Download Kunga Choden Bio -151 – Evolution Research Paper Evolution of

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Kunga Choden
Bio -151 – Evolution
Research Paper
Evolution of Carnivorous Plants
Other than sunlight and water, plants require many different chemicals for proper growth and
development. Over the course of their evolutionary history, different plants have evolved according to
different selective environments. Thus, we are able to find plants in both cold areas such as the tundra
or in hot places such as the desert; however, the form and function of these different plant groups
differ. Hence, it is reasonable to theorize that carnivorous plants evolved in environments that lacked
certain nutrients such as nitrogen and other essential metabolites. The extant carnivorous plants are
descendents of the first ancient carnivorous plants that had successfully adapted to digesting small
insects to fulfill their nutrient requirements in stressful environments. We know that carnivorous plants
trap insects and wait until the insects die and decay before they absorb the required nutrients. There
are many different enzymes involved in these digestive processes. A biologist at any level is aware that
the most valuable and limiting mineral element a plant requires is nitrogen. Thus, carnivorous plants
must derive nitrogen from their prey. There are four types of traps in seed bearing carnivorous plants in
the United States and Canada, which are closing traps, trap doors, pitfalls and flypaper, and they are
further divided into two main groups: the first two traps are active and the later two are passive form of
traps in carnivorous plants (Schnell, 1976). There are also many different variations of these types of
traps in carnivorous plants. Therefore, we are confident that carnivory in plants evolved more than
once.
Carnivorous plants are plants that catch or trap prey, absorb metabolites from their prey and
utilize these metabolites in their growth and development (Adamec, 1997). Darwin first studied
carnivorous plants in the summer of 1860 and was the first to publish a work covering carnivorous
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plants. In Insectivorous Plants, he points out that he was “surprised by finding how large a number of
insects were caught by the leaves of the common sun-dew (Drosera rotundifolia) on a heath in Sussex”
(Darwin, 1875). Darwin was also amazed by the number and size of insects (including small butterflies)
caught by these plants. He was impressed by the complexity of the glands and leaves that trapped the
insects and secreted various molecules to digest them. Darwin found out that the exterior tentacles
were excited when one touched the glands, yet the repeated drops of falling water had no effect. Thus,
he concluded that there must be selectivity and other mechanisms at play that controlled the inflexions
in these carnivorous plants. Moreover, Darwin conducted various experiments with temperature and
the introduction of different molecules such as salt and ammonia to see their effect and to understand
the biological machinery that governs the physiological function of these carnivorous plants. Darwin’s
pioneering work with carnivorous plants paved way for later experiments by other researchers on
carnivorous plants. Darwin knew through his theory of natural selection that carnivorous plants had a
selective advantage in their particular environments. There are many questions that surround the
nature of carnivorous plants including their relationship to other plants, their prey, their phylogeny, the
nutrients they absorb, the morphology of their structures and much more. This paper will study
carnivorous plants in an evolutionary context covering all of the major characteristics that make
carnivorous plants unique in the plant kingdom.
By the early 1940s, in Lloyd’s The Carnivorous Plants, the author had estimated there to be
around 450 or more species of carnivorous plants. This number had increased to 600 species in the late
1990s (Adamec, 1997). A question that arises is whether these carnivorous plants went through a
convergent or divergent evolution. Moreover, Lloyd discusses the existence of carnivorous fungi. Since
fossils of carnivorous plants are not well-preserved, there are no records of any ancient carnivorous
plants other than the extant ones.
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Phylogeny
Through sequence and morphological analysis of various different genes and structures, we
have found out that the ancestor of the first carnivorous plants were non-carnivorous angiosperms.
Albert et al. derived a phylogeny of carnivorous plants by using “the plastid gene rbcL [which] encodes
the large subunit of ribulose-1, 5-biphosphate carboxylase/oxygenase (RuBisCO), a prime enzyme in the
Calvin-Benson cycle” (Albert et al., 2009). Their phylogeny showed that carnivorous plants displayed
both homology and analogy in structural evolution. Therefore, one could see both convergent and
divergent evolution patterns in the derived phylogeny. It is also understood from the phylogeny that
carnivory is polyphyletic. For example, convergent evolution is observed between byblis and
Drosophyllum; this is similar to the polyphyletic relationships drawn from angiosperms regarding C4
photosynthesis. From the phylogenetic study we are also able to conclude that the special structure of
many of the carnivorous plants are homeotic and are likely to be products of different cell
differentiation rather than de novo structures.
