The critical stage for inducing oviparity and embryonic diapause in parthenogenetic Artemia(Crustacea: Anostraca): an experimental study

The brine shrimp Artemia exhibits two reproductive modes: 1) oviparity, producing diapause embryos;and 2) ovoviviparity, producing free-swimming nauplii. Previous studies have suggested the existence of a critical stage that determines the reproductive mode. Physicochemical factors, such as photoperiod, temperature, and salinity, have been suggested to irreversibly aff ect the reproductive mode of oocytes during this critical stage. In this study, experiments were carried out using a photoperiod and temperature-sensitive parthenogenetic Artemia clone where maternal Artemia were shifted bidirectionally between ovoviviparity (18 h L:6 h D, 27℃) and oviparity (6 h L:18 h D, 19℃) culture conditions. In the main experiment ( Artemia shifted at six diff erent stages including the post-larva Ⅱ to adult Ⅱ), the reproductive mode of fi rst brood was converted when shifting was performed on post-larva Ⅱ and Ⅲ but was not converted when females were shifted after post-larva Ⅲ. A supplementary experiment further revealed that the reproductive mode of fi rst brood could be altered when shifting females at an "early phase of postlarva Ⅳ", characterized by a developing ovisac reaching the middle of the third abdominal segment, ventral spines, and some oocytes growing larger than the others. In both experiments, reproductive modes of the second brood were signifi cantly aff ected when the shifting was performed on post-larva Ⅳ. These results suggest that the critical stage for inducing oviparity and embryonic diapause is at the previtellogenic stage of oocytes, or at maternal "early phase of post-larva Ⅳ" for the first-brood off spring. During this stage, diff erential gene expression patterns of the two destined oocytes may be triggered by the token stimuli signals received by the oocytes.

三北地区内陆盐水的浮游甲壳类

１９９０～１９９１年间在三北地区盐度０．５７～２１４ｇ／Ｌ的２８个内陆水体采集了浮游甲壳类水样．共检出４３种甲壳类，其中无甲类２种，枝角类２３种，桡足类１８种。根据出现率和平均密度，浮游甲壳类的主要种类有１２种，即无甲类：卤虫；枝角类：大型、蒙古裸腹、点滴尖额和矩形尖额；桡足类：细巧华哲水蚤、亚洲后镖水蚤、直刺北镖水蚤、近邻剑水蚤、短角异剑水蚤、扁平小剑水蚤和等刺温剑水蚤。结果表明，浮游甲壳类总种数随盐度增大而趋于下降，文中讨论了内陆盐水浮游甲壳类的种类组成、分布和盐度耐性以及种类多样性与盐度、碱度、ｐＨ、离子系数之间的关系等问题。

Limited long-distance dispersal success in a Western European fairy shrimp evidenced by nuclear and mitochondrial lineage structuring

Anostraca are known by their ability for long-distance dispersal,but the existence in several species of deep,geographically structured mtDNA lineages suggests their populations are subjected to allopatric differentiation,isolation,and prevalence of local scale dispersion.Tanymastix stagnalis is one of the most widespread species of Anostraca and previous studies revealed an unclear geographical pattern of mtDNA genetic diversity.Here,we analyze populations from the Iberian and Italian Peninsulas,Central Europe,and Scandinavia,with the aim to characterize the patterns of genetic diversity in a spatio-temporal framework using mtDNA and nuclear markers to test gene flow among close populations.For these aims we built a time-calibrated phylogeny and carried out Bayesian phylogeographic analyses using a continuous diffusion model.Our results indicated that T.stagnalis presents a deeply structured genetic diversity,including 7 ancient lineages,some of them even predating the Pleistocene.The Iberian Peninsula harbors high diversity of lineages,with strong isolation and recent absence of gene flow between populations.Dispersal at local scale seems to be the prevailing dispersal mode of T.stagnalis,which exhibits a pattern of isolation-bydistance in the Iberian Peninsula.We remark the vulnerability of most of these lineages,given the limited known geographic distribution of some of them,and the high risk of losing important evolutionary potential for the species.