The genus Cotoneaster (Rosaceae, Maloideae) is highly diverse, containing ≈400 species. Like other maloids, there is a high frequency of naturally occurring polyploids within the genus, with most species being tetraploid or triploid. Apomixis is also prevalent and is associated with polyploidy. The objective of this study was to estimate genome sizes and infer ploidy levels for species that had not previously been investigated as well as compare estimates using two fluorochromes and determine base pair (bp) composition. Chromosome counts of seven species confirmed ploidy levels estimated from flow cytometric analysis of nuclei stained with 4′,6-diamidino-2-phenylindole (DAPI). Monoploid (1Cx) genome sizes ranged from 0.71 to 0.96 pg. Differences in monoploid genome size were not related to current taxonomic treatment, indicating that while chromosome sizes may vary among species, there are no clear differences related to subgeneric groups. A comparison of DAPI and propidium iodide (PI) showed a difference in DNA staining in Cotoneaster comparable to other rosaceous species. Base pair composition (AT%) in Cotoneaster ranged from 58.4% to 60.8%, which led to overestimation of genome size estimates in many cases—assuming the estimates of the DNA intercalator are accurate. Our findings will inform breeders with regard to the reproductive behavior of potential parents and may be used to confirm hybrids from interploid crosses.
Joseph J. Rothleutner, Mara W. Friddle and Ryan N. Contreras
Joseph J. Rothleutner, Ryan N. Contreras, Virginia O. Stockwell and James S. Owen
Cotoneaster Medik. is a genus of ornamental landscape plants commonly affected by fire blight. Fire blight is a disease caused by the bacterial pathogen, Erwinia amylovora (Burrill) Winslow et al., that attacks a wide range of taxa in the apple subfamily (Maloideae; Rosaceae). To assess susceptibility of species and identify potential sources of resistance, we inoculated 52 taxa of Cotoneaster with E. amylovora. Disease severity was scored by percent shoot necrosis (lesion length/total shoot length). Disease screenings were conducted over 2 years and varying levels of susceptibility were observed. Some taxa were highly susceptible to fire blight and the disease resulted in whole plant mortality (C. rhytidophyllus Rehder & E.H. Wilson, C. rugosus E. Pritzel ex Diels, and C. wardii W.W. Smith). Other taxa repeatedly exhibited moderate to high levels of disease resistance [C. arbusculus G. Klotz, C. chungtinensis (T.T. Yu) J. Fryer & B. Hylmö, C. delsianus E. Pritzel var. delsianus, C. sikangensis Flinck & B. Hylmö, C. simonsii Baker, and C. splendens Flinck & Hylmö]. Ongoing studies are being conducted to determine if taxa with high levels of resistance under artificial inoculation will exhibit high levels of resistance in the field under natural disease pressure. Identifying sources of disease resistance will be useful for breeding programs to increase tolerance of these landscape plants with desirable horticultural characteristics to fire blight.