Carnivorous plant: Encyclopedia II - Carnivorous plant - Evolution

Carnivorous plant - Evolution

Elucidating the evolution of carnivorous plants is made difficult by the paucity of their fossil record. Very few fossils have been found, and all that do exist are either seed or pollen. However, much can be deduced from the structure of current traps. Pitfall traps are quite clearly derived from rolled leaves. The vascular tissues of Sarracenia show this quite clearly: the keel along the front of the trap contains a mixture of leftward and rightward facing vascular bundles, as would be predicted from the fusion of the edges of an adaxial (stem-facing) leaf surface. Flypapers also show a simple evolutionary gradient from sticky, non-carnivorous leaves, through passive flypapers to active forms. Molecular data show the Dionaea/Aldrovanda clade is closely related to Drosera, but the traps are sufficiently dissimilar to make the guess that snap-traps derived from very fast-moving flypapers which became less reliant on glue rather speculative.

There are over a quarter of a million species of flowering plants, but of these, only around five hundred are known to be carnivorous. True carnivory has probably evolved independently at least ten times; however, some of these 'independent' groups are probably descended from a recent common ancestor with a predisposition to carnivory. Some groups (the Ericales and Caryophyllales) seem particularly fertile ground for carnivorous preadaptation, although in the former case, this may be more to do with the ecology of the group than its morphology, as most of the members of this group grow in low-nutrient habitats such as heath and bog.

It has been suggested that all of the various trap types are modifications of a similar basic structure - the hairy leaf. Hairy (or more specifically, stalked-glandular) leaves have the ability to catch and retain drops of rainwater (especially if shield-shaped or peltate) in which bacteria can breed. Insects that land on the leaf can become mired by the surface tension of the water, and suffocate. The bacteria then begin the process of decay, releasing nutrients from the corpse, which the plant can absorb through its leaves. This foliar feeding can be observed in most non-carnivorous plants. Plants that were better at retaining insects or water therefore had a selective advantage, because they had access to more nutrients than less efficient plants. Rainwater can be retained by cupping the leaf, leading to pitfall traps. Alternatively, insects can be retained by making the leaf stickier by the production of mucilage, leading to flypaper traps.

The pitfall traps may have evolved simply by selection pressure for the production of more deeply cupped leaves, followed by 'zipping up' of the margins and subsequent loss of most of the hairs, except at the bottom, where they help retain prey.

The lobsterpot traps of Genlisea can be interpreted as pitchers formed from a Y-shaped leaf, that later specialised on ground dwelling prey; or as bladder traps whose prey-guiding protrusions form something more substantial than the net-like funnel found in most aquatic bladderworts. The twist is an adaptation that displays as much trapping surface as possible in all directions when buried in moss. In this case the hairs were also retained, but to a greater extent, since the trap was no longer held vertically, and could not rely on gravity to keep its prey in.

The traps of the bladderworts are more difficult to explain, but they may be derived from pitchers that specialised in aquatic prey when flooded, like Sarracenia psittacina does today. Escaping prey items in terrestrial pitchers have to climb or fly out of a trap, and both of these can be prevented by wax and tube narrowness. However, a flooded trap can be swum out of, so in Utricularia, a one way lid may have developed to form the door of a proto-bladder. Later, this may have become active by the evolution of a partial vacuum inside the bladder, tripped by prey brushing against trigger hairs on the door of the bladder.

Flypaper traps include the various true flypapers and the snap traps of Aldrovanda and Dionaea. The production of sticky mucilage is found in many non-carnivorous genera, so it is not difficult to see how the passive glue traps in Byblis and Drosophyllum evolved.

The active glue traps require a little more explanation. Rapid plant movement can be due to actual rapid growth, or it can be due to rapid changes in cell turgor, which allow cells to expand or contract by quickly altering their water content. Slow-moving flypapers like Pinguicula exploit growth, but the Venus flytrap uses more rapid turgor changes. In this plant, the movement is so rapid that glue has become unnecessary, and hence is no longer produced. The stalked glands that once made it (and are so evident in Drosera) have become the teeth and trigger hairs - an example of natural selection hijacking preexisting structures for new functions.

Recent taxonomic analysis of the relationships within the Caryophyllales indicate that the Droseraceae, Triphyophyllum, Nepenthaceae and Drosophyllum, whilst closely related, are embedded within a larger clade that includes non-carnivorous groups such as the tamarisks, Ancistrocladaceae, Polygonaceae and Plumbaginaceae. Interestingly, the tamarisks possess specialised salt-excreting glands on their leaves, as do several of the Plumbaginaceae (such as the sea lavender, Limonium), which may have been co-opted for the excretion of other chemical, such as proteases and mucilage. Some of the Plumbaginaceae (e.g. Ceratostigma) also have stalked, vascularised glands that secrete mucilage on their calyces and aid in seed dispersal and possibly in protecting the flowers from crawling parasitic insects. It is not unlikely that these are homologous with the tentacles of the carnivorous genera. It is possible that carnivory evolved from a protective function, rather than a nutritional one. The balsams (such as Impatiens), which are closely related to the Sarraceniaceae and Roridula similarly possess stalked glands.

The only traps that are unlikely to have descended from a hairy leaf/sepal of some sort are the carnivorous bromeliads (Brocchinia and Catopsis). These plants have just used the urn that is a fundamental part of the structure of a bromeliad for a new purpose, and built on it by the production of wax and the other paraphernalia of carnivory.

Adapted from the Wikipedia article "Evolution", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki