WELCOME

POLLINATION IN ORCHIDS

SACHIN HEGDE

SUNILKUMAR M T

MANJUNATH B K

SUNIL NAIK

Introduction• Orchidaceae is a diverse and

widespread family of flowering plants with blooms that are often colourful and often fragrant, commonly known as the orchid family.

• Along with the Asteraceae, they are one of the two largest families of flowering plants, with between 21,950 and 26,049 currently accepted species, found in 880 genera.

Eulophia zeyheriana Wahlenbergia cuspidata

Economic importance

• Perfumery• Horticulture• Use as food• Traditional medicinal uses

Scientific classification• Kingdom: Plantae• (unranked): Angiosperms• (unranked): Monocots• Order: Asparagales• Family: Orchidaceae

Juss.• Type genus: Orchis

Tourn. ex L.

Subfamilies: Apostasioideae  

Cypripedioideae Epidendroideae Orchidoideae Vanilloideae 

Distribution

• The following list gives a rough overview of their distribution:

• Oceania: 50 to 70 genera• North America: 20 to 26 genera• tropical America: 212 to 250 genera• tropical Asia: 260 to 300 genera• tropical Africa: 230 to 270 genera• Europe and temperate Asia: 40 to 60

genera

Distribution of Orchidaceae

Flowers• Orchidaceae are well known for the many

structural variations in their flowers.• Some orchids have single flowers, but most

have a racemose inflorescence, sometimes with a large number of flowers.

• The flowering stem can be basal, that is, produced from the base of the tuber

• As an apomorphy of the clade, orchid flowers are primitively zygomorphic (bilaterally symmetrical), with exceptions: like Mormodes, Ludisia and Macodes

• The orchid flower, like most flowers of monocots, has two whorls of sterile elements.

• The outer whorl has three sepals and the inner whorl has three petals.

• The medial petal, called the labellum or lip , which is always modified and enlarged, is actually the upper medial petal.

• However, as the flower develops, the inferior ovary  or the pedicel usually rotates 180 degrees, so that the labellum arrives at the lower part of the flower, thus becoming suitable to form a platform for pollinators.

• This characteristic, called resupination, occurs primitively in the family and is considered apomorphic, a derived characteristic all Orchidaceae share.

• The torsion of the ovary is very evident from the longitudinal section shown. Some orchids have secondarily lost this resupination, e.g. Zygopetalum and Epidendrum secundum.

• Apostasia and the Cypripedioideae have two stamens, the central one being sterile and reduced to a staminode

• All of the other orchids, the clade called Monandria, retain only the central stamen, the others being reduced to staminodes .

• The filaments of the stamens are always adnate (fused) to the style to form cylindrical structure called the gynostemium or column .

• The stigma is very asymmetrical, as all of its lobes are bent towards the centre of the flower and lie on the bottom of the column.

Viola aethnensis,

Viola aethnensis,Dactylorhiza sambucina

Dactylorhiza sambucina

• Pollen is released as single grains, like in most other plants, in the Apostasioideae, Cypripedioideae and Vanilloideae. In the other subfamilies, that comprise the great majority of orchids, the anther (3), carries and two pollinia.

• At the upper edge of the stigma of single-anthered orchids, in front of the anther cap, there is the rostellum, a slender extension involved in the complex pollination mechanism.

• As mentioned, the ovary is always inferior (located behind the flower). It is three-carpelate and one or, more rarely, three-partitioned, with parietal placentation (axile in the Apostasioideae).

Anthesis

Flowers of the unrewarding orchid Disa nervosa (a) mimic flowers of the rewarding iris Watsonia

densiflora (b)(Van der Pijil, L. 1972)

Pollination• The complex mechanisms which orchids have

evolved to achieve cross-pollination were investigated by Charles Darwin.

•  Pollinators are often visually attracted by the shape and colours of the labellum. Some Bulbophyllum species attract male fruit flies (Bactrocera  spp.)

• The flowers may produce attractive odours. • Nectar may be produced in a spur of the

labellum (8 in the illustration above), or on the point of the sepals, or in the septa of the ovary, the most typical position amongst the Asparagales.

In orchids that produce pollinia, pollination happens as some variant of the following sequence: When the pollinator enters into the

flower, it touches a viscidium, which promptly sticks to its body, generally on the head or abdomen.

