Cytology of the Ferox-Quercifolia-Stramonium Triangle in Datura

Abstract

An inbred tester race of each species was used in making these inter se crosses; results obtained apply to these tester races only. Instead of having the 24 chromosomes arranged as 12 bivalents, the stramonium-quercifolia hybrid showed 3 circles of 4 chromosomes each plus 6 bivalents; the stramonium- ferox hybrid, two circles of 4 each plus a circle of 4 with an attached bivalent plus 5 bivalents; the ferox-querci- folia hybrid, a "necktie" of 4 chromosomes plus 10 bivalents. The "necktie" is a limiting case of segmental interchange, namely, an exchange of only the humps which are located at the ends of certain chromosomes. As far as the ends of the chromosomes are concerned, D. ferox and D. quercifolia are closely related. Both show two types of segmental interchange previously found in races of stramonium from Peru. The third type of interchange is partly identical in ferox and quercijolia. In back-crosses of the stramonium-ferox hybrid it has given rise to two new configurations, explainable on the assumption of crossing over between homologous regions of a stramonium chromosome and that jerox chromosome which in terms of the stramonium tester is composed of parts of 3 chromosomes. Since configurations interpreted as due to segmental interchange have been found thus far in 8 inter se species crosses of Datura, such interchange seems to be of widespread occurrence.