HYBRIDIZATION IN TRADESCANTIA. III. THE EVIDENCE FOR INTROGRESSIVE HYBRIDIZATION

Abstract

PREVIOUTS STUDIES of the American species of Tradescantia have shown that interspecific hybridization is comparatively frequent between the eighteen or more species closely related to Tradescantica virginian-a (Anderson and Sax, 1936; Anderson and Woodson, 1935). Hybridizations which were inferred from herbariunm and field work have been produced experimentally in the breeding plot (Anderson, 1936a). Detailed morphological analyses of hybrid populations have shown that the ultimate effects of interspecific hybridization are various (Anderson, 1936c). Apparently the commonest result is that through repeated back-crossing of the hybrids to the parental species there is an infiltration of the germplasm, of one species into that of another. If, for instance, two species, 'A' and 'B' come into effective contact, they usually do sounder conditions which greatly favor either 'A' or 'B.' If 'A' and 'B' differ in habitat preferences, seldom or never is there a habitat equally acceptable to both; they usually meet, if at all, in a situation quite favorable to one of the species but just fairly so to the other. Therefore, if hybrids are produced, they tend to cross back to the more abundant species. The progeny of thes6 secondary hybrids are likewise crossed back again, and so on. The final result will depend upon the balance between the deleterious effects of the foreign germplasm and its advantageQus effects in the areas where the hybridization has taken place or to which the hybrids may spread. Preliminary analyses of a number of genera of the flowering plants (Anderson, 1936b; Riley, 1936, 1937; Goodwin, 1937; Delisle, 1937) have shown that while such is not the only effect of hybridization between species, it is certainly one of the commonest. We have therefore given it a distinctive name, introgressive hybridization. In discussing the effects of introgressive hybridization, we shall speak of the hybridization of one species into another rather than hybridization with another. This terminology is chosen as a matter of convenience in discussing particular cases and avoids needless repetition of explanatory phrases. The' application of the terms hybrid and species becomes a difficult problem in dealing with successive back-crosses between an original first generation hybrid and one of the parental species. The F is clearly entitled to the term hybrid, but among the progeny of its first cross back to the parent there, will be a number of individuals which resemble that species verv closely indeed, and each successive backcross will increase the percentage of these indistinguishable or almost indistinguishable mongrels. After a few back-crosses most of the individuals cannot be