| Most of the Cladocera are cyclical parthenogenesis,under favourable conditions,reproduction by parthenogenesis,when unfavourable environment occur,Cladocera turns to sexual reproduction and produces dormant eggs to through adverse environments.Therefore the reproductive method of Cladocera has an important role in adaptation to the environment.In this paper,Daphnia pulex which is a common species during the occurrence of cyanobacterial blooms was used as a research object,to study the role of reproductive method in adaptation to Microcystis aeruginosa.It was subcultured in different reproductive ways,its offspring were exposed to two different food conditions to evaluate its fitness,and microsatellite markers were used to analyze the clonal diversity of each generation.The role of sexual reproduction and parthenogenesis in the evolution of adaptation and resistance to cyanobacteria was studied from both phenotypic characteristics and genetic structure.The detailed results are as follows:Firstly,the role of sexual reproduction in the evolution of adaptation and resistance to Microcystin-producing Microcystis aeruginosa for Daphnia pulex was studied.We studied the life history responses of 125 D.pulex clones,they were hathed from different resting eggs collected from seven generations of sexual reproduction.One clone from F0 generation of sexual reproduction,eight clones from F1 generation of sexual reproduction,sixteen clones from F2 generation of sexual reproduction,twenty clones from F3 generation of sexual reproduction,twenty clones from F4 generation of sexual reproduction,twenty clones from F5 generation of sexual reproduction,twenty clones from F6 generation of sexual reproduction,twenty clones from F7 generation of sexual reproduction.Firstly exploring the role of sexual reproduction in the evolution of resistance to Microcystin-producing M.aeruginosa by life table experiment exposing clones to two diets,one composed only Chlorella pyrenoidosa,and the other a mixture of C.pyrenoidosa and toxic M.aeruginosa.The experimental results show that compared with the good food C.pyrenoidosa,M.aeruginosa can significantly reduce the intrinsic rate of increase.The resistance of F0 generation is 0.41 ± 0.02,the resistance of F1 generation was between 0.37 ± 0.09 and 0.95 ± 0.10,the resistance of F2 generation was between 0.57 ± 0.17 and 1.03 ± 0.08,the resistance of F3 generation was between 0.41 ± 0.06 and 0.87 ± 0.10,the resistance of F4 generation was between 0.34 ± 0.05 and 0.80 ± 0.06,the resistance of F5 generation was between 0.48 ± 0.04 and 0.84 ± 0.06,the resistance of F6 generation was between 0.38 ± 0.06 and 0.69 ± 0.06,the resistance of F7 generation was between 0.51 ± 0.03 and 0.79 ± 0.03.Average lifespan,generation time,intrinsic rate of increase,net reproduction rate,resistance and number of dormant eggs produced for every generation had clonal differences.Then ten pairs of microsatellite primers were used for each clone,and a total of 16 different clones were found in all generations of sexual reproduction.The experimental results imply that M.aeruginosa has a negative effect on the growth and reproduction of D.pulex.Almost all the clones produced more resting eggs under poor food conditions than those under good food conditions,suggesting that cyanobacteria can promote the formation of dormant eggs.D.pulex with single genotype produce different resistant clones by successive sexual reproduction,may provide sufficient raw materials for its evolution of resistance to M.aeruginosa,the resistance showing the trend of "rise-fall-fall-rise-fall-rise",there was no regular change in the first two generations,but the later five generations successively inheritance from one generation to next.Secondly,the role of parthenogenesis in the evolution of adaptation and resistance to Microcystin-producing Microcystis aeruginosa for Daphnia pulex was studied.In this experiment,a D.pulex hatched by a dormant egg was successively subcultured by parthenogenesis,every fourth generation for resistance testing and microsatellite markers,the results showed that the highest resistance was 0.909 ± 0.01 in the F40 generation and the lowest resistance was 0.32 ± 0.05 in the F24 generation,the resistance of D.pulex did not show regular changes with the development of parthenogenesis.Intrinsic growth rate under both bad and good food conditions show the trend of “rise-fall-rise-fall-rise-fall-rise”.Net reproductive rate under good food conditions show the trend of “rise-fall-rise-fall-rise”,and net reproductive rate under poor food conditions show the trend of “rise-fall-rise-fall-rise-fall-rise”.The average lifespan,generation time,intrinsic rate of increase,net reproductive rate,number of resting eggs and resistance varied significantly between different generations.A total of 9 different clones were found in all generations of parthenogenesis.The experimental results suggest that the successive parthenogenesis of D.pulex can also provide raw materials for evolution of resistance to M.aeruginosa,but it has less contribution than sexual reproduction,but once sexual reproduction takes more time,it is also relatively long,the time for once sexual reproduction is about 75 d-90 d,and the time for once parthenogenesis is about 4 d-6 d.In conclusion,sexual reproduction can provide the evolutionary basis for the evolution of resistance to cyanobacteria in D.pulex,and once sexual reproduction can lead to a great deal of genetic variation.Parthenogenesis can also provide the evolutionary basis for the evolution of resistance to cyanobacteria,but it is slower than sexual reproduction.Therefore,selective pressure can be added to future studies to select D.pulex with a highly tolerant genotype,which can coexists with cyanobacteria and plays a positive role in controlling cyanobacterial blooms. |
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