The origin of floral fragrance
after http://ivanov-petrov.livejournal.com/1539415.html?thread=76413271#t76413271
S: There are floral scents that are repulsive both to us and the majority of pollinating insects. For example, carrion flowers smell of decaying flesh; they attract only pollinators (e.g., flies) that are attracted to decaying flesh. However, most flowers explore "fruity" scents. Why are the carrion flowers so rare?
It is somewhat of a mystery to me, because flies have been around twice as long as bees, around 250 Myr. The evolutionary strategy of attracting "flies" should have been at least as rewarding as the strategy of attracting "bees." Did the flowering plants make nectar that attracted the first pollinators (that learned to associate it with a particular scent) or these pollinators were attracted to such scents because flowering plants co-opted pre-existing scents, like the present day carrion flowers that expoit the malodor of rotting flesh?
IP: первые опылители - жуки. Вместе с ними опылял всякий народ неспециального толка - пилильщики, скажем, даже прямокрылые. И вот потом, после этих первых опылителей, древних, пошла специализация - коэволюция цветов и насекомых, когда выбирались группы с цветочным постоянством (чтобы гад, пожрав пыльцы, не на кал летел, а на другой цветок этого вида). Именно вторая волна специализироанных опылителей - пчелы, шмели, частично бабочки и пр. - сделала современные цветки. А вот с трупами и калом - это уже отдельные соображения.
S. That what smells nice to pollinators also smells nice to us is easy to rationalize. The first primates were insectivors; flowers was where they caught pollinators, so flowers were associated with food. All such smells are invariably good, so flowers smell nice to us. Another reason could be that fruit that is produced by flowering plants (when it is sligtly fermented) smells just like flowers.
What I suspect might have happened is that in the pre-angiosperm world, there was already food that smelled like our flowers, and the first "nice" smelling flowers exploited these pre-existing scents to attract their first pollinators. Then the co-evolution started. The difficulty with my scenario is that nothing other than the flowers smells like flowers today. However, we are talking about 100 Mya, which was a long time ago. In principle, any biomass that is rich in starch and fermented by bacteria will have flowery scent. Perhaps some biodegrading insects were already attracted to such scents long before there were flowers. One possibility, for example is that the excrement of extinct large herbivores smelled this way. Herbivorous mammals consume a lot of cellulose, but still there is enough left for insects in their manure, and their feces have a tinge of fruity scent. Herbivorous dinosaurs had less advanced digestive systems, and their excrement would've contained more undigested starch. Many animals, including insects, process these feces. This makes sense, as the manure is baked in the sun and fermented by bacteria, then maggots infest it, thereby providing extra protein. Maybe what we take for "flowery smell" is, in fact, the smell of prehistoric manure. Then there is no fundamental difference between "nice" flowers and carrion flowers. Both groups originated through recruting insects in the same way. And then the producers of fruity excrement disappeared, while the new generation of (mammalian) herbivores had different digestive system. By that time, the co-evolution advanced and there was no pressure to change nectars.
IP: Мне это не кажется правдоподобным. Надо полагать, обмен рептилий не безумно отличается от прежнего. Мне кажется это избыточной гипотезой - к чему цветам пахнуть как навоз? цветковые формировались раньше, чем комплекс навозников. степи появились позже цветущих деревьев, трава - это молодая жизненная форма, деревья старше, в т.ч. покрытосеменные. то есть сначала появился ароматный нектар и красивые цветки. A уже потом возникала привлекательность запаха навоза для некоторых групп насекомых и оформлялись сами эти группы. И второе - зачем так сложно? предполагать, что навоз изначально симпатичен, а цветки подражает чему-то симпатичному... Это просто отодвигает причину, не помогая ее отыскать. кажется, можно проще - вот есть нектар, он пахнет собой - и опылители ищут этот нектар. А почему нектар пахнет именно так - это уже физиология растений, какие именно эфиры вырабатываются и почему.
S: It cannot be plant physiology. Plants can make ANYTHING. They make repellants (onions), pheromones, etc. Carrion plants have no difficulty making corpse-smelling amines. The building blocks of such molecules are being used to pack DNA, so they need just a few final touches. So this "easy to make" argument does not fly. To a plant and a fungus, everything is easy to make.
You are asking why would flowers smell like manure? For the same reason carrion flowers smell like carrion: to attract pollinators. The modern dung beetles appeared recently, however, there must've been other insects degrading excrement before this particular group.
