The Extraordinary Catasetum Orchids

Male  Catasetum osculatum

Male Catasetum osculatum

Orchids, in general, have perfect flowers in that they contain both male and female organs. However, in a family this large, exceptions to the rules are always around the corner. Take, for instance, orchids in the genus Catasetum. With something like 166 described species, this genus is interesting in that individual plants produce either male or female flowers. What's more, the floral morphology of the individual sexes are so distinctly different from one another that some were originally described as distinct species. 

Female  Catasetum osculatum

Female Catasetum osculatum

In fact, it was Charles Darwin himself that first worked out that plants of the different sexes were indeed the same species. The genus Catasetum enthralled Darwin and he was able to procure many specimens from his friends for study. Resolving the distinct floral morphology wasn't his only contribution to our understanding of these orchids, he also described their unique pollination mechanism. The details of this process are so bizarre that Darwin was actually ridiculed by some scientists of the time. Yet again, Darwin was right. 

Catasetum longifolium

Catasetum longifolium

If having individual male and female plants wasn't strange enough for these orchids, the mechanism by which pollination is achieved is quite explosive... literally. 

Catasetum orchids are pollinated by large Euglossine bees. Attracted to the male flowers by their alluring scent, the bees land on the lip and begin to probe the flower. Above the lip sits two hair-like structures. When a bee contacts these hairs, a structure containing sacs of pollen called a pollinia is launched downwards towards the bee. A sticky pad at the base ensures that once it hits the bee, it sticks tight. 

Male Catasetum flower in action. Taken from BBC's Kingdom of Plants.

Male Catasetum flower in action. Taken from BBC's Kingdom of Plants.

Bees soon learn that the male flowers are rather unpleasant places to visit so they set off in search of a meal that doesn't pummel them. This is quite possibly why the flowers of the individual sexes look so different from one another. As the bees visit the female flowers, the pollen sacs on their back slip into a perfect groove and thus pollination is achieved. 

Eulaema polychroma  visiting  Catasetum integerrimum

Eulaema polychroma visiting Catasetum integerrimum

The uniqueness of this reproductive strategy has earned the Catasetum orchids a place in the spotlight among botanists and horticulturists alike. It begs the question, how is sex determined in these orchids? Is it genetic or are there certain environmental factors that push the plant in either direction? As it turns out, light availability may be one of the most important cues for sex determination in Catasetum

31111938873_b2006358fc_o.jpg

A paper published back in 1991 found that there were interesting patterns of sex ratios for at least one species of Catasetum. Female plants were found more often in younger forests whereas the ratios approached an even 1:1 in older forests. What the researchers found was that plants are more likely to produce female flowers under open canopies and male flowers under closed canopies. In this instance, younger forests are more open than older, more mature forests, which may explain the patterns they found in the wild. It is possible that, because seed production is such a costly endeavor for plants, individuals with access to more light are better suited for female status. 

Catasetum macrocarpum

Catasetum macrocarpum

Aside from their odd reproductive habits, the ecology of these plants is also quite fascinating. Found throughout the New World tropics, Catasetum orchids live as epiphytes on the limbs and trunks of trees. Living in the canopy like this can be stressful and these orchids have evolved accordingly. For starters, they are deciduous. Most of the habitats in which they occur experience a dry season. As the rains fade, the plants will drop their leaves, leaving behind a dense cluster of green pseudobulbs. These bulbous structures serve as energy and water stores that will fuel growth as soon as the rains return. 

Catasetum silvestre in situ

Catasetum silvestre in situ

The canopy can also be low in vital nutrients like nitrogen and phosphorus. As is true for all orchids, Catasetum rely on an intimate partnership with special mychorrizal fungi to supplement these ingredients. Such partnerships are vital for germination and growth. However, the fungi that they partner with feed on dead wood, which is low in nitrogen. This has led to yet another intricate and highly specialized relationship for at least some members of this orchid genus. 

36793851562_606bc44817_o.jpg

Mature Catasetum are often found growing right out of arboreal ant nests. Those that aren't will often house entire ant colonies inside their hollowed out pseudobulbs. This will sometimes even happen in a greenhouse setting, much to the chagrin of many orchid growers. The partnership with ants is twofold. In setting up shop within the orchid or around its roots, the ants provide the plant with a vital source of nitrogen in the form of feces and other waste products. At the same time, the ants will viciously attack anything that may threaten their nest. In doing so, they keep many potential herbivores at bay.  

Female  Catasetum planiceps

Female Catasetum planiceps

To look upon a flowering Catasetum is quite remarkable. They truly are marvels of evolution and living proof that there seems to be no end to what orchids have done in the name of survival. Luckily for most of us, one doesn't have to travel to the jungles and scale a tree just to see one of these orchids up close. Their success in the horticultural trade means that most botanical gardens house at least a species or two. If and when you do encounter a Catasetum, do yourself a favor and take time to admire it in all of its glory. You will be happy that you did. 

Photo Credits: [1] [2] [3] [4] [5] [6] [7] [8] [9] 

Further Reading: [1] [2] [3] [4] [5]

The Amazing Orchid Bees

Bees are some of the most common organisms recruited by flowers to achieve sexual reproduction. Even the least plant savvy among us can tell you that. Individual species of flowering plants go to great lengths to stand out among the background of countless other flowering plants to ensure that bees (and other pollinators) visit their own kind. Some plants get very specific in this way, only attracting a few or even a single pollinator species to do their bidding. Nowhere is this more apparent than in the orchids. There are some orchids that have taken this specificity to the next level. Orchids in the subtribes Stanhopeinae and Catasetinae have turned the tables so that bees require their flowers for their own reproduction, a truly unique strategy for both plants and animals alike.

Decked out in metallic greens, blues, and reds, male Euglossine or orchid bees are a site for sore eyes. However, their behavior may be even more amazing. Before mating, male bees seek out special volatile compounds that they store in special pouches on their back legs. Just as teenage boys utilize various colognes, the male bees are using these scents to attract females. These compounds are not produced by the bees. Instead, they obtain them from the flowers of various species of orchid. Some of the most commonly encountered orchids that offer this service are those belonging to genera such as Stanhopea, Gongora, and Catasetum.

The male bees go to great lengths to track these flowers down. A single blooming orchid can come alive with the buzz of male bees vying for access to the scented compounds. They land on the flowers and begin scraping at the petals with special hairs on their legs. In the process, pollen sacs called "pollinia" get stuck to the backs of the bees. The next time a bee visits a flower it will brush some of the pollen off and reproduction for the orchid is achieved.

Specific species of bee require specific scents to attract females. Because of this, each orchid species often caters to only one species of bee, thus guaranteeing that those bees will only visit orchids of that species. In this way, precious pollen is not wasted. What's more, the influence these orchids have goes far beyond just helping male bees get laid. Euglossine bees are important pollinators throughout the rain forests that they live. Species like the Brazil nut tree (Bertholletia excelsa) rely on large bees such as Euglossines for pollination and will not produce seeds without them.

Photo Credit: Billtacular (http://bit.ly/1LuTn2Q), USGS Bee Inventory and Monitoring Lab (http://bit.ly/1AFD4G0), Ian Morton (http://bit.ly/1924m1R), barloventomagico (http://bit.ly/1xeYPAM), Quimbaya (http://bit.ly/1GXLr3D)

Further Reading:
http://books.google.com/books…

http://link.springer.com/article/10.1023%2FA%3A1021932131526