{{Short description|Hollow extension of a part of a flower}} thumb|Nectar spurs on ''Aquilegia.'' [[File:Side view of tropaeolum majus.jpg|thumb|Side view of ''Tropaeolum majus'', a plant with a nectar spur arising from the hypanthium of the flower.]] A '''nectar spur''' is a hollow extension of a part of a flower. The spur may arise from various parts of the flower: the sepals, petals, or hypanthium, and often contain tissues that secrete nectar (nectaries).<ref>{{Cite journal|last1=Antoń|first1=Sebastian|last2=Kamińska|first2=Magdalena|date=2015-11-01|title=Comparative floral spur anatomy and nectar secretion in four representatives of Ranunculaceae|journal=Protoplasma|language=en|volume=252|issue=6|pages=1587–1601|doi=10.1007/s00709-015-0794-5|issn=0033-183X|pmc=4628095|pmid=25772682|bibcode=2015Prpls.252.1587A }}</ref><ref name=":2" /> Nectar spurs are present in many clades across the angiosperms, and are often cited as an example of convergent evolution.<ref name=":0" />
== Taxonomic significance == thumb|The long tongue of a sphinx moth is depicted as reaching into the equally long-spurred orchid. 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'', an extremely similar species in section ''Piperia'' (Orchidaceae), by the unusually short length of its spur.<ref>Morgan & Ackerman, Lindleyana 5:205–211 (1990)</ref>
== Ecology and evolution == The presence of nectar spurs in a clade of plants is associated with evolutionary processes such as coevolution (two-sided evolution) and pollinator shifts (one-sided evolution).<ref name=":1">{{Cite journal|volume=3|pages=32–39|doi=10.1007/s12052-009-0192-6|year=2010|last1=Johnson|first1=Steven D.|title=Coevolution Between Food-Rewarding Flowers and Their Pollinators|journal=Evolution: Education and Outreach|last2=Anderson|first2=Bruce|doi-access=free}}</ref> 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.<ref name=":1" /> This variation in floral shape can restrict access of pollinators to nectar, limiting the range of potential pollinators.<ref>{{Cite journal|last1=Whittall|first1=Justen B.|last2=Hodges|first2=Scott A.|title=Pollinator shifts drive increasingly long nectar spurs in columbine flowers|journal=Nature|volume=447|issue=7145|pages=706–709|doi=10.1038/nature05857|pmid=17554306|year=2007|bibcode=2007Natur.447..706W |s2cid=4412955 }}</ref>
In a famous historical story, Darwin predicted that the ''Angraecum sesquipedale'', an orchid with an extremely long spur, must be pollinated by a pollinator with an equally long proboscis.<ref>{{Cite book|title=On the Various Contrivances by which British and Foreign Orchids are Fertilized by Insects, facsimile edn.|last=Darwin|first=Charles R.|year=1862}}</ref> The pollinator, the sphinx moth ''Xanthopan morganii'' ''praedicta'', was found and described 40 years after Darwin made his prediction.
