Genetic relationships among Aster species by multivariate analysis and AFLP markers

摘要

The genus Asters.l. (fam. Asteraceae; tribe Astereae) includes more than three hundred species, gynomonoecious, morphologically heterogeneous and geographically widespread, with centers of diversity in North America and Eurasia. The capitulum consists of a whorl of female flowers around a cluster of bisexual flowers but the advantage of this sexual system was only speculated (Bertin and Kerwin 1998). The taxonomic history of the genus is controversial due to the reliance on traditional taxonomic characters. Efforts to clarify phylogenetic relationships in Aster s.l. were based on morphological (Nesom 1994b), cytotaxonomic (Semple 1995) and molecular approaches using chloroplast DNA (cpDNA) and the internal transcribed spacers (ITS) of rDNA (Xiang and Semple 1996; Noyes and Rieseberg 1999). Unfortunately, there is disagreement of evolutionary relationships within Aster between ITS and cpDNA restriction site data. Chromosome number is extremely variable in Aster ranging from 2n=10 to 2n=112 and ploidy level reaches 2n=12x (Semple et al. 1983). The ancestral chromosomal base-number for the tribe Astereae is controversial. Molecular evidence by cpDNA analysis supported the theory of x=9 with a hypothesis of aneuploid reduction for lower base chromosome number (Semple 1995). The evolutionary significance of ploidy increase has also applied implications in Aster since achene weight, post-germination phenomena, seedling survival and vigour are associated with ploidy level (Chmielewski 1991). The wide geographical distribution of the Aster species in Old and New World indicates a range of adaptation to various habitats including harsh conditions. For instance, sea aster (A. tripolium) is a halophyte, tolerating 2–6% NaCl, which was used as source of aquaporin-homologue genes (Uno et al. 1998).A number of Aster species and their interspecific hybrids, both natural and artificial, are grown as ornamentals for cut-flowers, gardens, and, to a lesser extent, for pot plants. Moreover, extensive surveys have revealed an extremely rich chemical diversity in the family whose evolutionary success was suggested to be due to the highly diversified chemical defence system (Cronquist 1981; Jansen et al. 1991). The chemical diversity can help for assessing phylogenetic relationships although, the reliability of this approach is limited by parallel evolution of the same classes of secondary compounds throughout Asteraceae (Seaman 1982). Particularly in Aster, secondary metabolites, such as triterpenoid saponins, have been identified to inhibit tumoral cell development (Shao et al. 1997). Therefore, asters can also be an important source of pharmaceuticals. In this frame, a screening for callus culture and in vitro regeneration capacity was performed in Aster wild species (Cammareri et al. 2001a).At CNR Institute, a collection of Aster germplasm was established from European seed banks and private seed companies. The genetic diversity evidenced in this material, whilst interesting for breeding programmes aimed to Aster utilization as ornamental, provides useful chemical diversity for pharmaceutical purposes (Corea et al. Unpubl.). In this work, a sample of the germplasm collection was characterized for morphological traits of commercial interest, chromosome number and genetic diversity by AFLPs. The sampling took into account geographical origin of the species, morphological diversity and range of environmental adaptation.