Cytogenetics of Fraxinus mandshurica and F. quadrangulata: ploidy determination and rDNA analysis

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.