MEIOSIS IN AUTOPOLYPLOID CREPIS CAPILLARIS .2. AUTOTETRAPLOIDS

Meiotic chromosome pairing of autotetraploid Crepis capillaris was ana lysed by electron microscopy of surface-spread prophase I nuclei and c ompared with light microscopic observations of metaphase I chromosome configurations. Prophase I quadrivalent frequencies are high in all th ree tetrasomes. (A, D, and C) and partially dependent on chromosome si ze. At metaphase I quadrivalent frequencies are much lower and strongl y dependent on chromosome size. There is no evidence for multivalent e limination during prophase I in this system, and the reduction in mult ivalent frequency at metaphase I can be explained by an insufficiency of appropriately placed chiasmata. The high frequencies of prophase I quadrivalents far exceed the two-thirds expected on a simple model wit h two terminal independent pairing initiation sites per tetrasome, sug gesting that multiple pairing initiation occurs. Direct observations r eveal relatively high frequencies of pairing partner switches (PPSs) a t prophase I, which confirms this suggestion. The numbers of PPSs per tetrasome show a good fit to the Poisson distribution, and their posit ional distribution along chromosomes is random and nonlocalized. These observations favour a model of pairing initiation based on a large nu mber of evenly distributed autonomous pairing sites each with a unifor m and low probability of generating a PPS.