Population structure and evolution of virulence in the pinewood nematode

在木桐核基因组中发现了5个微卫星位点。其中4个是多态的，有助于线虫种群结构的分析。一项为期三年的研究表明，尽管每个位点的平均等位基因数量几乎不变，但等位基因的频率波动很大。种群内遗传多样性有一定的波动。线虫亚群内遗传多样性下降，导致亚群间遗传分化加剧。线虫种群的这种遗传变化与毒力（杀树能力）的变化无关。将毒株和无毒株同时接种在松柏树上，结果表明，在病株上，毒株特异的等位基因频率大于无毒株特异的等位基因频率，说明毒株比无毒株增殖更快。这表明松木线虫的有性繁殖满足了一个假设，即多重感染的毒性进化理论是建立在寄生虫和病原体无性繁殖的基础上的。该接种试验还显示了可能由于杂交而产生的新等位基因。遗传多样性的增加可能会增加毒力的差异。然而，用两个毒力不同的分离株进行了为期三年的接种实验，结果显示毒力没有增加。为了鉴定嗜木双歧杆菌（b.s yllophilus）和巨尖双歧杆菌（b.s mucronatus），我们开发了两个物种特异性PCR引物对，它们产生不同长度的DNA片段。在松木芽孢杆菌侵染松木芽孢杆菌前后，水分胁迫的缓解使松木芽孢杆菌幼苗在枯萎病中很少存活。

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.