ALL ABOUT ORCHIDS - YOUR ORCHID RESOURCE

ORCHID

Age and beauty: kin to both irises and onions, orchids have a long history and a large repertoire of enticing tricks, Natural History, by Kenneth M. Cameron

Ask a glamorous older woman her age and the secret to her beauty, and you're likely to get a

A few are parasites; lacking chlorophyll, they extract the necessary nutrients from the organism on which they have made their home. One Australian genus spends its entire life underground. Orchids come in every color except black, and though few have any fragrance, the ones that do run the gamut from the scent of chocolate to that of carrion.

The astonishing diversity of these plants is matched only by the complexity and unconventionality, of their lifestyles. Orchids are so unlike other flowering plants, in fact, that they seem to live in a kind of splendid isolation from the great hierarchy of other organisms. Darwin wrote a book on them--On the various contrivances whereby British and foreign orchids are fertilized by insects, and on the good effects of intercrossing.

The hook served as a kind of sequel to his Origin of Species, and was intended to clarify certain points crucial to the theory of natural selection. But only quite recently--and only because of the advent of powerful molecular techniques such as genetic sequencing--have plant biologists been able to reconstruct the history of the family to which these alluring flowers belong.

Darwin argued that natural selection cannot take place unless organisms cross with other individuals. The reason he gave is that the survival of individuals best adapted to prevailing ecological conditions--often called "survival of the fittest"--depends on the existence of a broad spectrum of characteristics to meet whatever those conditions throw at the individuals of a species. Sexual reproduction, with its radical reshuffling of genes in each new generation, gives rise to that variety.

Most plants--particularly the angiosperms, or flowering plants--possess both male and female parts, and so they can, in principle, fertilize themselves. The fact that they do not--indeed, that they have evolved a wide range of strategies for preventing self-fertilization--seems to support Darwin's reasoning. In his book on orchids he documents the elaborate frills and furbelows, gimmicks and traps, that lure and exploit insect pollinators, thereby ensuring cross-fertilization. Darwin's classic volume thus also lays the foundation for the study of the coevolution of plants and animals: how changes in one alter the other, leading to the ongoing evolutionary adjustment of both.

The blossoms of the orchid plant are simplified in certain respects but quite complex in others. Consider the architecture of the stamen, the flower's male component, and the pistil, its female component. Orchids belong to the class Liliopsida (informally called monocots), along with grasses and lilies, which both produce stamens in multiplies of three. But orchid flowers typically bear just one fertile stamen. Furthermore, that stamen is fused with the pistil, forming a bisexual structure called the column [see illustration on bottom of opposite page.
Pollen is produced within the anther at the apex of the column. Typically, the pollen grains adhere to one another, forming one or two small masses attached to a sticky pad--a complex structure called the pollinarium. Atop the pollinarium is the anther cap, a kind of hood that prevents self-pollination and is easily dislodged by an insect's body or a hummingbird's bill. Any visitor that comes in contact with the pollinarium's sticky pad ends up conveying the entire structure, pollen and all, to its next stopover--which may or may not be another orchid of the same species.

Because the pollinarium attaches to any visitor that dislodges the anther cap, the anther is empty when the insect or bird flies away. In other words, the orchid has a one-shot chance of effectively attaching the pollinarium to a visiting pollinator, and thence to another flower. Increasing the odds of success is the flower's labellum, or lip--usually the largest, most colorful, most elaborate petal--which serves as a landing platform for insects, and positions the apex of the column immediately above the potential pollinator's body. Instead of relying primarily on fragrance or nectar to attract and reward pollinators, orchids generally use color, shape, mimicry, and overall floral morphology to lure (though usually not to reward) them. All this reducing, restructuring, and fusing of the male and female floral organs, coupled with a lack of reward for the pollinators and a single chance of success, may seem a risky reproductive strategy--but evidently it works. Orchids, after all, are one of the most successful families of plants.

Once pollinated, the ovary of an orchid develops into a capsule filled with tens of thousands of microscopic seeds. Within each seed is an amorphous embryo made up of just a few cells; unlike the embryos of most seed plants, the orchid embryo is not provisioned with a food source. Furthermore, the orchid progeny are protected from the elements by nothing more than a paper-thin seed coat, leaving them vulnerable to damage and desiccation, and to attack by microorganisms. But the design has the great advantage of being economical, enabling the seeds to travel great distances.

