Taraxacum spp. (Asteraceae)



Taraxacum species in Pylon Meadow, Nuneham Courtney Arboretum. Taraxacum species inflorescence with flowers filled with beetles.

The vivid yellow dandelion inflorescence, borne on a naked stalk, emerging from a rosette of elongate, jagged leaves is one of the most recognisable plants in the northern hemisphere. A damaged dandelion will ooze bitter, white, sticky latex, which, as the species epithet suggests, has found long-standing use in medicine. Dandelion, a corruption of dent de lion (lion's tooth), is a powerful diuretic, as reflected in some common names. In Britain, descriptive names, such as piss-a-bed and shit-a-bed, have long since been lost from general usage, although in continental Europe equivalent names are still widely used.

Dandelions have perplexed biologists for generations. The plant's general uniformity belies complex cytological and morphological variation, leading many people to treat Taraxacum officinale as a species complex. Others choose to describe the multifarious morphological forms, producing something of a cottage industry in dandelion 'species' naming. Cytologically, the Taraxacum officinale complex comprises plants with two (diploid) and, usually, three (triploid) complete chromosome sets. Triploids are expected not to produce viable seed. However, slice the top off an unopened, triploid dandelion bud, separating the stigma and anther from the ovary, and viable seeds will develop. The seed cannot have developed by cross- or self-fertilisation. The answer to the conundrum is the peculiar process of apomixis, a form of sexual reproduction where seed develops directly from female tissue.

In triploid dandelions, apomixis is obligate, that is seed formation does not involve pollination; the male is irrelevant. In the late 1970s, the evolutionary biologist John Maynard Smith asked why apomictic dandelions continued to produce petals and pollen if there was no need to attract pollinators and pollen had no function. One hypothesis developed at about the same time Maynard Smith was puzzling over the problem. The American ecologist Daniel Janzen argued apomictic dandelions were superorganisms. Janzen used the analogy of a tree, with the tree being equivalent to an apomict and the genetically identical leaves equivalent to individual dandelion plants (clones). The main difference between tree leaves and dandelion clones is that the former are physically connected. It is now known apomixis is controlled by the 'apomixis gene' which has led to the intriguing suggestion, supported by empirical evidence, that, although individual apomicts may readily go extinct, over evolutionary time the apomixis gene will prevail. Furthermore, pollen is necessary for apomixis gene transmission. Hence retention of showy dandelion flowers is important for the maintenance of the apomixis gene.

Further reading

Majeský L et al. 2012. The pattern of genetic variability in apomictic clones of Taraxacum officinale indicates the alternation of asexual and sexual histories of apomicts. PLoS ONE 7: e41868.

van Dijk PJ et al. 2009. An apomixis-gene's view on dandelions. In Schön I et al. (eds) Lost sex: The evolutionary biology of parthenogenesis. Springer, pp. 475-495.

Stephen Harris