ROBERTSONIAN FUSION AND CENTRIC FISSION IN KARYOTYPE EVOLUTION OF HIGHER-PLANTS

Robertsonian fusion and centric fission are uniquely detectable in com parative studies of karyotype patterns. They are the most important ty pes of karyotype change in animals but seem to be relatively uncommon in higher plants. Both modify intra- and interchromosomal recombinatio n and linkage relationships and consequently patterns of genetic varia tion. When differentiating populations or species they can produce pos tmating barriers to gene flow. The number of reported cases of fusion or fission in higher plants has increased over the years but remains l ow, and most of these are casual comparisons of karyotypes without any follow-up investigation. This review focuses on more adequate studies made in a few groups. Studies in the Tradescantieae produce the stron gest evidence for fusion as a type of ortho-selection in the subfamily . Some species of Lycoris are also considered to have evolved their ka ryotypes in that way. Some genera of slipper orchids and the cycad gen us Zamia have populations where atypical chromosome number increase ca n be attributed to fission probably as a result of stressful influence s. It is suggested that fusion may have been involved in the evolution of many stable karyotypes and that fission is generally a secondary d estabilizing mechanism which may lead to refusion in the long term. Th eir proven incidence remains making it unwise to suggest that they hav e been major influences in karyotype evolution in higher plants.