Abstract
We benefited from the availability of a species-specific DNA marker to describe the morphometry of T. cordata × platyphyllos hybrids of an impressive age (ca. 150 years) grown in the Pazaislis baroque monastery yard in Central Lithuania. In an earlier study on a country-wide set of 543 T. cordata individuals from natural forest populations in Lithuania, we detected a nuclear microsatellite locus Tc8 well-differentiating between T. cordata and T. platyphyllos. The Tc8 locus contained a 140 bp allele in T. cordata (541 sampled individuals) and alleles above 160 bp in the two trees with a T. platyphyllos-like morphology (sampled in a national park). To verify the Tc8 locus as species specific, we sampled a further four T. platyphyllos-like individuals, which all contained the Tc8 locus alleles above 160 bp. We subsequently genotyped the six old-growth individuals from the Pazaislis monastery with mixed T. cordata × platyphyllos morphology. Results revealed that all six old-growth Tilia individuals from the Pazaislis monastery were heterozygous for the Tc8 locus with alleles of 140 bp (indicative of T. cordata) and 162 bp (indicative of T. platyphyllos). This finding confirms the morphological observations that these individuals are hybrids between T. cordata and T. platyphyllos. Additionally, the genotyping of a set of 14 nuclear microsatellite loci revealed that all six trees from the Pazaislis monastery are clones, possessing identical microsatellite genotypes. After the molecular identification, we morphotyped leaves, bracts, twigs, and nuts of the 6 old-growth T. cordata × platyphyllos hybrids from the Pazaislis monastery, 16 T. cordata old-growth trees, 4 T. × europaea var. europaea 'Pallida' trees growing near the Pazaislis monastery, and 4 mature T. platyphyllos trees from a nearby Girionys park. The morphotyping showed that T. cordata × platyphyllos hybrids may be the easiest to distinguish from T. cordata by raised and horizontally tertiary veins of leaves.