While leaving the flower, it pulls the pollinium out of the anther, as it is connected to the viscidium by the caudicle or stipe.

The caudicle then bends and the pollinium is moved forwards and downwards.

When the pollinator enters another flower of the same species, the pollinium has taken such position that it will stick to the stigma of the second flower, just below the rostellum, pollinating it. 

Deception mechanism in the Orchidaceae

Sl. No. mechanism Exploited insect behaviour

1 Generalised food deception

Food foraging

2 Batesian floral mimicry Food foraging

3 Brood-site imitation Oviposition

4 Shelter imitation Sleep/warmth

5 Pseudoantagonism Territoriality

6 Rendezvous attraction Sexual

7 Sexual deception Sexual

(Van der Cingel,1995)

BATESIAN FLOWER MIMICRY

Cochleanthus lipscombiaeEulalaema merianaClitoria javacensis (Fabaceae)

(Ackerman,1983)

Sexual deceptionPseudo-copulation

Ophrysis pollinated by several genera of solitary bees and wasps. It emits a pheromone that mimics the scent of a female pollinator. Males are highly attracted.

Their repeated attempts at copulation transport the pollinaria between orchids. Often these orchids bloom prior to the emergence of the real females.

Ophyrys insectifera (fly orchid)

which deceives male digger wasps

Mirror orchid (Ophrys speculum)

Which attracts “Scolid wasps”

Ophrys bombyliflora specifically attracts male solitary bees of the genus Eucera by chemical, visual, and tactile means, and does not

seemingly offer any other pollinator rewards (nectar/ pollen).

Source: http://www.botanicalgarden.ubc.ca/potd/2014/01/ophrys-bombyliflora.php

Orchid bees• The tribe Euglossini, in the

subfamily Apinae, commonly known as orchid bees or Euglossine bees.

• Most of the tribe's species are solitary, though a few are communal, or exhibit simple forms of eusociality.

•  The genera Exaerete and Aglae are cleptoparasites in the nests of other orchid bees.

• All except Eulaema are characterized by brilliant metallic coloration, primarily green, gold, and blue.

• Orchid bees range from 8 to 30 mm (0.3 to 1.2 in) long

• Females gather pollen and nectar as food from a variety of plants, and resins, mud and other materials for nest building.

Euglossine bees.

Scientific classification

• Kingdom: Animalia• Phylum: Arthropoda• Class: Insecta• Order: Hymenoptera• Suborder: Apocrita• Superfamily: Apoidea• Family: Apidae• Subfamily: Apinae• Tribe: Euglossini

Genera: Aglae Euglossa Eulaema Eufriesea ExaereteDiversity:200 species

Fragrance collection

• Male orchid bees have uniquely modified legs which are used to collect and store different volatile compounds (often esters) throughout their lives

• Several flowers from other plant families are also visited by the bees: Araceae, Gesneriaceae,Solanaceae, and Euphorbiaceae contain one or more species that attract male euglossines.

Modified legs

males lack corbiculae, they have characteristic enlarged hind tibiae. These unusual structures have a hole on the outer side, which provides

access to the spongy compartment within. These unusual structures play an important role in the

storage of fragrance.

• The chemicals are picked up using special brushes on the forelegs --- middle legs ---- hind legs, squeezing the chemicals past the waxy hairs which block the opening of the groove, and into a sponge-like cavity inside the hind tibia.

• The accumulated "fragrances" are evidently released by the males at their display sites, where matings are known to take place.

Fragrance collection

Male Behaviour

• Male orchid bees exhibit a peculiar behaviour of collecting fragrant volatile compounds from their environment.

• These compounds are meticulously collected, stored, and (presumably) presented to females by fanning their wings and “spray ventilating” their bouquet for the inspection of prospective mates.

• Fragrant compounds are collected by males with mop-like protrusions on their front tarsi.

• They are then transferred into the enlarged hind tibiae through the hole.

Nest structure• Plant resins are used by Euglossa as a

construction material, nest may be either concealed or aerial.

• In case of E. ignita, the bees actively make or enlarge cavities in masses of fern roots.

• Other nests are found in ground, termite nests or in artificial cavities.

• The nests are lined with resin and the entrance is reduced to a circular opening, which may be closed with resin at night.