The conclusion that convergent evolution exists in carnivorous plants is also supported by
Adams et al. 1977. The authors determine that three carnivorous pitcher plants, the Nepenthaceae,
Sarraceniaceae, and Cephalotaceae, all belong to three distinctly separate orders – it is an amazing find
since, “their basic morphology (both gross and microscopic)for performing the function of luring,
capturing, and digesting insects is quite similar” (Adams et al., 1977). All of the three families possess
some variation of the following structures: for luring they have nectar glands and toothed rim or
directive hair; for capturing prey they possess ablative wax, smooth surfaces, directive hair and
conductive funnel; and for digestion they have bacteria retentive hairs or digestive glands. It has been
demonstrated in many different studies that carnivorous plants that were subjected to similar selection
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are found in similar substrates (soil high in moisture and acidity and low in nitrogen and other nutrients)
(Adams et al. 1977). Therefore, the passive carnivory seen in these carnivorous plants from three
distinct orders are a result of similar selection.
Carnivorous plants that share similar morphologies with a common origin have also been
observed (Cameron et al.). In their study of snap-traps, the authors show that snap-traps in carnivorous
plants evolved only once. The phylogenetic data was based on the sequence analysis from nuclear 18S
and plastid rbcL, atpB, and matK genes. Their analysis concluded that Aldrovanda is sister to Dionaea,
and that both are snap-trapping carnivorous plants. The authors also report a similar morphological
similarity between the two groups such as the, “rapid transmission (6-17cm/s) of action potentials
between excitable cells” used in the snap-traps. From their phylogenetic tree they also conclude that
snap-traps of Androvanda and Dionaea had a common terrestrial ancestor that possessed flypapertraps. It is also shown that pitfalls of pitcher plant genus Nepenthes also evolved from a flypapertrapping ancestor. By looking at the non-carnivorous out-groups (such as Plumbago, Tamarix, Frankenia)
that also posses structures and functions similar to carnivorous plants including multicellular glands that
produce mucilage, salt or other compounds, they concluded that modification of these pre-existing
structures and co-option of trigger hairs could have produced the first carnivorous fly-traps. Moreover,
they report that in flooded areas, these carnivorous plants were able to capture prey and grow
underwater. Thus, one can imagine these plants living in a permanently aquatic environment. Another
study by Rivadavia et al. shows that close relationships can be derived between the multi-cellular hairs
of Drosera and Aldrovanda and Dionaea. Furthermore, the origin of these unique hairs and glands of
carnivorous plants are traced back to adhesive glands in Plumbaginaceae (Rivadavia et al., 2003). Their
results suggest that the flypaper morphology of Drosera and the snap trap system of Dionaea and
Aldrovanda were established early in the evolutionary history of these carnivorous plant taxa. The
phylogenetic tree obtained through sequence analysis of various genes is further refined by looking at
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the chromosome number and diversity that were caused by aneuploidization and polyploidization,
which might give rise to new species in the different carnivorous plant groups.
The Feeding Ecology
It has been shown that all carnivorous plants are able to digest both terrestrial and winged
insects and uptake nitrogen, phosphorus, sulfur, potassium, calcium and magnesium from their prey
(Adamec, 1997). In a study on subarctic carnivorous plant, lower rates of photosynthesis compared to
non-carnivorous plants have been reported (Mendez & Karlsson, 1999). A weak correlation between
nitrogen content and photosynthesis has been also demonstrated in the study. However, the authors
also report that an increased rate of photosynthesis is possible as a result of obtaining essential
nutrients through carnivory. They also show that the increased reproduction observed in carnivorous
plants is an indirect consequence of increased photosynthetic capability through carnivory. Over the
course of their evolution, carnivorous plants have developed varying dependence on prey for required
nutrients. In some cases, carnivorous plants can digest the carbon from the decaying body of their prey.