I do not think much is known about dinosaur digestion, but the stench of feces is mainy from sulfur-containing metabolites. Herbivores excrete few of those, as cystine is scarce in plants. I am not suggesting that the manure smelled exactly like flowers. I am suggesting that it had (among other odoriferous molecules) scent molecules that were similar to those presently produced by "nice" flowers. This idea may look contrived, but how do you imagine evolution of carrion flowers? I tried to imagine such a scenario. Then I asked myself: how do I know that "nice" scents did not originate through the same scenario? The first flowers would face the same problem as todays' carrion flowers: recruting insects that so far targeted other food sources. Providing food (if you can get away with cheating) is wasteful. Perhaps the first flowers were cheaters.
IP: Вы говорите: растения легко сделают любой запах. Допустим. И далее: они сначала копировали запах каких-то привлекательных отходов. Почему? Откуда этот ход мысли? Ведь нет же никакой проблемы курицы и яйца, и мы не пытаемся объяснить, как самый первый организм-опылитель привлечен первым цветком. Мы говорим совсем иначе: насекомые "всегда" если растения, разные их части. И листья, и шишки у голосемянных. и вот появляются первые цветковые - их все равно едят. У шишек ели пыльцу - и в цветковые лезут насекомые, чтобы их поесть, богатая энергией пыльца. Там вообще нет проблемы привлечения - сами лезут. Есть другое: растения смогли за счет избыточной продукции обеспечить полезную себе функцию - они производят пыльцы больше чем надо, и потому могут пользоваться опылителями. то же с листьями - Вы помните - растения, которых объедают листогрызущие насекомые, дают больше биомасссы, чем растения, которых никто не ест. Это обычный ход растения. насекомые всюду и всегда берут численностью. растения всегда берут стратегически тем, что могут завалить проблему биомассой. они будут производить пищу - пусть их едят - и будут пользоваться своими поедателями. а уже потом, на основе этой связи, выделяется более тонкая дифференциация - можно делать специальные отношения с данными опылителями, минимизировать эту вообще-то затратную стратегию. И тут уже наступает время тонких игр - можно привлекать только шмелей, не пуская пчел (длины хоботка, нектар на дне глубокого цветка); можно привлекать только падальных мух запахом падали и т.п.
S: Every day I pass by a Gingko tree. In the fall, the fallen fruit of this tree reeks of butyric acid (that smells like rancid butter). The fruit makes butyl esters, then symbiotic bacteria living on the tree infest the fruit before it falls to the ground and ferment these esters to butyric acid. There have been many explanations for this offensive smell, but one of these explanations is that the scent targets an extinct animal that dispersed the seeds. The fruit contains urushiol; no modern animal can consume it without poisoning to death. So here you are: it is one of the oldest relict cycads, and it produces carrion smell to attract its seed dispersers. There is a precedent in the gymnosperms.
I can believe that some insects had pre-adaptations to become pollinators of angiosperms. What I do not understand, though, is why would flowers produce specifically "fruity" smells in order to attract these ancient pollinators? Any scent signal would do - correct? Why was it, specifically, this fruity smell? Think about it this way. Carrion smell is just as good a scent signal as any. But if you choose this signal, in addition to plant biomass degraders (that were attracted to pollen anyway, just as you say) you would also attract animal refuse degraders. Would not it be better to attract as many potential pollinators as possible? The only resolution of this paradox is to assume that the first pollinators were recruited not only by food that was produced by the plant, but also by a familar scent signal.
OK, you do not like the excrement idea. But this does not mean that the fruity smell did not exist before the flowers. The rancid butter smell of Gingko existed to attract an animal that found this scent as pleasing as we find the smell of roses. I am suggesting that flowering plants co-opted a pre-existing scent signal that targeted ancient insects. The problem is that no non-flowering plant produce such fruity scents. The only way I can imagine "natural" evolution of this scent is processing plant matter in a digestive system of an animal - or by bacterial fermentation. Gymnosperms do not make fruity smells, but digested and pooped out gymnosperms do make such smells. Hence the excrement idea.
Your scenario is fine except it provides no explanation as to why flowers have fruity fragrances. It could have been any scent signal. If it can be any, why not all flowers are carrion flowers?
