Oenothera is a genus of about 145 species of herbaceous flowering plants native to the Americas. It is the type genus of the family Onagraceae. Common names include evening primrose, suncups, and sundrops. They are not closely related to the true primroses (genus Primula).
The species vary in size from small alpine plants 10 centimeters tall, such as O. acaulis from Chile, to vigorous lowland species growing to 3 meters, such as O. stubbei from Mexico. The leaves form a basal rosette at ground level and spiral up to the flowering stems. The blades are dentate or deeply lobed (pinnatifid). The flowers of many species open in the evening, hence the name "evening primrose". They may open in under a minute. Most species have yellow flowers, but some have white, purple, pink, or red. Most native desert species are white. Oenothera caespitosa, a species of western North America, produces white flowers that turn pink with age. One of the most distinctive features of the flower is the stigma, which has four branches in an X shape.
Oenothera flowers are pollinated by insects, such as moths and bees. Like many other members of the Onagraceae, however, the pollen grains are loosely held together by viscin threads, so only insects that are morphologically specialized to gather this pollen can effectively pollinate the flowers. Bees with typical scopa cannot hold it. Also, the flowers open at a time when most bee species are inactive, so the bees which visit Oenothera are generally vespertine temporal specialists: bees that forage in the evening. The seeds ripen from late summer to fall.
Oenothera are used as food plants by the larvae of some Lepidoptera species. The flower moths Schinia felicitata and S. florida both feed exclusively on the genus, and the former is limited to O. deltoides.
In the wild, evening primroses act as primary colonizers, quickly appearing in recently cleared areas. They germinate in disturbed soils, and can be found in habitat types such as dunes, roadsides, railway embankments, and waste areas. They are often casual and are eventually outcompeted by other species.
The genus Oenothera may have originated in Mexico and Central America, and spread farther north in North America and into South America. With the advent of international travel, species are now found in most temperate regions of the world. In Europe alone there are about 70 introduced species of Oenothera. During the Pleistocene era a succession of ice ages swept down across North America, with intervening warm periods. This occurred four times, and the genus experienced four separate waves of colonization, each hybridizing with the survivors of previous waves. This formed the present-day subsection Euoenothera. The group is genetically and morphologically diverse and the species are largely interfertile, so the species boundaries have been disputed amongst taxonomists.
This pattern of repeated colonizations resulted in a unique genetic conformation in the Euoenothera whereby the chromosomes at meiosis can form circles rather than pairs. This is the result of several reciprocal translocations between chromosomes such that the pairing occurs only at the tips. This phenomenon apparently has non-Mendelian genetic consequences; with this mode of chromosome segregation and a system of balanced-lethal genes, genetic recombination is prevented and the plants display the hybrid vigor of heterosis. This resulted in the evolution of many sympatric races in North America east of the Rocky Mountains. Analysis of the cytology of these races and of artificial hybrids between them increased understanding of the genetic and geographic evolution of the Euoenothera. This subject was a major area of genetic research during the first half of the 20th century.
Evening primroses were originally assigned to the genus Onagra, which gave the family Onagraceae its name. Onagra (meaning "(food of) onager") was first used in botany in 1587, and in English in Philip Miller's 1754 Gardeners Dictionary: Abridged. The modern name Oenothera was published by Carolus Linnaeus in his Systema Naturae. Its etymology is uncertain, but is believed to be derived from the Greek words ονος θηρας (onos theras), meaning "donkey catcher", or οινος θηρας (oinos theras), meaning "wine seeker". In addition, the Latin oenothera means "a plant whose juices may cause sleep".
In 1905, while studying the genetics of Oenothera lamarckiana, Hugo de Vries discovered a variant with a chromosome number of 2n = 28 compared with 2n = 24 for O. lamarckiana. DeVries was unable to breed this variant with O. lamarckiana. He named the variant Oenothera gigas.
Evening primrose oil is sometimes marketed as a cancer remedy, but according to the American Cancer Society there is very little evidence for its effectiveness, and "neither GLA nor other GLA-rich supplements (such as evening primrose oil) have been convincingly shown to be useful in preventing or treating any other health conditions." A Cochrane meta-analysis published in April, 2013, found evening primrose oil and borage oil to be ineffective for eczema.
A number of perennial members of the genus are commonly cultivated and used in landscaping in the southwestern United States. Popular species include tufted evening primrose (Oenothera caespitosa), Mexican evening primrose (Oenothera berlanderii), and Saltillo evening primrose (Oenothera stubbei).
The first plants to arrive in Europe reached Padua from Virginia in 1614 and were described by the English botanist John Goodyer in 1621. Some species are now also naturalized in parts of Europe and Asia, and can be grown as far north as 65°N in Finland. The UK National Council for the Conservation of Plants and Gardens, based at Wisley, maintains an Oenothera collection as part of its National Collections scheme.
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