Molecular approaches. A contribution to the classification of the genus Rhododendron

Rhododendrons and azalea are the most popular flowering, broadleaf evergreens and landscape plants in the Northern Hemisphere. The cultivated types are hybrids obtained by means of complicated cross-combinations of many species, mainly with Asian origin. The first recorded cross was made in 1800 between R. ponticum and R. nudiflorum. During the end of the 18th century the hybridisation activity reached remarkable levels. As for most of the cultivated tree species, the identification of rhododendrons is convoluted and the same cultivar name has been accidentally given to more than one genotype. Conversely, some cultivars have several synonymous names. A better understanding of the effectiveness of the different DNA-based markers is an important step towards plant germplasm characterization and classification. Comparisons of the performance of several molecular markers in assessing genetic diversity have demonstrated that correlations between the genetic relationships revealed by different polymorphism techniques can vary widely. For this reason AFLP, STMS and EST markers were compared to determine their relative efficiencies in a study of genetic diversity among 84 evergreen azalea, an important source of germplasm almost unknown. Three AFLP, 6 STMSs and 9 EST primer pairs were used. Statistical analyses were carried out in order both to compare markers and to establish horticultural classification. High level of polymorphism was observed for all three marker systems, but AFLPs appeared to be the most efficient marker system due to their highest polymorphism detection capacity. Similarity matrices produced for each marker technique showed weak, yet significant, correlations (statistic g) when Mantel test was applied. The highest correlations were observed between AFLP and STMS. The Analysis of Molecular Variance showed that for all markers the genetic diversity was mainly attributable to differences among cultivars within horticultural groups. By means of assignment test and cluster analyses cultivars were grouped with a remarkable efficiency in their source population. In conclusion, the selected AFLPs thanks to their high polymorphism detection capacity and genome coverage resulted to be the most appropriate markers for studying the origin of azalea hybrids. On the other hand, STMS and EST markers appeared to be the most appropriate markers for paternity assessment of narrow genetic relationships. Then, in order to characterized and classified an important rhododendron collection, 4 STMSs, developed in R. catawbiense were applied to 33 accessions collected in the Burcina Park (Northern Italy). Fingerprinting revealed a total of 79 alleles with an average of 19.75 alleles per locus. PCo analysis was firstly performed only on 17 morphological traits but could not differentiate hybrids. Indeed, no significant correlation between molecular and morphological data was detected. Probably caused by the STMSs, considered as neutral markers, while morphological traits are subject to environmental factors. However, combining STMS and morphological data sets allowed to obtain a more comprehensive representation of the genetic relationships among plants. In particular, some hybrids with unknown parents were grouped closed to R. catawbiense such as ‘Memoire de Dominique Vervaene’. Thus STMss appeared to give useful information for horticultural classification.