An Analysis of Chiasma Pairs Showing Chromatid Interference in Trillium Erectum L.

NTIL comparatively recently there has been no compelling evidence U that genetic “interference” involves more than the occurrence of one crossover reducing the chance of another occurring in nearby regions. Genetic data from Drosophila have generally been interpreted as indicating that there is no “chromatid interference,” that is, that a crossover between two of the four chromatids does not influence which two are involved in the next. The occurrence of the first type of interference (now known as “chiasma interference”) can be tested in any organism having a sufficient number of marker genes. Chromatid interference can be detected genetically in organisms such as certain fungi and mosses in which the four products of the meiotic divisions remain associated, thus making it possible to determine the genetic constitution of the four strands involved in two adjacent chiasmata, or in such abnormal stocks as “attached-X” Drosophila (and the X” or “closed X” arising from it) in which a permanent association a t the right end of the two X chromosomes makes i t possible to recover two of the four strands of a bivalent in the female progeny. The occurrence of more than 50 percent recombination, which has been reported in Pisum and Viola, would in itself indicate chromatid interference but the evidence in these cases requires further cytogenetic examination. HEARNE and HUSKINS (1935) found definite cytological evidence of chromatid interference in chiasma formation in Melartoplus femur-rubrum. Recently, LINDEGREN and LINDEGREN (1937, 1939) have found genetic evidence of chromatid interference in crossing-over in Neurospora crassa. Trillium erectum has advantages over any other material yet studied for detailed cytological analysis of chromatid relationships and possible genetic interpretations, and a study of it was therefore initiated (HUSKINS et al. 1938) soon after chromatid interference was found in Melanoplus. Evidence is here presented showing that there are complexities in chiasma formation that have not previously been described or considered in analyses and theories of crossing-over.