Estimation of the introgression level in Populus nigra L. populations by means of isozyme gene markers

Three enzyme systems (6-PGD, LAP, GPI) were tested in the progeny of a cont rolled crossing of Populus x canadensis x Populus nigra with the object of distinguishing hybrids in a population of P. nigra. Segregation ratios of 1 :1 for 6-PGD and LAP were determined among phenotypes corresponding to P, n igra (homozygote) and P. x canadensis (heterozygote). The segregation ratio for GPI was 3:1 in favour of the homozygote. No linkage was found between the polymorphic loci of 6-PGD and LAP. Experimental results proved that it is possible to identify hybrids of the P. x canadensis x P. nigra type from spontaneous populations of P. nigra. These results enabled analyses of thr ee populations of P. nigra. In the generative progeny of the female trees o f P. nigra no hybrids were ascertained. Hybrids morphologically different f rom P. nigra were identified in a population of two-years-old seedlings. In a population of trees 10 to 18 years of age 9.7% were hybrids with P. x ca nadensis yet riot different from P. nigra morphologically. The extent of in trogression in the progeny of female P. nigra is discussed.