S: Мне кажется. ваша гипотеза может быть проверена - по косвенным свидетельствам. 1) существуют реконструкции массовых пород в лесах разных эпох. Можно представить себе реконструкцию запахов. Хвойные пахнут падалью7 в самом деле? Это можно установить. Никакого другого метода, чем метод актуализма - заключения от того. что известно. к тому. что не известно. у нас нет. И минимизация необоснованных догадок. Может в п.1 получить запах лесов до появления покрытосеменных. 2) можно восстановить энтомофауну к моменту появления покрытосеменных. то есть это уже сделано. я имею в виду - просто найти работы и прочитать, какие отряды, в каком обилии. И по современным представителям насекомые с точностью до отряда сейчас примерно те же) - понять. так кто же там был и как себя вел и опылял или ел. Например, этот п.2 позволит проверить высказанную Вами добавочную гипотезу - что растения могли бы привлекать еще и тех, кто на падаль, раз уж все равно. какой запах выбрать. Там можно проверить и понять. осмыслено ли такое поведение. 3) совсем по вашей части - тут я ничего сделать не могу. Разобрать химию производства всех этих ароматических эфиров в цветах - и посмотреть. из чего они получаются, на что увязаны, сравнить со стадиями эволюции растительности и попытаться понять, насколько древние это запахи. например. посмотрев разнообразие запахов на разнообразие растений, примитивных и продвинутых. грубо говоря - как пахнет магнолия, как - осока. Думаю, по результатам таких проверок доказано ничего не будет - но зато будут отсеяны некоторые гипотезы.
S: A molecular biologist friend of mine suggested an explanation that makes a lot of sense. All flowers cannot be carrion flowers. Carrion is scarce in nature and finding carrion requires extreme sensitivity to carrion scents. If all flowers begin to smell like carrion, carrion insects would not be able to locate carrion and become extinct. Without such pollinators, there is no point in smelling like carrion. So the majority of flowers cannot smell like carrion.
This addresses one of your points. Doubling on carrion insects benefits a plant only if few other flowering plants pursue the same strategy, so an insect mistakes a flower for carrion. Having the specific scent signal, by contrast, makes it possible for a flower to co-exist with many other types of flowers. It is the adaptation selected by long-term advantage for angiosperms as a class rather than individual plants. The cheaters cannot occupy a large niche and they would be replaced. The straight players prevail.
This sounds right to me, but it does not quite explain chosing the fruity scents as a signal to pollinators. Well, at least it answers the original question. It also explains why my idea about excrement origin of fruity smell is likely to be incorrect. It could have worked only when flowering plants began colonizing the planet. The advantage of cheating would be short lived.
IP: Если честно, мне это объяснение совершенно не нравится. Примерная причина такой моей реакции: дело заключается во взаимодействии множества факторов; объяснение пытается сделать ситуацию простой, не обращая внимания на массу специфических эффектов, которые есть и которые могли бы быть. То есть это объяснение, на мой взгляд, обладает исключительно риторической убедительностью - тем, кто любит аргументы такого сорта, оно кажется пригодным. Для меня аргументами будут те, что я пытался назвать - указание на группы, которые тогда ели пыльцу голосеменных и покрытосеменных (предположим. жуки и скорпионницы). наличие среди них форм. которые питаются падалью. Выяснение, что за падаль, как находят - по современных формам. Реконструкция следствий и проверка на имеющемся материале цветков. Вот из чего-то такого вырастет эволюционный сценарий, который будет мне казаться убедительным - пока не появится другой сценарий, опирающийся на иные факты.
S: OK, another friend pointed me to a paper
http://onlinelibrary.wiley.com/doi/10.1111/j.1461-0248.2010.01451.x/full
on comparative analysis of floral scents in angio- and gymno- sperms and insects. It's a crossover between mine and your ideas - and I think I got my answer.
One component of the fruity scent (monoterpenes) had been produced by gymnosperms before the flowering plants, while another component (benzenoids) had been produced by insect and non-insect arthropods THEMSELVES.
It turns out that insect grazing on gymnosperms & their gymnosperm hosts do share a lot of scent molecules with flowering plants. I reinvented the bicycle: the question has been asked before, and the dilemma that bothered me has been also recognized. There is significant overlap between floral and insects scent molecules, so either the plants mimicked insect scents to attract pollinators or the herbivorous insects used scent molecules extracted from plants to meet their own needs, and they were subsequently recruted by flowering plants through this pre-existing dependence. I was right (there were pre-existing source of flowery/fruity scents) but you were also right (the origin of these scents is interaction of gymnosperms and their herbivores).
