The Genus Collinsia. XV. A Cytogenetic Study of Three Fertile Interspecific Hybrids with an Interchange Complex of Six Chromosomes

Abstract

1. The frequency of plants with an interchange complex of six chromosomes was significantly greater than 50% in F2 and F3 populations from Collinsia corymbosa x C. multicolor while in F2 populations from C. corymbosa x C. tinctoria and from C. multicolor x C. heterophylla it was approximately 50%. The frequency of plants with the interchange complex in populations from crosses between (C. corymbosa x C. multicolor) and either a parental or a nonparental species depended on which species was used as seed-parent. 2. The interchange complex displayed a directed orientation at metaphase I in F2 and F3 plants from C. corymbosa x C. multicolor, in F2 plants from C. corymbosa x C. tinctoria, and in plants from crosses between the former interspecific hybrid and either a parental or a non-parental species. 3. Chiasma frequency at metaphase I was similar in plants with and without the interchange complex. 4. Plants of the F2 and F3 populations from the three interspecific hybrids and of populations from crosses between (C. corymbosa x C. multicolor) and a parental species were fertile, but plants from crosses between the interspecific hybrid and a non-parental species were sterile. 5. The reduced fertility or sterility of plants from crosses between the interspecific hybrid and either the parental or a non-parental species was attributed to "cryptic structural hybridity." 6. The absence of predicted interchange complexes in plants from crosses between (C. corymbosa x C. multicolor) and a non-parental species was explained on the basis of localized chiasmata and chromosome breakage in the chiasma-forming segments.