Actually, and counterintuitively the seed's exposure to microbial attack is no bad thing. To germinate at all, the seed must first be invaded by a fungus. Once the orchid embryo makes a cellular connection with a fungus, the immature seedling begins to siphon off essential nutrients from its fungal host. In other words, the orchid seedling becomes a parasite on the fungus. The orchid may carry on with this living arrangement until it develops leaves capable of photosynthesis, making it able to manufacture food on its own. Alternatively, the orchid may continue to feed off its host for the rest of its life, without ever producing green chlorophyll. This strategy is called myco-heterotrophism, and orchids are its most common practitioners.
Clearly, orchids are an exceptional family of plants. Classifying them within the standard system of taxonomy, and thereby indicating their closest relatives, has been a matter of considerable controversy among botanists. Some have focused on the orchid seed as the basis for classification, placing Orchidaceae alongside other mycoheterotrophs. Others have focused on the flower, and considered the family closely related to the lilies. Still others have placed them in their own unique order, Orchidales, which just sweeps the controversy under the rug.
Recently, however, cutting-edge techniques of molecular biology have offered an entirely new basis for classification. The most important of those techniques is DNA sequencing--the process of determining the exact order of the nucleic acids that collectively constitute the genes comprising the genome of the plant. Biologists have already sequenced the complete genomes of more than a dozen multicellular organisms, including mice, rice, and human beings. But the sequence of even a small portion of an organism's genome can help reveal fundamental aspects of its natural history--the path the organism took to get to where it is today.

The genetic approach, then, is amplifying the practice of classification, rooting it in history. Systematics--the broad investigation into the discovery, naming, and cataloging of biodiversity--has become not just a collector's passion, but a fundamental pursuit in biology'. For the plant systematist, DNA sequences yield crucial information about the ancestors and closest living relatives of orchids, when and where orchids evolved, and why they came to have such a complex lifestyle.

Genetic analysis shows that Orchidaceae is a member of the order Asparagales, to which the agave, asparagus, hyacinth, iris, and onion families also belong. The orchid family, moreover, was the first of those groups to branch off on its own. But because orchids have left almost nothing in the fossil record, determining a date for their origin has not been straightforward.
Some biologists, therefore, have turned to an investigative tool known as a "molecular clock," whose ticking is based on the assumption that DNA mutates at a fairly constant rate. The clock is usually calibrated by comparing its (admittedly speculative) readings with the independently agreed upon dates of some widely recognized fossils from various plant or animal families. Molecular clocks are not without their critics, but one such clock has enabled botanists to calculate that Orchidaceae may have branched of prior to 100 million years ago, around the end of the Early Cretaceous epoch--much earlier than traditionally thought.

One reason that age estimate seems surprisingly ancient to many botanists is that most major orchid groups occur in either the Old World or the New World, but rarely in both. A plant group native to the continents in one of those regions, but absent from those in the other, might be expected to have established itself more recently than 100 million years ago--that is, after South America and Africa (which were once part of the supercontinent Gondwana) had fully separated, thereby preventing further exchanges of organisms between the two landmasses.

Two other reasons for thinking orchids are of relatively recent origin are that most are epiphytes (and thus presumably arose after the emergence of forests full of flowering plants), and most sustain complex relationships with certain insect groups (bees, for instance, didn't become pollinators until the advent of flowering plants). And sure enough, most horticulturally popular orchids evolved not so very long ago. Yet DNA data from several small groups--particularly the one to which the genus Vanilla (source of the much-loved flavoring) belongs--support the idea that orchids in general are of ancient origin.

Vanilloid orchids, which encompass some fifteen genera that form the subfamily Vanilloideae, have always posed an enigma to orchidologists. They in corporate certain advanced features--some are climbing vines, some have winged seeds, most have highly elaborate flowers--as well as features that usually occur in more primitive orchids: they are terrestrial, their pollen grains are not lumped together on a pollinarium, and the fusion of their stamens and pistil is less complete than in most other orchids. DNA sequencing, in fact, shows that Vanilla and its close relatives diverged from other orchid lineages early on.

Furthermore, vanilloid genera today are distributed across the tropical belt of the Southern Hemisphere: Africa, eastern Australia, the Pacific island of New Caledonia, South America, and Southeast Asia (especially Papua New Guinea, but also Indonesia and Malaysia), all of which were once part of Gondwana. Although significant rifting began in Gondwana about 165 million years ago, it was not until about 100 million years ago that Africa and South America became distinct continents; Antarctica, Australia, and New Caledonia, however, re-roamed in contact until as recently as 85 to 90 million years ago. If the orchid family evolved on Gondwana prior to 100 million years ago, the ancestors of the vanilloid orchids would have had plenty of time to spread across the supercontinent before it broke apart, and their family tree should reflect that historical pattern of continental breakup. Indeed, that's precisely what the DNA data show.