Hence, the possibility of seeing a heterotrophic carnivorous plant in the future is also possible.
Carnivorous plants capture prey of diverse orders including arachnids (Zamora, 1990). These
carnivorous plants are both generalists and specialists. Their diet consists of a main prey and various
other insects. By conducting experiments with mimic-traps, it has been shown that, “there does not
seem to be a taxonomical selection in the diet” (Zamora, 1990). Prey selection is viewed as being
dependent on size, which is directly related to the capturing mechanisms such as the stickiness of the
mucilage on the flypaper traps. Therefore, evolutionary speaking these carnivorous plants have selected
against smaller insects. In the future, the study of these carnivorous plants should pay special attention
to the size of the prey and the adhesive system of the respective plant.
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Carnivorous plants display highly adaptive features. One example is Nepenthes rafflesiana,
which utilizes two pitchers. The high pitcher captures aerial prey and the low pitcher specializes in
capturing ground prey. The differentiation of these two pitcher types might have some evolutionary
significance. The species has evolved so that visual and olfactory stimuli are present in the high pitchers
(low density of prey) and no such stimuli are detected in the lower pitchers (high density of prey and
other insects). Contrary to the forms of prey discrimination found in Drosera and Sarracenia regarding
UV patterns, these visual cues as mechanisms of prey attractions are not observed in Nepenthes (Joel et
al., 1985). The reason proposed by the authors for the absence of these stimuli in the lower pitcher is
that excessive holding of many prey and other insects can actually result in the death of the pitcher.
They also suggest that carnivorous plants might also display mutualistic behavior since they don’t
consume all prey that passes across the leaves of the plants.
Environmental effects
The effect of the environment on the success of carnivorous plants is significant, as mentioned
earlier in the paper. The factors that contribute to the success of these plants concern the effect of
certain parameters such as temperature, nitrogen, soil humidity and light. The availability of prey, which
is most prevalent in the shadiest regions, also has a significant impact on the success of these plants
(Alcala and Dominguez, 2003). Prey abundance is directly dependent on the environment; therefore, the
success of carnivorous plants, prey availability and the environmental conditions are directly related to
each other. Because different populations of carnivorous plants capture different prey, one can see a
difference in the taxonomic composition of the prey and the prey actually captured by different
carnivorous plants. Contrary to the other findings, Alcala and Dominguez 2003 state that no solid
evidence supporting the capture of certain prey due to the color and smell of leaves has been
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established. Because the plants in the shadiest populations with high amounts of prey captured cannot
take advantage of the high-nutrient environment, the authors suggest that carnivory cannot substitute
for low levels of photosynthetic activity in these environments. The authors also make the claim that
carnivory is adaptive only in nutrient-poor, well lit and moist environments. A tradeoff is observed
between highly-lit areas with low prey, high temperature and dry soil, and one with high water and prey
availability, but not well lit environments. Hence, different types of carnivorous plants are observed in
these two different ecological niches, and the authors claim that intermediates of the two extreme
conditions provide the most suitable habitat for the most successful carnivorous plants. If this is true,
then one should be able to find the successful carnivorous plants in intermediate environments, and
plants in the two extreme environments should demonstrate specialized form and function relating to
that particular environment. This evolutionary strategy might also effect the interaction of these
different types of carnivorous plants with their prey.
Some carnivorous plant such as Pinguicula vallisneriifolia have become specialized due to their
unique environments such as rocky substrates and therefore are more successful in these environments
than in nutrient rich soil (Zamora, 1997). The authors demonstrate that P. vallisneriifolia has therefore
evolved to have a higher level of dependence on prey for nutrients than the soil. Thus, one might be
able to find unique characteristics in the roots of these plants in regard to form and function compared
to other carnivorous or non-carnivorous plants. Similar findings are also observed in studies done on
two sympatric species of Drosera (Thum, 1976). Leaves of Drosera rotundifolia are found lying flat on
the ground whereas Droserfa intermdia stands in an upright position. Thus, two microhabitats are
established for these two closely related plants – one specializes in aerial traps whereas the other
specializes in ground traps. These two different species also exist on different substrates. D. intermidia is
often found on water, thus, not accessible for terrestrial arthropods, and D. rotundifolia is observed
covered with moss and small bushes making it hard for aerial insects to land on it. These two sympatric
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species might have diverged due to disruptive selection. Also, the preference of different arthropods for
the two different species might have also resulted in the establishment of these two distinct carnivorous
plants in different ecological niches. The authors make the assertion that benefits of one year can carry
on to the next year via the size of the winter bud in these plants. Consequently, the evolutionary
selections for these microhabitats are under very strong selection.
