Evolution and Variation in <i>Trillium</i>

Abstract

The mechanisms of sterility were investigated with three interspecific hybrid species of Trillium found in Japan.There were three possible mechanisms concerned in sterility operative during microsporogenesis of these plants: 1) univalent formation, 2) non-dis-junction of dicentric and acentric chromatids produced in bivalents formed by a pair of corresponding chromosomes belonging to a pair of different genomes included in the hybrids, and 3) random assortment of chromosomes in dyads due to random disjunction of the bivalents formed by heterogenetic chromosome pair.The first mechanism is effective in hybrids with an odd number of different gametic sets. The latter are operative irrespective of polyploid of hybrids.It was assumed from the above that all possible interspecific hybrids of Japanese Trillium, both living and extinct, are sterile due to the above mentioned mechanisms.The origin of the dicentric chromatids, which are the cause of the second sterile mechanism, was discussed on the basis of comparative examinations of the types and frequencies of the dicentrics in all the fertile and sterile species of Japanese Trillium. The conclusion was reached that a majority of the dicentrics originate from inversion crossing-over at prophase. The simultaneous chromatid breakages at chiasmata in metaphase bivalents and the subsequent illegitimate reunion of the broken ends sometimes deform the configuration of chromatids at chiasmata in such a way as that described by Matsuura (1950). But the deformation seems to be facilitated only when the dicentrics produced by inversion crossing-over pre-exist.