Nectar spurs have been cited as prime examples of "key innovations" that may promote diversification, and play a part in the adaptive radiation of clades.<ref name=":0" /> Columbines (''Aquilegia'') have been studied in depth for the link between their floral nectar spurs and their rapid evolutionary radiations.<ref name=":0">{{Cite journal|last=Hodges|first=Scott A.|year=1997|title=Floral Nectar Spurs and Diversification|jstor=2475168|journal=International Journal of Plant Sciences|volume=158. No. 6, Supplement: Morphology and Evolution of Flowers|issue=6|pages=S81–S88|doi=10.1086/297508|bibcode=1997IJPlS.158S..81H |s2cid=84429142 }}</ref> However, there has also been some refutation to this idea recently, suggesting that the adaptive radiation of ''Aquilegia'' may have been due more to climate and habit than to the varying lengths of the nectar spurs.<ref>{{Cite journal|last1=Bastida|first1=Jesús M.|last2=Alcántara|first2=Julio M.|last3=Rey|first3=Pedro J.|last4=Vargas|first4=Pablo|last5=Herrera|first5=Carlos M.|date=2009-12-04|title=Extended phylogeny of Aquilegia: the biogeographical and ecological patterns of two simultaneous but contrasting radiations|journal=Plant Systematics and Evolution|language=en|volume=284|issue=3–4|pages=171–185|doi=10.1007/s00606-009-0243-z|issn=0378-2697|hdl=10261/36746|s2cid=5926462 |hdl-access=free}}</ref><ref>{{Cite journal|last1=Donoghue|first1=Michael J.|last2=Sanderson|first2=Michael J.|date=2015-07-01|title=Confluence, synnovation, and depauperons in plant diversification|journal=New Phytologist|language=en|volume=207|issue=2|pages=260–274|doi=10.1111/nph.13367|pmid=25778694|issn=1469-8137|doi-access=free|bibcode=2015NewPh.207..260D }}</ref><ref>{{Cite journal|last1=Fior|first1=Simone|last2=Li|first2=Mingai|last3=Oxelman|first3=Bengt|last4=Viola|first4=Roberto|last5=Hodges|first5=Scott A.|last6=Ometto|first6=Lino|last7=Varotto|first7=Claudio|date=2013-04-01|title=Spatiotemporal reconstruction of the Aquilegia rapid radiation through next-generation sequencing of rapidly evolving cpDNA regions|journal=New Phytologist|language=en|volume=198|issue=2|pages=579–592|doi=10.1111/nph.12163|pmid=23379348|issn=1469-8137|doi-access=free|bibcode=2013NewPh.198..579F }}</ref>
== Underlying development and genetics == In terms of development, the varying lengths of nectar spurs has been found to be based solely on the anisotropic elongation of cells.<ref>{{Cite journal |last1=Puzey |first1=Joshua R. |last2=Gerbode |first2=Sharon J. |author-link2=Sharon Gerbode |last3=Hodges |first3=Scott A. |last4=Kramer |first4=Elena M. |last5=Mahadevan |first5=L. |date=2012-04-22 |title=Evolution of spur-length diversity in Aquilegia petals is achieved solely through cell-shape anisotropy |journal=Proceedings of the Royal Society of London B: Biological Sciences |language=en |volume=279 |issue=1733 |pages=1640–1645 |doi=10.1098/rspb.2011.1873 |issn=0962-8452 |pmc=3282339 |pmid=22090381}}</ref> 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'' and ''Aquilegia''.<ref>{{Cite journal|last1=Glover|first1=Beverley J.|last2=Airoldi|first2=Chiara A.|last3=Brockington|first3=Samuel F.|last4=Fernández-Mazuecos|first4=Mario|last5=Martínez-Pérez|first5=Cecilia|last6=Mellers|first6=Greg|last7=Moyroud|first7=Edwige|last8=Taylor|first8=Lin|date=2015-05-01|title=How Have Advances in Comparative Floral Development Influenced Our Understanding of Floral Evolution?|journal=International Journal of Plant Sciences|volume=176|issue=4|pages=307–323|doi=10.1086/681562|bibcode=2015IJPlS.176..307G |s2cid=59126705 |issn=1058-5893|url=https://www.repository.cam.ac.uk/handle/1810/248357}}</ref> Studies in model plant ''Antirrhinum'' and ''Arabidopsis'' identified that type I KNOX SHOOTMERISTEMLESS (STM) genes play a role in the development of spur-like structures.<ref>{{Cite journal|last1=Golz|first1=John F.