Jaffe et al. find that limiting factor in the evolution of carnivorous plants is not insect
degradation (Jaffe et al., 1992). Bacteria on these plants are able to digest the dead prey. In some cases
such as with ants, it might be possible that the proteolytic enzymes within the ants’ bodies digest the
ants, and no enzyme production is required in these ant-trapping plants. Thus, the need for an enzyme
is usually favorable in environments with a rich population of various species of arthropods. The
majority of the Heliamphora species capture ants because of their small size and specific odors which
attract this particular prey. On the other hand, H. tatei is the only species of Heliamphora group that
traps flying insects. Therefore, it is more reasonable to assume that the carnivory in these species
evolved first by trapping ants and later moved on to other types of arthropods by developing more
specialized mechanisms of prey capture instead of the other way around. The authors also support the
major claim that the limiting factor in the growth of carnivorous plants is not nutrients, but light.
Studies have also demonstrated that carnivorous plants are affected by competition and
disturbance (Brewer, 1998). Competitions from live and dead vegetation are shown to have significant
effects on the reproductive success of a germinating carnivorous plant. Furthermore, carnivorous plants
are viewed as good colonizers of disturbed areas as they wait for disturbances such as fires to grow and
develop more organs and flora (Brewer, 1998 & Brewer, 1999). Further researches are required to
determine if carnivorous plants originated from non-carnivorous plants who were also good colonizers
or if they acquired the trait after the carnivorous trait was established.
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Interaction with other organisms
For any sized pitcher plants, there are always larger size classes of insects who do not fall prey to
the traps (Gibson, 1991). Gibson shows that the differential escape various insects is dependent
positively correlated with size. The bigger an insect is, the farther it can crawl towards the edge of the
leaves. Because of the different sizes of insects are trapped by pitfalls and adhesive traps and snap traps,
different sorts of evolutionary selection pressure is applied towards the different groups and niches of
insects. It is also believed that large pitcher plants have evolved to capture larger insects. Thus, just as in
an evolutionary arms race between a prey and predator, Carnivorous plants and their prey have
selected against certain traits in the morphologies of both plants and their prey.
Mites are also found on the leave hairs of many carnivorous plants – these mites are scavengers
and display mutualistic behaviors (Antor and Garci, 1995). These mites feed on the dry left over after the
plant absorption of nutrients and might also remove the fungal parasites on the leaves surface. The
authors claim that these types of mutualistic behavior are not new, and are observed in many different
plants.
Conclusions
Carnivorous plants evolved from non-carnivorous angiosperms that possess certain carnivory
traits such as secretion of mucilage. Carnivory of plants have reversed the long-held position of plants as
prey and the animals as predators. Evolution has produced these carnivorous plants because they had
conferred a selective advantage in nutrient-poor environments. There is a trade-off present in the areas
these plants can exist. They must take into account the both the effect of sunlight, which is obviously
higher in well lit places, and the abundance of prey, which is higher in shady environments. These
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carnivorous plants do not consume all their prey, they select against small insects and for larger ones.
The reason might be that if they had captured all the prey that came across, they might not have been
pollinated by insects. Future experiments can focus on whether these carnivorous plants capture prey
that has a high content of certain nutrients since energy must be invested especially in active traps to
capture prey. Other than morphological changes, carnivorous plants have also evolved characteristics in
the smell and color of the leaves produced, they type of mucilage and enzymes produced to maximize
efficiency for capturing prey. One is can also see that both antagonistic and mutualistic behaviors are
observed in these carnivorous plants.
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