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Abstract. Different coevolutionary characteristics and behaviors of pollinators and. angiosperms


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Taylor Janecek Insect Behavior Abstract Different coevolutionary characteristics and behaviors of pollinators and angiosperms Angiosperms are relatively new on the evolutionary
Taylor Janecek Insect Behavior Abstract Different coevolutionary characteristics and behaviors of pollinators and angiosperms Angiosperms are relatively new on the evolutionary time scale, only appearing about 140 million years ago. This was the first type of plant that evolved to be extremely dependent on other animals. Some angiosperms entire life cycles require animal assistance, whether that is pollination, or dispersal. Some seeds can only sprout after passing through an animal s digestive system. Even though dispersal is important, insects have by far influenced angiosperm evolution the most, and vice versa. Insects of various orders pollinate, including but not limited to Coleoptera, Lepidoptera, Hymenoptera, Diptera and Thysanura. Most beetles that pollinate have chewing mouthparts, so they are mostly pollen feeders. Lepidopteron adults only drink the nectar, and hymenopterans will both drink the nectar and eat the pollen (usually storing for later consumption). Thysanurans can spend their entire life-cycle perched upon the cup of the flower. Page 1 The pollinators and the pollinated have coevolved amazing characteristics to find one another. Flowers can have distinct coloration, which sometimes translates into ultraviolet (bees and other pollinators can see this). Some flowers have coevolved with only one species of insect (Darwin's Xanthopin morgani which has an extremely long proboscis to reach the nectar of the very long flower of the comet orchid.). Even more amazing are some other orchids that disguise their flowers as female wasps. Male wasps are seduced by these flowers, and attempt to mate with them, unknowingly allowing the orchid to attach pollen to it. In this review article I will explore many different characteristics of pollination, and how angiosperms and insects evolved together mutually (usually). Pollinator and angiosperm relationships are extremely important to understand, as we rely on both of them for most of our food and can thank this relationship for the multitude of flowers we see today. This paper is centered on the behaviors that pollinators evolved along with their plant counterparts, and delves into the particulars of a few papers with very interesting elaborate experiments along with some supporting articles. Introduction The earliest fossil of flowering plants was recorded to be no older than 135 million years. However, scientists believe that angiosperms appeared much earlier, Page 2 around 200 million years ago. Logic would dictate that angiosperm diversity was much lower than it is today. Back then, wind would have been the main pollinator like you see today in other seed plants. Their diversity is miniscule compared to angiosperms. Angiosperm species diversity is estimated to be around 250,000 to 400,000. The intimate relationships among insect pollinators and flowers are astounding. Insect pollinators are thought to be the main cause of this massive speciation and diversification of angiosperms, but also the insects themselves. The main orders of pollinating insects are Hymenoptera (Bees), Lepidoptera (Butterflies), Diptera (flies), and Coleoptera (Beetles). There are also sparse pollinators in the other orders (Thysanura) but these four make up the main ones. In a study based off of published and unpublished community-level surveys of pollination systems, it is estimated with much firmer evidence then before that animal pollinated species range from 78% in temperate zones to 96% in tropical regions, which is about 87.5% of all the species of angiosperms(ollerton, J., Winfree, R., & Tarrant, S. 2011). The most famous and one of the most intriguing examples is Darwin's moth ( Xanthopin morgani ) and it's interaction with comet orchids. The length of the flower is cm long, and Darwin simply upon seeing this, knew that there had to be an insect with a proboscis just as long. Nectar is located at the bottom of the flower, where the pollen containing parts are on the outside (anthers). This is one of those examples about the intimacy of pollination and insects. Some flowers and insects have coevolved, and have depended on each other for so long, that only that insect and that flower can successfully achieve pollination. Page 3 (Orchid Photo courtesy of CCS Bio Blog In the photograph above you can see the very long comet orchid corolla Another interesting example of this intimate relationship (and could be considered even more intimate) is the wasp and the orchid. Hammer orchid and Thynnid wasps have evolved together to the point of female mimicry. Female wasps are flightless, and wait on top of plants for males. Hammer orchids have evolved an almost perfect deceit. The orchid's labellum looks exactly (to the wasp) like the abdomen of a female Thynnid. If this wasn't enough already, the plant also releases a hormone that mimics Thynnid females as well. Males usually carry off their new found partner, but when they land on the labellum of this orchid and try to pull the 'female' off, the morphology of the flowered has evolved a lever-like action to it. When the male tries to pull the 'female' off of the plant, it bends a part of the plant that contains the pollen, and Page 4 puts the sticky pollen packet on the wasp. Some orchids place them on their heads, backs, and abdomens. Other flowers don't make such a splendid display of smells and visual signals. Abutilon pictum is a species of bell flower that emits a pheromone that attracts dipterans into its trap (Urru, I., Stensmyr, M. C., & Hansson, B. S. (2011). The pheromones would be akin to rotting flesh to a human, but smells of a delicious feast for a fly s young. The flower lures the insect into a chamber where it becomes trapped and covered in pollen, and is then released soon thereafter. Pollination isn't always mutualistic. (Photo courtesy of BBC Nature) In the photograph above you can see the orchid's female mimicry of the wasp, and the two small pollen packets that get placed on the insect when it tugs at the 'female' Discussion An example of an angiosperm and a pollinator working together to achieve a common goal regardless of a nectar robber Page 5 Although not mentioned in the introduction, there is also another completely different behavioral interaction between insects and angiosperms called nectar robbing. Nectar robbing is fairly self-explanatory. Nectar is stolen from the plant without actually pollinating at all. A prime example comes from the paper Nectar Robbing and Pollination of Lantana Camara (Verbenaceae) by Edward M. Browns (1976). The interaction between a stingless bee (Trigona fulviventris), a nectar-foraging-perforating robber of L. Camara. The bee's behavior is interesting because bees and angiosperms have coevolved in a more mutualistic way, but this happens to be a behavior that simply robs the flower of its nectar. That's insects for you, if there's a niche it s already taken by an insect. The study was performed on the development of 72 flowers, with six inflorescences each on a different plant that was noted. Just to be sure that the flowers actually contained nectar, they were pressed against a hard surface so that a drop of nectar oozed out. Plants were artificially pollinated in the lab, and four plants were used in cross pollination and three plants in self-pollination. The flower heads only last about three days time. 95% of the flowers opened at dawn whilst the remaining did sometime during the day, and about 80% of them actually contained nectar at dawn, and 25 to 40 percent contained it during the day. This was a very important point to mention, because if the flower didn't have nectar there was no point for the robbers to rob them (even if it is unbeknownst to them, it s a waste of energy). Throughout the day they change colors, and it turns out that it was indicative of how much pollen/nectar the flowers contained. They turn yellow at dawn and gradually become reddish-orange as the day goes on. Yellow flowers had the most nectar and pollen, and logically the darker colored ones had less. This is also an important point, and shows that flowers have to Page 6 continuously restock their flowers with pollen/nectar, and reminds the researcher just how costly being a flowering plant can be. An important note, 35% of the 39 flowers which were artificially cross pollinated produced seeds, and none of the 36 which were self-pollinated did. This is important to observe because it shows that these angiosperms rely heavily on their pollinating comrades. Recalling Darwin's comet orchids, these lantana also have specialized structures (corollas) that restrict their nectar only to insects that have a long proboscis. The robber (T. fulviventris) cannot reach the nectar by use of a long proboscis. The bee bites a small hole into the base of the corolla for a second or two, and drinks the nectar for up to 30 seconds. The flowers that were robbed usually contained no nectar. Interestingly, the surrounding red-orange flowers related to decreased nectar robbing to the central yellow ones. Various species of butterflies also visited these flowers, but their behavior gave some insight to the relationship between them. 99% of them only visited the yellow flowers, and almost completely avoided the darker colored ones. Note that this is not indicative of butterflies solely visiting these because there is a decreased chance that they had been robbed, 32% of them (yellow flowers) already had been. This is an interesting point. As we speak of the evolutionary intimacy of insects and angiosperms, one could hypothesize that they visit these flowers because they most likely contain nectar. The darker colored flowers usually contained negligible amounts of nectar, whether they were robbed or visited by a pollinator. Wrapping up this paper comes with