...Two evolutionary scenarios may potentially account for associations between floral and insects scent molecules, namely coevolution and one-sided evolution. Coevolution between pollinators and flowers makes the assumption that (1) ‘floral’ scent compounds have primarily evolved in the pollinators, and (2) scent production in plants and pollinators evolved at approximately the same time. However, none of the compounds was produced exclusively by pollinator groups. Additionally, the insects’ use of ‘floral’ aromatics precedes the evolution of flowers, because aromatics are widespread in both early (e.g. Hemiptera) and late (e.g. Lepidoptera) diverging insect orders. Some floral aromatics are even used in non-insect arthropods.
....As an alternative, one-sided evolution predicts that one partner has evolved similarities to a pre-existing pattern in the other partner. One-sided evolution can principally occur in insects and plants, however a scenario where insects evolve onto patterns of plants seems unlikely as it assumes, similar to coevolution, that ‘floral’ compounds have evolved primarily in pollinator insects, which is not supported by my dataset. In the reverse scenario, floral signals evolved to match pre-existing sensory signals of their pollinators during the relatively recent radiation of angiosperms. This ‘pre-existing bias’ hypothesis thus assumes that (1) floral scent is associated with insect volatiles in both pollinator and non-pollinator groups, since their commonness should not be influenced by an association with flowers, and (2) insect volatiles should be evolutionarily older than floral scent. In support of the pre-existing bias hypothesis, floral aromatics were correlated with aromatics from pollinators and herbivores and marginally correlated with insects with no association to plants. Additionally, ‘floral’ aromatics appear to have originated in arthropods well before the angiosperm radiation.
...Pre-existing bias may be an important selective force for the evolution of mutualisms. Invoking pre-existing bias as the driver of floral scent evolution suggests that plants exploited sensory or neuronal preferences of their pollinators outside the context of sexual selection by mimicking volatiles with established receptors and perceptual preferences. In a mutualistic relationship, the operators (in this case, the pollinators) are selected to respond to a mimetic signal, even if it is not identical to the model. Therefore, this scenario does not predict an exact one-to-one match between insect and floral volatile chemistry. Instead, it suggests a loose overlap with floral scents that contain both insect-like and plant-specific components. This pattern is prevalent in floral bouquets.
PS: I've learned my lesson, and I understand the problem better. Floral scents is the result of convergence of two interdependent biochemistries that have been mimicking and exploiting each other from the time immemorial. This interdependency started long before the angisperms and their pollinators evolved.
That we would eventually enjoy the fragrance of flowers had been preordained when the first arthropods started to graze on the first land plants.
Blessed is the one who grants sweet fragrance to fruit.
בָּרוּךְ אַתָּה יְיָ אֱלֹהֵֽינוּ מֶֽלֶךְ הָעוֹלָם,
הניתן רוח טוב כפירות
S: There are floral scents that are repulsive both to us and the majority of pollinating insects. For example, carrion flowers smell of decaying flesh; they attract only pollinators (e.g., flies) that are attracted to decaying flesh. However, most flowers explore "fruity" scents. Why are the carrion flowers so rare?
It is somewhat of a mystery to me, because flies have been around twice as long as bees, around 250 Myr. The evolutionary strategy of attracting "flies" should have been at least as rewarding as the strategy of attracting "bees." Did the flowering plants make nectar that attracted the first pollinators (that learned to associate it with a particular scent) or these pollinators were attracted to such scents because flowering plants co-opted pre-existing scents, like the present day carrion flowers that expoit the malodor of rotting flesh?
IP: первые опылители - жуки. Вместе с ними опылял всякий народ неспециального толка - пилильщики, скажем, даже прямокрылые. И вот потом, после этих первых опылителей, древних, пошла специализация - коэволюция цветов и насекомых, когда выбирались группы с цветочным постоянством (чтобы гад, пожрав пыльцы, не на кал летел, а на другой цветок этого вида). Именно вторая волна специализироанных опылителей - пчелы, шмели, частично бабочки и пр. - сделала современные цветки. А вот с трупами и калом - это уже отдельные соображения.
S. That what smells nice to pollinators also smells nice to us is easy to rationalize. The first primates were insectivors; flowers was where they caught pollinators, so flowers were associated with food. All such smells are invariably good, so flowers smell nice to us. Another reason could be that fruit that is produced by flowering plants (when it is sligtly fermented) smells just like flowers.
