Evolution of polyploidy in cultivatedNarcissus subgenusNarcissus

Abstract

With the availability of vegetatively-propagated ancestral cultivars ofNarcissus together with their modern descendants, the majority of which have known dates of introduction, the evolution of polyploidy in cultivation during the past hundred years is traced using previously published chromosome counts. From an original range of largely diploid species triploid cultivars arose initially, followed later by tetraploids. After an early gradual increase which was similar to those of diploids and triploids the rate of introduction of new tetraploid cultivars accelerated rapidly from the 1920's onwards, with the result that tetraploids now comprise nearly 75% of the cultivars of which the chromosome numbers are known. Higher polyploid cultivars are very rare (5x) or unknown (6x, 8x etc.). The horticulturally optimal tetraploid level inNarcissus differs from the optimal levels of ploidy inHyacinthus (3x–4x) andTulipa (2x) which have growth habits that are similar to those ofNarcissus and which are also subject to similar breeding and propagation methods, but the optimal chromosome numbers of all three genera are comparable (2n=24−31). As their chromosomes are of an equivalent order of size there is thus an optimal DNA amount affecting growth rate and horticultural value in these genera which must be approximately the same in each one, but which is reached at different levels of ploidy depending on differences in basic chromosome number. A loss of vigour and/or growth rate rate with a consequent lack of favourable artificial selection probably occurs if the optimal DNA amount is greatly exceeded.