Life cycle

Life cycle of bees in general

Single mating in orchid bees• Strictly neotropical orchid bees

(Euglossini), monandry (Strassmann, 2001)

• Kin selection theory-- natural selection resulting from altruistic behaviour by animals towards members of the same species, esp their offspring or other relatives (Queller and Strassmann, 1998; Boomsma, 2007).

• Males forage for volatile chemicals (fragrances) in a time-consuming and risky manner (Eltz et al., 1999).

Altruism= behaviour by an animal that may be to its disadvantage but that benefits others of its kind, as a warning 

cry that reveals the location of the caller to a predator.

• Flowers of orchids and other plants, as well as decaying wood or fruits, serve as natural sources of such fragrances, which consist mostly of terpenoids and aromatics.

• The male perfume is likely to function as a species specific chemical signal analogous to endogenous sex pheromones (Vogel, 1966; Zimmermann et al., 2006).

• In addition, the individual perfume of a male could represent a fitness indicator, giving an approaching female the possibility to evaluate a male’s quality and to choose her best mate.

Fragrance collection, storage, and

accumulation by individual maleorchid bees• Individually marked males of two species of

Euglossa were sighted repeatedly and over considerable periods of time (up to 44 days) at artificial fragrance baits exposed on Barro Colorado Island (BCI), Panama.

1. Do male Euglossini forage for fragrances over long periods of their lifetime?

2. Are individual males interested in collecting a variety of compounds as expected from observations at the species level (Ackerman, 1989)?

3.What quantities and qualities of fragrances are found in the hind tibiae of individual males captured at fragrance baits?

4. What happens to the fragrances once they have been transferred into the hind tibiae? Are males able to retain the volatiles over time, or are the fragrances lost, chemically modified or exposed during display?

5. Is there a relationship between the age of individual bees and the quantity of fragrances in their hind tibiae?

Mark-Recapture at Artificial Fragrance Baits.

• A total of eight compounds was used: 1,8-cineole (c), benzyl acetate (ba), methyl salicylate (ms), p-dimethoxybenzene (p-db), vanillin (v), p-cresyl acetate (p-ca), terpinene-4ol (t-ol), and t-methyl cinnamate (t-mc).

• These compounds were previously known to be good attractants for a range of euglossine bee species.

(Ackerman, 1989; Whitten, unpublished observations).

• Fragrances were simultaneously exposed on herbarium blotter pads in the radio tower clearing on BCI on 25 days (for 2-4 hr between 9:00 and 14:00 hr) between May 31 and July 23 in 1994 (every second day, except on rainy days)

• The fragrances can be stored very efficiently in the hind tibiae over long periods of time. This superior storage capability is probably achieved through the combined effects of the retaining properties of the non polar carrier lipids secreted from the labial glands (Whitten et al., 1989) and the strong capillary forces imposed by the sponge-like morphology of the invaginated cuticle inside the hind tibia that serves as a storage container

( Vogel, 1963, 1966; Cruz-Landim et al., 1965).• There is a positive relationship between the

individual fragrance quantity and the established age correlate wing wear in Euglossa cognata.

Results

Trichocentrum pumilum: an orchid

pollinated by oil-collecting bees• The reproductive biology, reward production

and pollination mechanism of Trichocentrum pumilum were studied .

• Trichocentrum pumilum blooms in spring. • Each paniculate inflorescence bears an

average of 85 flowers that present a central yellow callus and finger-like trichomes on the lateral lobes of the lip.

• A lipoidal substance is produced and stored among these trichomes. • In the studied population, T.

Pumilum is exclusively visited and pollinated by two bee species:

Tetrapedia diversipes and Lophopedia nigrispinis

Pollinaria are deposited on mouthparts of bees during collection of the lipoidal substance from the lateral lobes of the labellum.

Trichocentrum pumilum is self-incompatible and pollinator-limited.

Natural fruit set was low (9%, compared to 45% in experimentally cross-pollinated flowers). Potentially viable seed exceed 97% in fruits obtained through cross-pollination and in natural conditions (open pollination).

Trichocentrum pumilum. (A) Flower in frontal view. (B) Pollinarium

in dorsal and lateral views. (C) Tetrapedia diversipes visiting a flower. (D) A

detail of the head of T. diversipes showing a pollinarium attached on clypeus

region. Scale bars: A = 1 mm; B = 0.5 mm; C–D = 1 mm.