What I suspect might have happened is that in the pre-angiosperm world, there was already food that smelled like our flowers, and the first "nice" smelling flowers exploited these pre-existing scents to attract their first pollinators. Then the co-evolution started. The difficulty with my scenario is that nothing other than the flowers smells like flowers today. However, we are talking about 100 Mya, which was a long time ago. In principle, any biomass that is rich in starch and fermented by bacteria will have flowery scent. Perhaps some biodegrading insects were already attracted to such scents long before there were flowers. One possibility, for example is that the excrement of extinct large herbivores smelled this way. Herbivorous mammals consume a lot of cellulose, but still there is enough left for insects in their manure, and their feces have a tinge of fruity scent. Herbivorous dinosaurs had less advanced digestive systems, and their excrement would've contained more undigested starch. Many animals, including insects, process these feces. This makes sense, as the manure is baked in the sun and fermented by bacteria, then maggots infest it, thereby providing extra protein. Maybe what we take for "flowery smell" is, in fact, the smell of prehistoric manure. Then there is no fundamental difference between "nice" flowers and carrion flowers. Both groups originated through recruting insects in the same way. And then the producers of fruity excrement disappeared, while the new generation of (mammalian) herbivores had different digestive system. By that time, the co-evolution advanced and there was no pressure to change nectars.
IP: Мне это не кажется правдоподобным. Надо полагать, обмен рептилий не безумно отличается от прежнего. Мне кажется это избыточной гипотезой - к чему цветам пахнуть как навоз? цветковые формировались раньше, чем комплекс навозников. степи появились позже цветущих деревьев, трава - это молодая жизненная форма, деревья старше, в т.ч. покрытосеменные. то есть сначала появился ароматный нектар и красивые цветки. A уже потом возникала привлекательность запаха навоза для некоторых групп насекомых и оформлялись сами эти группы. И второе - зачем так сложно? предполагать, что навоз изначально симпатичен, а цветки подражает чему-то симпатичному... Это просто отодвигает причину, не помогая ее отыскать. кажется, можно проще - вот есть нектар, он пахнет собой - и опылители ищут этот нектар. А почему нектар пахнет именно так - это уже физиология растений, какие именно эфиры вырабатываются и почему.
S: It cannot be plant physiology. Plants can make ANYTHING. They make repellants (onions), pheromones, etc. Carrion plants have no difficulty making corpse-smelling amines. The building blocks of such molecules are being used to pack DNA, so they need just a few final touches. So this "easy to make" argument does not fly. To a plant and a fungus, everything is easy to make.
You are asking why would flowers smell like manure? For the same reason carrion flowers smell like carrion: to attract pollinators. The modern dung beetles appeared recently, however, there must've been other insects degrading excrement before this particular group.
I do not think much is known about dinosaur digestion, but the stench of feces is mainy from sulfur-containing metabolites. Herbivores excrete few of those, as cystine is scarce in plants. I am not suggesting that the manure smelled exactly like flowers. I am suggesting that it had (among other odoriferous molecules) scent molecules that were similar to those presently produced by "nice" flowers. This idea may look contrived, but how do you imagine evolution of carrion flowers? I tried to imagine such a scenario. Then I asked myself: how do I know that "nice" scents did not originate through the same scenario? The first flowers would face the same problem as todays' carrion flowers: recruting insects that so far targeted other food sources. Providing food (if you can get away with cheating) is wasteful. Perhaps the first flowers were cheaters.
IP: Вы говорите: растения легко сделают любой запах. Допустим. И далее: они сначала копировали запах каких-то привлекательных отходов. Почему? Откуда этот ход мысли? Ведь нет же никакой проблемы курицы и яйца, и мы не пытаемся объяснить, как самый первый организм-опылитель привлечен первым цветком. Мы говорим совсем иначе: насекомые "всегда" если растения, разные их части. И листья, и шишки у голосемянных. и вот появляются первые цветковые - их все равно едят. У шишек ели пыльцу - и в цветковые лезут насекомые, чтобы их поесть, богатая энергией пыльца. Там вообще нет проблемы привлечения - сами лезут. Есть другое: растения смогли за счет избыточной продукции обеспечить полезную себе функцию - они производят пыльцы больше чем надо, и потому могут пользоваться опылителями. то же с листьями - Вы помните - растения, которых объедают листогрызущие насекомые, дают больше биомасссы, чем растения, которых никто не ест. Это обычный ход растения. насекомые всюду и всегда берут численностью. растения всегда берут стратегически тем, что могут завалить проблему биомассой. они будут производить пищу - пусть их едят - и будут пользоваться своими поедателями. а уже потом, на основе этой связи, выделяется более тонкая дифференциация - можно делать специальные отношения с данными опылителями, минимизировать эту вообще-то затратную стратегию. И тут уже наступает время тонких игр - можно привлекать только шмелей, не пуская пчел (длины хоботка, нектар на дне глубокого цветка); можно привлекать только падальных мух запахом падали и т.п.
