Research On Reproduction Ways, Genetic Variation And Ploidy Level Change Of Potato Phytophthora Infestans

The oomycete Phytophthora infestans is the pathogen of potato late blight.It has led to the Irish Great Famine in Ireland and continues to threaten the global food security.At present,people generally use chemical agents,agricultural cultivation measures and disease-resistant crops to prevent and control P.infestans,but the effect is not prominent.It is possible to achieve good results by understanding its entire life cycle,and then selecting a weak point for prevention or control,which is a common method for the control of other plant pathogen.As are other oomycetes,P.infestans is regarded to be diploid during the vegetative phase of its life cycle,although some studies reported trisomy and polyploidy.People also found that the ploidy level of P.infestans has been changing along with its evolution,and the dominant strains have also been in transition.It was assumed that it may change its genetic variability through changes in ploidy levels to adapt to environmental changes.Studies on different reproductive patterns in the life cycle of P.infestans have revealed that they can reproduce both sexually and asexually.It is speculated that the rapid propagation of the former help them expand the population,and the latter undergoes chromosome recombination,which help them generate genotypic diversity.Combining these two advantages,P.infestans has high potential to adapt to various environments.From the above,the ploidy level of P.infestans seems to have a certain relationship with its reproductive ways,and they have a strong correlation with the environmental adaptability of P.infestans.However,there is almost no systematic study on the relationship between the ploidy level of P.infestans and its reproductive ways and there were also few reports on the genetic diversity,environmental adaptability mechanisms and data support for them of P.infestans with different ploidy levels.Using microsatellite fingerprinting,genome-wide assessment of single nucleotide polymorphisms,nuclear DNA quantification,and microscopic counting of chromosome numbers,this study assessed the ploidy levels of a comprehensive selection of isolates and found the relationship between their ploidy levels and reproductive ways,genetic diversity and environmental adaptability which have strong data support.This study also performed a series of stress induction experiments on one triploid and one diploid and did resequencing analysis,conjecturing and validating the pattern of change in ploidy level of P.infestans.The main results are as follows:1.Ploidy variation of P.infestans in nature relative to their reproduction ways.All progenies from sexual populations were found to be diploid,in contrast nearly all dominant asexual lineages,including the most important pandemic clonal lineages US-1 and 13_A2 were triploid.2.Such triploids possess significantly more allelic variation than diploids.This happens not only at the whole genome level,but also in all coding regions and on the large repertoire of putative effector proteins coding regions,such as RXLR and CRN genes.More variation in genome in part reflects higher adaptability and “error-tolerant rate” in nature conditions.3.When exposed to artificial stress conditions,triploid genotype isolate NL08452 underwent a process of rapid chromosome loss and ploidy reduction should occur just before or at the beginning of mutation accumulation.Shift of ploidy level reflects that under stressful conditions,triploids show strong genome plasticity which may be a strategy to adapt to the environment change.4.Each haplotype of triploid genotype isolate NL08452 was not evolved independently so its genome constitution should be “AAB” rather than “ABC”.P.infestans appears to maintain genome-wide heterozygosity after the ploidy reduction but the isolate that underwent this process did not show obvious reduction in growth or fitness to the environment.This study revealed the relationship between the ploidy level,reproductive ways and genetic diversity of P.infestans.It also conjectured and validated the relationship between ploidy level changes and environmental adaptability,which laid a theoretical foundation and offered a new direction for the prevention and control of potato late blight.