Genecological investigations on zinc plants. V. Barriers to gene flow limiting the introgression of Viola arvensis (Murr.) into Viola calaminaria Lej.ssp. westfalica (Lej.) Ernst

Hybrids of V. arvensis (annual) and V. calaminaria (perennial) are regularly found on an open pit mine near Blankenrode (W. Germany). Morphological as well as karyological studies show that these hybrids represent different generations of selling and/or backcrossing to V. calaminaria. The hybrids show a low tolerance to zinc. To study the gene flow the reproductive strategy of the parent species, their Fi and F2 hybrids was compared in a normal soil. The progeny of natural hybrids with V. calaminaria was investigated under zinc stress. The results show that Fi and F2 hybrids have a higher fitness than V. calaminaria if both are grown in normal soil. This is caused by hybrid vigour and by a combination of the reproductive strategies of the parent species. When the progenies of hybrid plants are cultivated together with V. calaminaria in soil with zinc added to it, no difference in reproductive capacity can be shown. Prolonged observations in the field have shown that the longevity of established hybrid plants is only slightly less than that of non-hybrids. The observations are in accord with the assumption that a dynamic equilibrium exists between gene flow and selection. The consequences of this assumption for the genetic variation in V. calaminaria are discussed.