So orchids have been around for a long time. But the same holds true for many other families of flowering plants. What, then, has enabled orchids to become so diverse? Extreme specialization in tandem with specific insect pollinators--an elaborate ballet of coevolution--is usually cited as the primary driving force. And that is almost certainly an important factor. But the DNA data suggest an alternative, though complementary, explanation.

Biologists often specify the evolutionary relationships among organisms via a treelike diagram called a cladogram. In essence, a cladogram is a map of history as well as kinship [see bottom illustration on page 28]. Genetic change takes place through time, and on the orchid's genetic tree, one of the most recent (sometimes represented as one of the shortest) branches includes more than 85 percent of all orchid species.

That kind of pattern is generally a sign that a single momentous event or a decisive biological innovation has taken place--say, a drought that led to desertification, or a petal transformed into a vessel for nectar. Such changes often lead to increased evolutionary activity and speciation: in effect, an evolutionary big bang.

The plant systematist seeking to explain such a pattern would logically look for the distinguishing features of the plethora of orchids populating that single branch of the cladogram. Could the branching mark a shift from one kind of pollinator to another? Does it signal some innovation in the structure of the orchid's flower, fruit, leaf, pollen, seed, or stem? All are logical possibilities.
But the DNA evidence--from five of the orchids' chloroplast genes--actually points elsewhere. It turns out that the branching records a divergence between species that dwell almost exclusively on the ground (terrestrials) and species that dwell almost exclusively in trees (epiphytes). Obviously that's a major shift, and, not surprisingly, it was accompanied by changes in orchid physiology. Stems became specialized for water storage. Roots developed to absorb water from the atmosphere, hold it like a sponge, and resist desiccation. Leaves learned to perform photosynthesis in sunny, windy, drying conditions. And so quite possibly this change in both habit and habitat--even more than coevolution with pollinators--drove the evolution of the biological innovations and the new orchid lineages.

One mustn't rush to conclusions, though. A transition from a terrestrial to an epiphytic lifestyle might have led to an explosion of diversity in orchid species, but that doesn't imply the shift is always strictly a one-way trip, or that pollinators haven't played a major role in the evolution of orchids. Quite the contrary. Several otherwise epiphytic orchid groups appear to have descended from the trees back to the ground ("back" because orchids originally started out on the ground). Once again, DNA evidence has helped resolve the question.

Consider the evolutionarily advanced group of orchids known as Malaxideae, which traditionally includes at least three genera: Oberonia, Malaxis, and Liparis. All Oberonia species are epiphytes, whereas all Malaxis species are terrestrials. Liparis, however, includes nearly equal numbers of epiphytes and terrestrials. The traditional classification of malaxids is based on the assumption that epiphytism was its ancestral condition, and that members of two genera independently adopted a terrestrial lifestyle. But now DNA sequences from both the nucleus and the chloroplasts of more than fifty malaxid species show that all the epiphytic species are derived from a single common ancestor, and all the terrestrial species are derived from another. In other words only one evolutionary event brought these orchids down from the trees again.

Such new hypotheses about the relations among species challenge the traditional basis for classification: the architecture of the flowers. For centuries, botanical taxonomists have focused on reproductive structures, such as flower parts, fruits, pollen, and seeds. Their underlying assumption was that the visible forms of vegetative structures, such as leaves, roots, and stems, are subject to considerable change because of the plant's need to adapt to particular environments, and thus those forms are unreliable indicators of kinship. That process, called "convergent evolution," is exemplified in the remarkable similarities among the thorny, leafless stems of various unrelated desert plant families, such as cacti, milkweeds, and spurges.

Yet the DNA data seem to indicate that orchids--whose flowers readily change color, form, shape, and size as a result of the selective pressure of specific pollinators--disobey that "rule." Flowers can be misleading. The mode of growth--whether terrestrial or epiphytic--and the structure of the leaves and stems turn out to be the better indicators of malaxids' (as well as some other orchids') evolutionary history.

Biologists generally maintain that hierarchical classification systems should be "natural"--that is, based on evolutionary relations rather than on some shared attribute such as flower color, leaf shape, or geography. To some extent, a plant's name and its placement in the hierarchy should enable one to infer part of its evolutionary history. And as hypotheses about evolutionary relationships change, so must the names.

Darwin and countless other biologists of the past made huge strides in understanding the natural history and evolution of orchids. Some of their hypotheses, however, were based on educated speculation and have quite recently been shown to be in error. New genomic data and high-speed computers have helped contemporary investigators propose more objective and