Genome Size Estimates for Cotoneaster Spp

Cotoneaster is a diverse genus that comprises ~400 species of highly variable plants; from ground covers to trees.  Cotoneasters are valued in the landscape because of their ability to grow in poor soils, response to pruning and tolerance of urban environments.  Breeding efforts to select superior plants have lead to relatively few introductions.  Hybridization has been hindered by the fact that apomixis is widespread in the genus.  Approximately 85% of species are triploid (2n = 3x = 51) or tetraploid (2n = 4x = 68) that exhibit apomixis.  New introductions with improved disease resistance and novel ornamental characteristics would increase interest in cotoneasters as landscape plants.  While information on chromosome numbers of many species is available, there have been few reports of genome sizes for cotoneasters.  Data on the relative genome sizes of species in different taxonomic groups could provide insight into the potential for crossing distantly related species.  Genome sizes were estimated for 20 species of Cotoneaster using flow cytometric analysis of DAPI stained nuclei using maize (Zea mays 'CE-777') as an internal standard.  Three replicates of each species were used to calculate mean holoploid nuclear genome size.  Nuclear genome sizes were translated to monoploid (1Cx) values from previously reported ploidy levels.  Monoploid genome sizes ranged from 1.01 to 1.50 pg with a mean of 1.14 pg.  Monoploid values were subjected to ANOVA and mean separation using Tukey's HSD.  Significant differences were observed among species' monoploid genome size estimates.  Based on monoploid genome size, it does not appear that chromosome size is correlated to taxonomy.  Ploidy levels were inferred for four species for which reports were lacking.  Cotoneaster arbusculus, C. hebephyllus, and C. rhytidophyllus were determined to be tetraploid and C. buxifolius was found to be triploid.  Multiple ploidy levels have been reported for C. congestus and C. bacillaris; however, both of our accessions were found to be triploids.  To verify flow cytometry results, chromosome counts were performed for two species via light microscopy of metaphase cells by squashing meristematically active root tips.  At least five cells were counted for each species.  The chromosome number for C. buxifolious was found to be 2n = 3x = 51, and is the first report of chromosome number and ploidy level for this species.  Chromosome counts also confirmed previous reports that C. henryanus is a diploid specides 2n = 2x = 34.