Plant 294

Oenothera species (Onagraceae)

Evening primrose

The American genus Oenothera contains about 140 herbaceous species. Those species that are classified into section Oenothera have special places in the history of genetics and evolutionary biology.

In the 1880s, the Dutch botanist Hugo de Vries, one of the scientists who draw attention to Gregor Mendel's fundamental genetic results, began research on naturalised, weedy populations of North American Oenothera lamarckiana growing in the Netherlands. He wanted to know why individuals, which were usually uniform and bred true, occasionally produced individuals with dramatically different appearances to the other plants in the population. His interest in the origin of species led him to call the new sorts species.

By 1901, de Vries thought he had an answer; his Mutation Theory of Evolution. The new sorts had arisen by large mutations (a word he introduced into the biological lexicon). Moreover, de Vries argued that all species arose episodically, through distinct jumps that affected many characters together, rather than through the gradual accumulation of changes argued by Charles Darwin.

Oenothera became a favourite model plant for European and North American researchers as the new science of genetics established itself in the first half of the twentieth century. In the 1920s, the German physiologist Otto Renner observed that species in section Oenothera, despite selfing, were heterozygous, and that combinations of characters were inherited as unchanging units. The American botanist Ralph Cleland showed that inheritance of the units was due to the unusual way in which Oenothera chromosomes behave during sex-cell formation. Groups of chromosomes (called Renner complexes) remain joined together. Different species in section Oenothera have different sorts of Renner complex.

Here was a mechanistic explanation of de Vries' observations. Section Oenothera species, usually isolated from each other in North America, came into contact with each other in Europe, crossed and produced new combinations of Renner complexes. The new combinations dramatically altered the plant's appearance. Each new combination, which breeds true, is essentially clonal, and is considered a new species; in Europe over 85 such species are now recognised.

The genus has also shaped our notion of how chloroplasts are inherited. However, unlike most flowering plants, the chloroplasts of an Oenothera seedling come from both its male and female parents, rather than just its female parent.

Oenothera species, with their large, showy, insect-pollinated flowers, widely grown in European gardens since the early seventeenth century, are today naturalised in many parts of Europe and Asia.

Further reading

Cleland RE 1972. Oenothera: cytogenetics and evolution. Academic Press.

Holsinger KE and Ellstrand NC 1984. The evolution and ecology of permanent translocation heterozygotes. The American Naturalist 124: 48-71.

Murphy RJ 2016. Evening-primroses (Oenothera) of Britain and Ireland. Botanical Society of the British Isles.

Stephen Harris