Ashes (Fraxinus spp.) are important hardwood tree species in rural, suburban, and urban forests of the eastern USA. Unfortunately, emerald ash borer (EAB, Agrilus planipennis) an invasive insect pest that was accidentally imported from Asia in the late 1980s–early 1990s is destroying them at an alarming rate. All North American ashes are highly susceptible to EAB, although blue ash (F. quadrangulata) may have some inherent attributes that provide it some protection. In contrast Manchurian ash (F. mandshurica) is relatively resistant to EAB having coevolved with the insect pest in its native range in Asia. Given its level of resistance, Manchurian ash has been considered for use in interspecies breeding programs designed to transfer resistance to susceptible North American ash species. One prerequisite for successful interspecies breeding is consistency in chromosome ploidy level and number between the candidate species. In the current study, we cytologically determined that both Manchurian ash and blue ash are diploids (2n) and have the same number of chromosomes (2n = 2x = 46). We also characterized these species’ ribosomal gene families (45S and 5S rDNA) using fluorescence in situ hybridization (FISH). Both Manchurian and blue ash showed two 45S rDNA and one 5S rDNA sites, but blue ash appears to have an additional site of 45S rDNA. The 5S rDNA in both species is colocalized interstitially with one of the 45S rDNA sites. The copy number of these two ribosomal gene families in Manchurian ash were observed to be quite varied, which indicates the species are still undergoing evolutionary homogenization.

灰烬（Fraxinus spp。）是美国东部农村，郊区和城市森林中重要的硬木树种。不幸的是，1980年代末至1990年代初从亚洲意外进口的一种入侵性虫害“翠绿色ash虫（EAB，Agrilus planipennis）”正在以惊人的速度销毁它们。尽管蓝灰（F. quadrangulata）可能具有某些内在属性，但可以提供一些保护，但所有北美灰烬对EAB的敏感性都很高。相比之下，满洲灰（F. mandshurica）对EAB与其在亚洲本地范围内的害虫共同进化具有相对抗性。鉴于其抗性水平，满洲灰已被考虑用于种间育种计划，旨在将抗性转移至易感的北美灰种。成功进行种间繁殖的先决条件是染色体倍性水平和候选物种之间的数目保持一致。在当前的研究中，我们从细胞学上确定满洲灰和蓝灰均为二倍体（2 n），并且具有相同数量的染色体（2 n  = 2 x = 46）。我们还使用荧光原位杂交（FISH）对这些物种的核糖体基因家族（45S和5S rDNA）进行了表征。满洲人和蓝灰都显示了两个45S rDNA和一个5S rDNA位点，但是蓝灰似乎还有一个45S rDNA的位点。两个物种中的5S rDNA与45S rDNA位点之一在间隙内共定位。观察到满洲灰中的这两个核糖体基因家族的拷贝数变化很大，这表明该物种仍在进行进化同质化。