Development of molecular markers to assess the level of introgression of Populus tremula into P. alba natural populations

Abstract

Morphological traits traditionally adopted to discriminate between Populus alba L. and P. tremula L. have frequently led to misclassification of their spontaneous hybrid P. · canescens Sm. Moreover, they may not be of any help in cases of spontaneous backcross phenomena. These limitations can be overcome by molecular markers, which are not environmentally influenced nor subjectively assessed. In this study, the effectiveness of amplified fragment length polymorphism (AFLP) and simple sequence repeat (SSR) markers in species and hybrid discrimination was evaluated by analysing a set of reference samples of P. alba,P. tremula and P. · canescens. Species-specific and speciesindicative AFLPs, as well as diagnostic SSR alleles, were recorded in both P. alba and P. tremula reference samples. The results allowed a clear distinction between the two poplar species and their hybrid. Using these diagnostic markers, a natural population of P. alba trees sampled along the Ticino river basin in northern Italy was analysed, and P. · canescens individuals, intermingled with P. alba trees, were detected.

Admixture in European Populus hybrid zones makes feasible the mapping of loci that contribute to reproductive isolation and trait differences

“…alba and P. tremula share a wide sympatric or parapatric distribution across large parts of Central and Southern Europe and hybrid zones often form between them (Rajora and Dancik, 1992;Fossati et al, 2004;Lexer et al, 2005). A recent sequencing survey of nuclear genes in one of the two species (P. tremula;Ingvarsson, 2005) indicates that LD may often not extend beyond a single gene, suggesting that the detection of genetic associations in wild intra-specific populations may be difficult.…”

Admixture as the basis for genetic mapping

“…Natural admixed populations of these species contain a high proportion of recombinant, backcrossed genotypes and occur in multiple locations throughout Europe [4,43,45]. A first survey allowed the refinement of statistical approaches to detect locus-specific effects in genomic studies of hybrid zones [4].…”

Genomic Admixture Analysis in European Populus spp. Reveals Unexpected Patterns of Reproductive Isolation and Mating

“…These are defined by the three river drainage systems in which they are situated: Danube (Northeastern Austria), Ticino (Northwestern Italy), and Tisza (Northeastern Hungary) ( Table 1). The three hybrid or admixture zones were previously discovered with the help of genetic markers (Bartha 1991;Fossati et al 2004;Lexer et al 2005). The main focus of this article is on the Danube hybrid zone; the Ticino and Tisza populations were used as additional ''replicates'' to check whether genotypic patterns observed for the Danube could be generalized to other localities.…”

“…Statistical analyses: Descriptive statistics: Since European hybrid zones of P. alba and P. tremula were characterized for their genetic diversity and structure elsewhere (Fossati et al 2004;Lexer et al 2005), descriptive population genetic analyses were kept to a minimum here: populations were characterized for their gene diversity equivalent to expected (H E ) heterozygosity, observed (H O ) heterozygosity (for fully codominant markers), inbreeding coefficient (F IS ) and allelic richness corrected for sample size by rarefaction, and the following measures of genetic divergence: F ST , Hedrick's (2005) G9 ST , and the allele frequency differential (d). These calculations were carried out using the FSTAT software (Goudet 1995) or manually on the basis of the output of this program.…”

“…The two species are highly differentiated for neutral DNA-based markers (Lexer et al 2007) and numerous phenotypic and ecological traits (Lexer et al 2009). Mosaic hybrid zones between these species often form in riparian habitats (Lexer et al 2005; hybrids sometimes referred to as P. 3 canescens) and have been proposed as potential ''mapping populations'' for identifying QTL and genes of interest in evolutionary biology (Lexer et al 2007;Buerkle and Lexer 2008) and breeding (Fossati et al 2004;Lexer et al 2004). Previous studies of these hybrid zones were conducted with a relatively small number of genetic markers and without making use of linkage information; the genomic composition of hybrid zones between these species has never been studied with a genomewide panel of codominant markers with known linkage relationships.…”