Chromosome banding patterns in Lettuce species (Lactuca sect. Lactuca, Compositae)

Chromosome banding patterns obtained with C- and N- banding, and AgNO3 staining were studied in somatic metaphase complements of four Lactuca species. L. sativa and L. serriola have almost identical chromosome morphology, and L. saligna differs only slightly from them, but L. virosa is quite distinct from the other species. A gross comparison of the banded karyotypes suggests a closer relationship of L. saligna to L. sativa/serriola than to L. virosa. Our data agree with the results of previous crossing experiments in these species but conflict partly with recent RFLP data which indicate a closer phenetic relationship of L. saligna to L. virosa than to L. sativa/serriola. Such a discrepancy may be explained assuming that domestication of L. sativa/serriola resulted in an increased selection pressure on unique DNA sequences as demonstrated by the RFLP data. Differential evolution of specific heterochromatin classes (and presumably of highly repetitive DNA classes), as revealed by chromosome banding techniques was not linked to domestication. Thus the disparity in conclusions about relationship (in terms of genetic similarity) as based on the different experimental approaches reflects a non-parallel evolution of highly repetitive vs. unique DNA classes.