Population structure and evolution of virulence in the pinewood nematode

在松材线虫核基因组中发现了5个微卫星位点。其中4个具有多态性，有助于线虫种群结构的分析。三年的研究表明，虽然每个位点的平均等位基因数几乎没有变化，但等位基因频率波动很大。在整个群体内遗传多样性波动较小。从个别枯死树木或媒介昆虫采集的线虫亚群内的遗传多样性降低，导致亚群间的遗传分化增加。线虫种群的这种遗传变化与其毒力（杀死树木的能力）的变化无关。同时接种的毒力和无毒菌株对松thunbernitus树表明，特异性的等位基因的频率为有毒的菌株比那些特定的无毒菌株在病树，表明有毒的菌株比无毒的增殖更快。这表明，有性繁殖的松材线虫满足了一个假设，即在无性繁殖的寄生虫和病原体中建立了多重感染毒性的进化理论。该接种实验还显示可能由于交换而出现新的等位基因。遗传多样性的增加可能会增加毒力的差异。然而，三年的接种实验，使用两个不同的菌株在毒力没有增加。去辨认B。xylophilus和B.为进一步研究拟短额真核生物的遗传多样性，开发了两对能产生不同长度DNA片段的种特异性PCR引物。B感染前后水分胁迫下的松弛。木虱在密花杨幼苗上很少能在枯萎病中存活。

Five microsatellite loci were found in the nuclear genome of Bursaphelenchus xylophilus. Four of them were polymorphic and shown to help the analysis of nematode population structure. A three-year study indicated that frequencies of alleles fluctuated greatly although the mean number of alleles per locus was almost unchanged. There was a little fluctuation in the genetic diversity within a total population. The genetic diversity within nematode subpopulations sampled from individual dead trees or insect vectors decreased, which caused the genetic differentiation to increase between subpopulations. Such genetic change in nematode population had no relation to the change in the virulence (the capability of killing tree). Simultaneous inoculations of virulent and avirulent isolates on Pin us thunbergii trees showed that frequencies of alleles specific to virulent isolates were greater than those specific to avirulent isolates in diseased trees, indicating that virulent isolates proliferated more rapidly than avirulent ones. This shows that the pinewood nematode reproducing sexually satisfies one of the assumptions on which evolutionary theories of virulence by multiple infections are built in parasites and pathogens reproducing asexually. This inoculation experiment also showed appearance of new alleles possibly due to crossing over. The increase in genetic diversity may raise the difference in virulence. However, a three-year inoculation experiment by using two isolates differing in virulence showed no increase in virulence. To identify B. xylophilus and B. mucronatus, two species-specific PCR primer pairs that produce DNA fragments of different lengths were developed. Relaxation in water stress before and after the infection of B. xylophilus on P. densiflora seedlings rarely survived wilt disease.