Non-Reduction Of Dna Content In Polyploids Of The Genus Agave
ML Robert1, L Hanson2, MD Bennett2 and IJ Leitch2
1 Centro de Investigacion cientifica de Yucatan, Ap. 87, Cordemex 97310, Merida, Yucatan, Mexico
2 Jodrell Laboratory, Royal Botanic Gardens, Kew, Richmond, Surrey TW9 3DS, UK
Although the agaves of Yucatan, Mexico, have been a source of hard fibre for the cordage industry, few have been genetically improved and some are now endangered by the destruction of their habitats. In order to tackle both problems it is necessary to have a more precise knowledge of their genetic characteristics.
C-values and ploidy levels reported for some species show discrepancies that need clarification. We used flow cytometry to measure the C-values of cultivated and wild ecotypes of henequen (Agave fourcroydes Lem.) and its close relatives (A. angustifolia Haw. and A. sisalana Perine), all used for hard fibre production. The well-known diploids, A. tequilana Weber and the hybrid 11648 [(A. amaniensis x A. angustifolia) x A. amaniensis], were also included for reference purposes.
Absolute C-values cannot be established, since they vary depending on the nature of the internal standards used, but we found that DNA content increased proportionally in a polyploid series irrespective of the standard used. All the plants had C-values in simple proportion to the diploid value, producing a series of 2x, 3x, 4x, 5x and 6x, showing that there is no significant loss, if any, of DNA in the polyploids.
No diploids were found, only one triploid and several tetraploids, pentaploids and hexaploids. This suggests a strong selection pressure for polyploidy and raises questions about their origin and the role of vegetative propagation in the stabilisation of the triploids and pentaploids. The discrepancies with the C-values previously reported and the difficulties posed by the classification of the agaves are also discussed.