluca scintillans is reviewed in this paper. Noctiluca scintillans is one of the most prominent ＂red tide＂ organisms found in all Chinese coastal waters. There are two forms, red Noctiluca and green Noctiluca. The former is heterotrophic and broadly distributed in temperate and sub-tropical coastal waters. The latter contains a photosynthetic symbiont and can survive without an external food supply. It is mainly limited to the tropical west Pacific, parts of the Arabian Sea, the Gulf of Oman, and the Red Sea. The complete life cycle of N. scintillans consists of binary fission and sexual reproduction. A small fraction of trophonts transform into gametogenic cells, which undergo two successive nuclear divisions. The products of nuclear division migrate to the cell surface with a small amount of cytoplasm and further synchronously divide six to eight times. A large number of gametes, which possess some noctilucid-like characters, such as a transverse groove, a longitudinal groove, and two mother cell ghost. The zygote, which results from the fusion of two isogametes, gives rise flagella, are released from the to miniscule trophonts, which undergo change in shape and reduction in flagellum number and acquire a tentacle and crust. Gamete fusion in N. scintillans is not followed by formation of a non-motile resting cyst or by meiosis. To date, there is some controversy regarding the process of sexual reproduction, i.e., gametogenesis and the development from zygote to trophont. There is a strict program for the formation of gametogenic cells, which starts to operate in every cell after a defined number of binary fissions. However, the presence of gametes can reset this program and keep trophonts in binary fission instead of proceeding to gametogenesis. Sexual reproduction is common when populations are increasing but rare during population decline both in the field and in the culture. Thus, sexual reproduction is considered to contribute to the population growth of N. scintillans. Gametes were found all year round in coastal waters of Sagami Bay, and its peak abundance was observed to occur just before or at the same times as the peak abundance of the trophonts. It is suggested that sexual reproduction enhances the formation of N. scintillans ＂red tides＂ as intensive multiplication may occur under high-density conditions. The primitiveness of the noctilucid flagellum can be regarded as an ancestral character of dinoflagellates. If the gametes are likely to reflect the primary attributes of trophonts, it could be concluded that N. scintillans is an ancestral form of dinoflagellates. The methodology of investigating this sexual reproduction, including collection and cultivation, observation by light microscopy, quantification of gametes, etc., is briefly introduced in this article. Studies on physiology, population dynamics, and bloom formation of N. scintillans in Chinese coastal waters are summarized, and future research topics are briefly discussed.