Brasier Clive, Franceschini Selma, Forster Jack, Kirk Susan
Forest Research, Alice Holt Lodge, Farnham, Surrey GU10 4LH, UK.
J Fungi (Basel). 2021 Jun 6;7(6):452. doi: 10.3390/jof7060452.
In the 1970s, clones of the two subspecies of , subsp. (SSAM) and subsp. (SSNU) began to overlap in Europe, resulting in hybrid swarms. By 1983-1986, hybrids with high, SSAM-like growth and pathogenic fitness comprised 75% of popula-tions at Limburg, Netherlands and Orvieto, Italy. We resampled these populations in 2008 to examine trends in hybrid fitness traits. Since preliminary sampling in 1979-1980, locus frequency had increased from ~0% to ~32% at Orvieto and 5% to ~43% at Limburg, and vegeta-tive incompatibility type frequency had changed from near clonal to extremely diverse at both sites. This represents an enormous increase in outcrossing and recombination potential, due in part to selective acquisition (under virus pressure) of and loci from the resident and in part to SSAM × SSNU hybridisation. Overt virus infection in the 2008 samples was low (4%), diagnostic SSAM and SSNU and loci were recombinant, and no isolates exhib-ited a parental SSAM or SSNU colony pattern. At both sites, mean growth rate and mean patho-genicity to 3-5 m clonal elm were high SSAM-like, indicating sustained directional selection for these characters, though at Orvieto growth rate was slower. The once frequent SSNU-specific colony dimorphism was largely eliminated at both sites. Perithecia formed by Limburg isolates were mainly an extreme, long-necked SSNU-like form, consistent with transgressive segregation resulting from mismatch of SSAM and SSNU developmental loci. Orvieto isolates produced more parental-like perithecia, suggesting the extreme phenotypes may have been se-lected against. The novel phenotypes in the swarms are remodelling in Europe. Locally adapted genotypes may emerge.
20世纪70年代，榆枯萎病菌的两个亚种，即榆枯萎病菌荷兰亚种（SSAM）和榆枯萎病菌北美亚种（SSNU）的克隆菌株开始在欧洲重叠，形成了杂交群体。到1983 - 1986年，具有类似SSAM的高生长和致病适应性的杂交种在荷兰林堡和意大利奥尔维耶托的种群中占比约75%。我们在2008年对这些种群重新采样，以研究杂交适应性性状的趋势。自1979 - 1980年初步采样以来，奥尔维耶托的mat位点频率从约0%增加到约32%，林堡从5%增加到约43%，营养体不亲和类型频率在两个地点都从近乎克隆型变为极其多样。这代表着异交和重组潜力的巨大增加，部分原因是（在病毒压力下）从本地的榆枯萎病菌中选择性获得mat和het位点，部分原因是SSAM×SSNU杂交。2008年样本中的明显病毒感染率较低（约4%），诊断性的SSAM和SSNU的mat和het位点是重组的，没有分离株表现出亲本SSAM或SSNU的菌落模式。在两个地点，平均生长速率和对3 - 5米克隆榆树的平均致病性都高度类似SSAM，表明对这些性状持续存在定向选择，尽管在奥尔维耶托生长速率较慢。曾经常见的SSNU特有的菌落二态性在两个地点基本消失。林堡分离株形成的子囊壳主要是一种极端的、长颈的类似SSNU的形式，这与SSAM和SSNU发育位点不匹配导致的超亲分离一致。奥尔维耶托分离株产生的亲本样子囊壳更多，表明极端表型可能已被选择淘汰。杂交群体中的新表型正在重塑欧洲的榆枯萎病菌。可能会出现局部适应的基因型。
