The Mutual Similarity of Reciprocal Crosses

Abstract

In his pioneering hybridization experiments with two species of tobacco (Nicotiana paniculata and N. rustica) Koelreuter (1763, Fortsetzung, p. 24) concluded that it was a matter of indifference which of these species was taken as the mother and which as the father. In both cases, the same result was obtained; the F_l-generation of N. paniculata ♀ × N. rustica ♂ was not distinguishable from that of the reverse or reciprocal cross N. rustica ♀ × N. paniculata ♂. Although he found deviations from this rule of the similarity of reciprocal crosses in some other hybridization experiments, however, these were not sufficient to prove the rule invalid. His successors could only corroborate his findings, that in general no difference worth mentioning occurred between reciprocal crosses. Hence Focke, in his meritorious ‘Pflanzenmischlinge’ (1881, p. 479) comes to the conclusion ‘that in general, in true species of the vegetable kingdom, the shape-determining ability of the male and the female element are absolutely equal at fertilization’. Nevertheless this rule does not have so firm a foundation as the previous one. There are many exceptions to Focke’s rule which later turned out to have a fundamental significance (Pellew, 1929). We find important examples in which both of the reciprocal hybridization products are dissimilar. Quoting again from rich Oenothera-investigations we find (besides the strong linkage of genes in groups and dissimilarity with in F_l-generations) another striking phenomenon in many of the species of Oenothera investigated. This is the phenomenon of heterogamy (De Vries, 1911, p. 99). In 1903 for instance De Vries had already concluded (1903, p. 471) that the hybrids Oenothera biennis ♀ × O. muricata ♂ showed many more ‘muricata’ characteristics than the reciprocal cross O. muricata ♀ × O. biennis ♂ which tends to resemble the biennis-plants (figure 64).