|last2=Keck|first2=Emma J.|last3=Hudson|first3=Andrew|date=2002-04-02|title=Spontaneous mutations in KNOX genes give rise to a novel floral structure in Antirrhinum|journal=Current Biology|volume=12|issue=7|pages=515–522|issn=0960-9822|pmid=11937019|doi=10.1016/S0960-9822(02)00721-2|doi-access=free|bibcode=2002CBio...12..515G }}</ref> These type I KNOX STM genes also play important roles in the development of the growing tip of the plant, the shoot apical meristem, by controlling cell division and prolonging indeterminate growth.<ref>{{Cite journal|last=Barton|first=M. K.|date=2010-05-01|title=Twenty years on: The inner workings of the shoot apical meristem, a developmental dynamo|journal=Developmental Biology|series=Special Section: Morphogenesis|volume=341|issue=1|pages=95–113|doi=10.1016/j.ydbio.2009.11.029|pmid=19961843|doi-access=}}</ref> Subsequent gene expression studies confirmed that orthologues of the type I KNOX genes are expressed in the petals of ''Linaria'', a genus of plants with a spur arising from the ventral petal.<ref>{{Cite journal|last1=Box|first1=Mathew S.|last2=Dodsworth|first2=Steven|last3=Rudall|first3=Paula J.|last4=Bateman|first4=Richard M.|last5=Glover|first5=Beverley J.|date=2011-11-01|title=Characterization of Linaria KNOX genes suggests a role in petal-spur development|journal=The Plant Journal|language=en|volume=68|issue=4|pages=703–714|doi=10.1111/j.1365-313X.2011.04721.x|pmid=21790812|s2cid=25765402 |issn=1365-313X|doi-access=free}}</ref> However, the type I KNOX homologues were not differentially expressed during spur development on the petals of ''Aquilegia'', while certain TCP genes instead were suggested to play a role.<ref>{{Cite journal|last1=Yant|first1=Levi|last2=Collani|first2=Silvio|last3=Puzey|first3=Joshua|last4=Levy|first4=Clara|last5=Kramer|first5=Elena M.|date=2015-03-22|title=Molecular basis for three-dimensional elaboration of the Aquilegia petal spur|journal=Proceedings of the Royal Society of London B: Biological Sciences|language=en|volume=282|issue=1803|article-number=20142778|doi=10.1098/rspb.2014.2778|issn=0962-8452|pmc=4345449|pmid=25673682}}</ref> These results suggest that nectar spurs may represent a case of convergent evolution 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'', ''Aerangis'', ''Neofinetia'', Piperia * On petals: ''Aquilegia'', ''Delphinium'',{{efn|Delphinium has two spurs on the upper petals and one spur on the upper sepal. The sepaline spur is not a nectar spur because it has no nectar.<ref>{{Cite journal|last1=Jabbour|first1=Florian|last2=Renner|first2=Susanne S.|date=2012-11-01|title=Spurs in a spur: Perianth evolution in the Delphinieae (Ranunculaceae)|url=https://www.journals.uchicago.edu/doi/10.1086/667613|journal=International Journal of Plant Sciences|volume=173|issue=9|pages=1036–1054|doi=10.1086/667613|bibcode=2012IJPlS.173.1036J |s2cid=59128580 |issn=1058-5893|url-access=subscription}}</ref>}} Lentibulariaceae, ''Viola'', Fumarioideae * On sepals: ''Impatiens''<ref>{{Cite journal|last1=Travers|first1=Steven E|last2=Temeles|first2=Ethan J|last3=Pan|first3=Irvin|date=2003-02-01|title=The relationship between nectar spur curvature in jewelweed (Impatiens capensis) and pollen removal by hummingbird pollinators|journal=Canadian Journal of Botany|volume=81|issue=2|pages=164–170|doi=10.1139/b03-014|bibcode=2003CaJB...81..164T |issn=0008-4026}}</ref> * From hypanthium: ''Tropaeolum''<ref name=":2">{{Cite journal|last=Ronse Decraene|first=L|date=2001-11-01|title=Floral Developmental Evidence for the Systematic Relationships of ''Tropaeolum'' (Tropaeolaceae)|journal=Annals of Botany|language=en|volume=88|issue=5|pages=879–892|doi=10.1006/anbo.2001.1525|issn=0305-7364|doi-access=free|bibcode=2001AnBot..88..879R}}</ref>
== Notes == {{notelist}}
== References == {{Reflist}}
Category:Plant morphology