A nectar spur is a hollow extension of a part of a flower. The spur may arise from various parts of the flower: the

sepal A sepal () is a part of the flower of angiosperms (flowering plants). Usually green, sepals typically function as protection for the flower in bud, and often as support for the petals when in bloom., p. 106 Etymology The term ''sepalum'' ...

petal Petals are modified leaves that form an inner whorl surrounding the reproductive parts of flowers. They are often brightly coloured or unusually shaped to attract pollinators. All of the petals of a flower are collectively known as the ''corol ...

s, or

hypanthium In angiosperms, a hypanthium or floral cup is a structure where basal portions of the Sepal, calyx, the petal, corolla, and the stamens form a cup-shaped tube. It is sometimes called a floral tube, a term that is also used for corolla tube and cal ...

, and often contain tissues that secrete nectar (nectaries). Nectar spurs are present in many clades across the

angiosperms Flowering plants are plants that bear flowers and fruits, and form the clade Angiospermae (). The term angiosperm is derived from the Greek words (; 'container, vessel') and (; 'seed'), meaning that the seeds are enclosed within a fruit. T ...

, and are often cited as an example of

convergent evolution Convergent evolution is the independent evolution of similar features in species of different periods or epochs in time. Convergent evolution creates analogous structures that have similar form or function but were not present in the last comm ...

Taxonomic significance

Spur length can be an important diagnostic character for taxonomy, useful in species identification. For example, Yadon's piperia can be distinguished from ''

Platanthera elegans The genus ''Platanthera'' belongs to the subfamily Orchidoideae of the family Orchidaceae, and comprises about 150 species of orchids. The members of this genus, known as the butterfly orchids or fringed orchids, were previously included in the ...

'', an extremely similar species in section '' Piperia'' (

Orchidaceae Orchids are plants that belong to the family (biology), family Orchidaceae (), a diverse and widespread group of flowering plants with blooms that are often colourful and fragrant. Orchids are cosmopolitan distribution, cosmopolitan plants that ...

), by the unusually short length of its spur.

Ecology and evolution

The presence of nectar spurs in a clade of plants is associated with evolutionary processes such as

coevolution In biology, coevolution occurs when two or more species reciprocally affect each other's evolution through the process of natural selection. The term sometimes is used for two traits in the same species affecting each other's evolution, as well a ...

(two-sided evolution) and

pollinator A pollinator is an animal that moves pollen from the male anther of a flower to the female carpel, stigma of a flower. This helps to bring about fertilization of the ovules in the flower by the male gametes from the pollen grains. Insects are ...

shifts (one-sided evolution). Like variations in floral tube length, variation in nectar spur length has been associated with variation in the lengths of organs on the primary pollinators of the plants, whether being the tongues of moths, the proboscis of flies, or the beaks of hummingbirds. This variation in floral shape can restrict access of pollinators to nectar, limiting the range of potential pollinators. In a famous historical story, Darwin predicted that the ''

Angraecum sesquipedale ''Angraecum sesquipedale'' , also known as Darwin's orchid, Christmas orchid, Star of Bethlehem orchid, and king of the angraecums, is an epiphytic orchid in the genus ''Angraecum'' endemic to Madagascar. The orchid was first discovered by the Fre ...

'', an orchid with an extremely long spur, must be pollinated by a pollinator with an equally long proboscis. The pollinator, the

sphinx moth The Sphingidae are a family of moths commonly called sphinx moths, also colloquially known as hawk moths, with many of their caterpillars known as hornworms. It includes about 1,450 species. It is best represented in the tropics, but species ar ...

''Xanthopan morganii'' ''praedicta'', was found and described 40 years after Darwin made his prediction. Nectar spurs have been cited as prime examples of “

key innovation In evolutionary biology, a key innovation, also known as an adaptive breakthrough or key adaptation, is a novel phenotypic trait that allows subsequent radiation and success of a taxonomic group. Typically they bring new abilities that allows the ta ...

s” that may promote diversification, and play a part in the

adaptive radiation In evolutionary biology, adaptive radiation is a process in which organisms diversify rapidly from an ancestral species into a multitude of new forms, particularly when a change in the environment makes new resources available, alters biotic int ...

of clades. Columbines (''Aquilegia'') have been studied in depth for the link between their floral nectar spurs and their rapid evolutionary radiations. However, there has also been some refutation to this idea recently, suggesting that the adaptive radiation of ''

Aquilegia ''Aquilegia'', commonly known as columbines, is a genus of perennial flowering plants in the family Ranunculaceae (buttercups). The genus includes between 80 and 400 taxa (described species and subspecies) with natural Species distribution, rang ...

