A variety of karyotypic differences were seen among six Lathyrus species. In part these differences are the result of different numbers of metacentric and acrocentric chromosomes in the complement but there were also clear differences in the number(s) and position(s) of secondary constrictions. Silver staining and in-situ hybridization were used to identify the nucleolar organizer regions (NORs). Silver staining also occurred at the centromere in most species and in addition at a number of non-centromeric sites in L. tingitanus. Some of these additional sites coincide with allocyclic regions revealed by the use of air-dried preparations. Unusual extended' centromeres were observed in these air-dried preparations of two of the species.

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2C nuclear DNA amounts for 24 species of Lathyrus (section Lathyrus) were determined using flow cytometry. A greater than two-fold variation was observed, ranging from 10.2 pg in L. basalticus to 24.2 pg in L. latfolius. In general, the perennial species had greater DNA amounts than the annuals. Significant intraspecific variation was observed in five species of Lathyms (from 10.1% in L. annuus to 28% in L. tingitanus). A positive correlation was observed between DNA values obtained by flow cytometry and those previously determined by microdensitometry. Finally, the distribution of DNA amounts in species within section Lathyrus appears to be continuous. 0 1997 The Linnean Society of London

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