Reconstruction of the past plankton community in Lake Biwa using Daphnia resting eggs

为了明确琵琶湖浮游生物群落在过去100年的变化，我们对琵琶湖沉积物中的支海生物遗骸进行了古湖泊学分析，并对水蚤休眠卵进行了遗传分析。用137Cs和210Pb测定了湖泊沉积物的年代。为了进行遗传分析，我们开发了一种从水蚤休眠卵中提取DNA的新方法。采用PCR-RFLP方法，根据mtdna 12S rRNA序列和核DNA- its区域进行水蚤物种鉴定。结果揭示了浮游动物群落的历史变化。1900年以前，mendotae被认为是美洲大陆的特有种，在该湖大量分布，是唯一的透明水蚤种。1990年以后，出现了galeata，并与mendotae杂交。因此，目前琵琶湖透明水蚤种群主要由门尾水蚤、galeata水蚤及其杂种水蚤组成。对该种群的详细分析表明，mendotae和galeata以透明水蚤（Hyalodaphnia）为主要优势种，其存在平衡偏离了Hardy-Weinberg平衡，表明mendotae和galeata是作为单一物种在湖中孤立存在的。除上述品种外，1999年出现了突发性和丰富性分布。遗传分析表明，该种群在遗传上与北美西海岸湖泊的种群最接近，表明琵琶湖的白斑蝶是最近从北美入侵的。对枝海化石的古湖泊学分析进一步表明，水蚤（主要为门多塔水蚤）种群密度有所增加，近20年来不产休眠卵。实验分析表明，静息产蛋量的减少不是由于水质的变化。自1980年以来，水蚤在冬季也大量出现，静息产卵量的减少似乎是由于冬季食物和水温变化引起的自然选择的结果。这些结果表明，琵琶湖浮游动物群落在过去的100年里发生了很大的变化，并且经常受到各种类型的环境干扰，包括富营养化、物种入侵和水温变化。少

To clarify changes in the plankton community for the past 100 years in lake Biwa, we performed paleolimnological analyses on cladoceran remains and genetic analyses on daphnia resting eggs in die lake sediments. Dating of the lake sediments was determined using 137Cs and 210Pb. For genetic analyses, we developed a new method to extract DNA efficiently from the resting eggs of Daphnia species. Identification of Daphnia species was then made based on sequences of mt DNA 12S rRNA and nuclear DNA-ITS regions with PCR-RFLP method.The results revealed historical changes in the zooplankton community. Before 1900, D. mendotae which have been thought as endemic species in Norm America were abundantly occurred and sole Hyalodaphnia species in this lake. After 1990, however, D. galeata was occurred and produced hybrids with D. mendotae. Thus, at present, Hyalodaphnia populations in Lake Biwa compose of D. mendotae, D. galeata and their hybrid. Detailed analysis of the present population showed th … More at D. mendotae were most dominant Hyalodaphnia species and that existence balance of these species (and hybrid) deviated from the Hardy-Weinberg equilibrium, suggesting that D. mendotae and D. galeata are isolated as single species within the lake. Other than these species, D. pulicaria occurred suddenly and abundantly in 1999. Genetic analyses showed that this population is genetically most close to those in lakes at west coast of North America, suggesting that D. pulicaria occurred in Lake Biwa is a recent invader from North America.Paleolimnological analyses on cladoceran remains further revealed that population density of Daphnia species (mainly D. mendotae) has increased, they did not produce resting eggs for the last 20 years. Experimental analysis indicates mat this decrease in resting egg production is not due to changes in water quality. Since Daphnia became to appear abundantly even in winter since 1980, this decrease in resting egg production seems to be due to a results of natural selection caused by changes in food and water temperature regimes in winter. These results suggest that the zooplankton community in Lake Biwa has largely changed for the past 100 years and has received frequently various type of environmental disturbances including eutroophication, species invasions and shift in water temperature regimes. Less