'' may have been due more to climate and habit than to the varying lengths of the nectar spurs.

Underlying development and genetics

In terms of development, the varying lengths of nectar spurs has been found to be based solely on the

anisotropic Anisotropy () is the structural property of non-uniformity in different directions, as opposed to isotropy. An anisotropic object or pattern has properties that differ according to direction of measurement. For example, many materials exhibit ver ...

elongation of cells. However, it still remains to be understood which genes underlie the elongation of cells to form a spur. Are the same genes being co-opted over and over again across the angiosperms to form spurs, or are there several developmental pathways to make a spur. The genetic basis underlying the development of nectar spurs has been explored in several clades of plant families, such as ''

Linaria ''Linaria'' is a genus of almost 200 species of flowering plants, one of several related groups commonly called toadflax. They are annuals and herbaceous perennials, and the largest genus in the Antirrhineae tribe of the plantain family Plant ...

'' and ''Aquilegia''. Studies in model plant ''

Antirrhinum ''Antirrhinum'' is a genus of plants in the Plantaginaceae family, commonly known as dragon flowers or snapdragons because of the flowers' fancied resemblance to the face of a dragon that opens and closes its mouth when laterally squeezed. They ...

'' and ''Arabidopsis'' identified that type I KNOX SHOOTMERISTEMLESS (STM) genes play a role in the development of spur-like structures. These type I KNOX STM genes also play important roles in the development of the growing tip of the plant, the

shoot apical meristem In cell biology, the meristem is a structure composed of specialized tissue found in plants, consisting of stem cells, known as meristematic cells, which are undifferentiated cells capable of continuous cellular division. These meristematic ce ...

, by controlling cell division and prolonging indeterminate growth. Subsequent gene expression studies confirmed that

orthologue Sequence homology is the biological homology between DNA, RNA, or protein sequences, defined in terms of shared ancestry in the evolutionary history of life. Two segments of DNA can have shared ancestry because of three phenomena: either a speci ...

s of the type I KNOX genes are expressed in the petals of ''

'', a genus of plants with a spur arising from the ventral petal. However, the type I KNOX homologues were not differentially expressed during spur development on the petals of ''

'', while certain TCP genes instead were suggested to play a role. These results suggest that nectar spurs may represent a case of

on the genetic level, where the nectar spur has developed through different developmental pathways.

List of plants with nectar spurs

The following is an incomplete list of plant clades with nectar spurs. * Orchids: ''Satyrium'', ''Disa'', ''

Angraecum ''Angraecum'', also known as comet orchid, is a genus of the family Orchidaceae native to tropical and South Africa, as well as Sri Lanka. It contains 223 species. Etymology Despite the genus's distribution being largely confined to Africa and i ...

'', '' Aerangis'', '' Neofinetia'', Piperia * On petals: ''

'', ''

Delphinium ''Delphinium'' is a genus of about 300 species of annual and perennial flowering plants in the family (biology), family Ranunculaceae, native species, native throughout the Northern Hemisphere and also on the high mountains of tropical Africa. T ...

'',

Lentibulariaceae Lentibulariaceae is a family of carnivorous plants containing three genera: '' Genlisea'', the corkscrew plants; ''Pinguicula'', the butterworts; and ''Utricularia'', the bladderworts. The genera ''Polypompholyx'' (two species of pink petticoats ...

, ''Viola'',

Fumarioideae Fumarioideae is a subfamily of the family Papaveraceae (the poppy family). It was formerly treated as a separate family, the Fumariaceae (the fumitory, fumewort or bleeding-heart family). It consists of about 575 species of herbaceous plants in 20 ...

* On sepals: ''

Impatiens ''Impatiens'' is a genus of more than 1,000 species of flowering plants, widely distributed throughout the Northern Hemisphere and the tropics. Together with the genus ''Hydrocera'' (one species), ''Impatiens'' make up the family (biology), f ...

'' * From hypanthium: ''

Tropaeolum ''Tropaeolum'' , commonly known as nasturtium (; literally "nose-twister" or "nose-tweaker"), is a genus of roughly 80 species of annual and perennial herbaceous flowering plants. It was named by Carl Linnaeus in his book ''Species Plantarum'', ...

Notes

References

{{Reflist Plant morphology