some interesting points. There are three types of nectar thievery. The first type (nectarforaging robber) restricts its visits to individual flowers reducing outcrossing. The second (nectar-foraging-perforating) which doesn't restrict the insect to individual flowers, and is Page 7 observed from T. fulviventris, perforating a hole which it can drink through. The third is called pollen-foraging robbery. The insect is restricted to a single flower where it eats the pollen. It can be assumed that these flowers and its pollinators (and robbers) have coevolved, and thus the flower produces more nectar then necessary. It's also apparent that cross pollination is the only type of pollination that works for this plant, increasing their reliance on their pollinators. Perhaps the way that the flowers are situated (yellows in center, with peripheral oranges) is a combination of coevolution with the plant's robbers and pollinators. It's thought that the butterflies are 'let known' by the plant which ones to come and collect by the color itself. There were many interesting points in this well done paper, and indicative of insect coevolution with angiosperms, and how they perhaps evolved together in order to better meet each others needs in the face of these robbers. Being choosey with a sphinx moth; the combination of olfactory and visual stimuli on pollinator preference Evolution can start with a behavior. The behavior of pollinators can either greatly influence the evolution/physiology/morphology of an angiosperm, or the morphology/physiology of a plant can influence the evolution of a behavior towards the pollinator. Once some interaction that takes place consistently between the two, coevolution is soon to follow (relatively). Taking a look at a paper by Geraldine A. Wright and Florian P. Schiestl (2009) we can look into the more cognitive and learning behaviors of pollinating insects through vision and olfaction. Physiology/morphology Page 8 also plays a considerable role in pollinator preference (Kuniyasu Momose, Teruyoshi Nagamitsu, Tamiji Inoue 2006) especially in thrips because they are small and tend to live their lives actually on/in the flowers, and preferentially choose flowers with morphology that would make a suitable home. Being small also opens up another niche for both thrips and plants to coevolve smaller receptacles for their pollen. A sphinx moth (Manduca sexta) was the center of the study. The objective was to see the preferential decision making of these free flying moths, and the importance of olfactory and visual cues in these decisions. The moths were presented with a control flower that had its olfactory and visual cues easily accessible-a regular flower. Cloth bagged flowers that could still release their scent, and paper replica flowers that had no scent but the visual cue was out in the open. They were presented in random mixed variations. The moths did not try to feed from the paper replicas nor did they from the bagged flowers. Floral scent was placed on the replica, and the moths responded by attempting to feed on them. Oddly enough, with simply vegetative odors placed on them they also attempted to feed, but not as strongly as the floral scents. The wave lengths of each important facet were recorded (Figure 1). Scent and a floral representation seemed to be the only thing that influenced the preferential decision making of these moths to feed. This was indicative of preference for both stimuli. Not to be too meticulous, the study group they used had prior experience to feeding on Datura wrightii flowers, and that this preference for both stimuli may be a learned behavior. An interesting experiment, that leads right into another. One could rear captive moths on specific flowers, and use study groups of moths that have only been raised feeding on scentless, colorless, and regular flowers, with basically the same objective as the prior experiment but more precise. Page 9 (Figure 1) The sphinx again; the particulars of learning food sources through scent and reward The same species of sphingid moths (Manduca sexta) can also associate scent with reward (Jeffrey A. Riffell, Hong Lei, Leif Abrell, and John G. Hildebrand 2012). The specifics of the relationships between innate-preference and the ability to learn through olfactory conditioning for this moth have been unclear. We know that these moths will try to feed on flowers of D. wrightii when a flower replica is doused in floral odors, but learning about how this pollinator operates in regards to its antennal olfactory lobes, and how those lobes affect the behavior of the moth is a much more fundamental question. To start the experiment, flowers that were consistently visited by M. sexta were noted, and flowers that were visited by more generalized pollinators were also noted. The researchers collected volatile organic compounds and analyzed them through gas chromatography and spectrometry. They found that the scent profiles of these flowers varied qualitatively and quantitatively-but most of these flowers had their scent profiles Page 10 dominated