A numerical analysis of karyotypes and DNA amounts in lettuce cultivars and species (<i>Lactuca</i>subsect.<i>Lactuca</i>, Compositae)

Abstract

Karyotype and relative DNA content were used to characterize Lactuca sativa, L. serriola, L. saligna and L. virosa and to determine their evolutionary relationships. In these species karyotype analyses requiring the identification of the homologues are unreliable, because not all chromosomes can be distinguished by their length and centromere position, and no useful additional cytological markers are available. Therefore the karyotypes were established using numerical parameters describing the whole metaphase complement rather than the individual chromosomes, namely: intra- and interchromosomal asymmetry index, total chromosome length and area, and number of discernible satellites. The karyotype data were supplemented with data on relative DNA content. No significant differences were found between L. sativa and L. serriola, whereas L. saligna differed significantly from L. sativa/serriola only in its relative DNA amount. L. virosa differed from L. saligna and L. sativa/serriola for all parameters. The largest differences were found between L. saligna and L. virosa, although both have asymmetric karyotypes compared to L. sativa/serriola. Since asymmetric karyotypes in Compositae tribe Cichorieae (including Lactuca) are considered to be derived it follows that L. saligna and L. virosa are advanced species that evolved in different directions.