by methyl benzoate, benzyl alcohol and benzaldhyde. They then conducted electrophysiological experiments on the antenna and antennal lobes (AL). When they were stimulated by VOCs (volatile organic compounds) the anterior AL responded strongly. They responded to the floral odors that the moth usually visited (very rich aromas), but did not respond well to flowers (varying richness) that other pollinators usually visited. Olfactory response doesn't necessarily mean a behavioral response. Remember, moths that have been in the wild (learned naturally) and moths that have been conditioned to floral scents are two different situations. This experiment is very similar to the moth experiment described earlier; they took naive moths into an artificial setting in order to fully understand their pollination behavior. Using a very similar method, paper flowers were covered with the oxygenated VOCs that elicited a strong response earlier in the free flying moths. Once again, a strong response to the aromatic-rich scent was observed. Other paper flowers were covered in scents of flowers not usually pollinated by these moths, and that did not elicit a strong response as well. Since this is being characterized as an innate behavior, this result was assumed. Laboratory reared moths were then used in order to experiment on this behavior more. The captive moths were fed on agave, (Agave palmeri) a flower usually pollinated by bats. When hawkmoth flowers were introduced to the agave experienced moths, they elicited a response much like the wild moths. More experiments were performed in order to understand how the olfactory processes odor information, using multichannel antennal lobe recording and olfactory conditioning. The moths were taught and conditioned to associate VOCs with a nectar reward. When there was a reward associated with this scent, their AL olfactory neurons fired much more rapidly, and when Page 11 there was not a reward the neurons did not elicit such a response and did not learn to associate reward with the particular floral scent. This evolution of behavior is very interesting, and compliments the prior sphinx moth experiment very well. Association of flower shape and a flower scent, and then also associative learning of VOCs and nectar rewards. Floral odors have been studied extensively along with floral patterns in relation to pollinator preference. Honey bees are much more generalized in their foraging. They have a lot of mouths to feed so no need to be picky. After their waggle-dance, the bee gives out a sample of nectar from the food source (Keith D. Waddington 1982). A gustatory stimulus is also an important honest indicator to the rest of the hive of the quality of the food source. A classic story; the milkweed bug and the mantis The ability to learn in insects is an interesting topic, and one of the most memorable situations that this occurred in is the milkweed bug and the praying mantis experiment (May R. Berenbaum and E. Miliczky 1984). The mantis species Tenodera ardifolia sinensis was used in an experiment to observe the learning capabilities in insects. Milkweed bugs (Oncopeltis fasciatus) are a common insect in the family lygaeidae. They feed on milkweed (Asclepius syriaca), a plant that is well known for its toxic cardenolides. She fed two groups of these bugs, food that had these toxins and food that didn't have these toxins to another group. She then fed these milkweed bugs to the mantis in a laboratory. After eating and regurgitating enough of these milkweed bugs, the mantis would not eat them anymore, regardless even if they were the Page 12 harmless ones. The mantis associated the toxins with the aposematic coloration of the bugs, much like how the sphinx moth associated a reward with certain scents. The use of different pollinator species, which can vary substantially in their floral preferences and sensory systems, is regarded as the main source of divergent selection underlying the remarkable diversification of angiosperm flowers (de Jager, M. L. & A. G. Ellis. 2012). Gender specific preference in pollination is a subject that may make you think of mosquitoes (Culicidae). In many mosquito species, males drink the nectar of flowers (pollination can occur), whilst female mosquitoes are the infamous bloodsucking pests of the world (even though both male and female mosquitoes can drink nectar). This is a huge gender divergence, but necessary for the female to attain nutrients for her eggs, and in the process can vector many pathogens such as west nile virus (Hubálek, Z., & Halouzka, J. 1999) and malaria. Gender differences in decision making and preference; the long story of a devious plant In fact, when speaking of gender specific preference of a pollinator, a bombylid (bee fly) called Megapalpus capensis, we can observe some interesting behaviors that, when pollinating, differ by gender. In this experiment (De Jager et al 2012) the bombylid was chosen as the study species, and its quarry was the self-incompatible daisy Gorteria diffusa
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