S: Every day I pass by a Gingko tree. In the fall, the fallen fruit of this tree reeks of butyric acid (that smells like rancid butter). The fruit makes butyl esters, then symbiotic bacteria living on the tree infest the fruit before it falls to the ground and ferment these esters to butyric acid. There have been many explanations for this offensive smell, but one of these explanations is that the scent targets an extinct animal that dispersed the seeds. The fruit contains urushiol; no modern animal can consume it without poisoning to death. So here you are: it is one of the oldest relict cycads, and it produces carrion smell to attract its seed dispersers. There is a precedent in the gymnosperms.
I can believe that some insects had pre-adaptations to become pollinators of angiosperms. What I do not understand, though, is why would flowers produce specifically "fruity" smells in order to attract these ancient pollinators? Any scent signal would do - correct? Why was it, specifically, this fruity smell? Think about it this way. Carrion smell is just as good a scent signal as any. But if you choose this signal, in addition to plant biomass degraders (that were attracted to pollen anyway, just as you say) you would also attract animal refuse degraders. Would not it be better to attract as many potential pollinators as possible? The only resolution of this paradox is to assume that the first pollinators were recruited not only by food that was produced by the plant, but also by a familar scent signal.
OK, you do not like the excrement idea. But this does not mean that the fruity smell did not exist before the flowers. The rancid butter smell of Gingko existed to attract an animal that found this scent as pleasing as we find the smell of roses. I am suggesting that flowering plants co-opted a pre-existing scent signal that targeted ancient insects. The problem is that no non-flowering plant produce such fruity scents. The only way I can imagine "natural" evolution of this scent is processing plant matter in a digestive system of an animal - or by bacterial fermentation. Gymnosperms do not make fruity smells, but digested and pooped out gymnosperms do make such smells. Hence the excrement idea.
Your scenario is fine except it provides no explanation as to why flowers have fruity fragrances. It could have been any scent signal. If it can be any, why not all flowers are carrion flowers?
S: Мне кажется. ваша гипотеза может быть проверена - по косвенным свидетельствам. 1) существуют реконструкции массовых пород в лесах разных эпох. Можно представить себе реконструкцию запахов. Хвойные пахнут падалью7 в самом деле? Это можно установить. Никакого другого метода, чем метод актуализма - заключения от того. что известно. к тому. что не известно. у нас нет. И минимизация необоснованных догадок. Может в п.1 получить запах лесов до появления покрытосеменных. 2) можно восстановить энтомофауну к моменту появления покрытосеменных. то есть это уже сделано. я имею в виду - просто найти работы и прочитать, какие отряды, в каком обилии. И по современным представителям насекомые с точностью до отряда сейчас примерно те же) - понять. так кто же там был и как себя вел и опылял или ел. Например, этот п.2 позволит проверить высказанную Вами добавочную гипотезу - что растения могли бы привлекать еще и тех, кто на падаль, раз уж все равно. какой запах выбрать. Там можно проверить и понять. осмыслено ли такое поведение. 3) совсем по вашей части - тут я ничего сделать не могу. Разобрать химию производства всех этих ароматических эфиров в цветах - и посмотреть. из чего они получаются, на что увязаны, сравнить со стадиями эволюции растительности и попытаться понять, насколько древние это запахи. например. посмотрев разнообразие запахов на разнообразие растений, примитивных и продвинутых. грубо говоря - как пахнет магнолия, как - осока. Думаю, по результатам таких проверок доказано ничего не будет - но зато будут отсеяны некоторые гипотезы.
S: A molecular biologist friend of mine suggested an explanation that makes a lot of sense. All flowers cannot be carrion flowers. Carrion is scarce in nature and finding carrion requires extreme sensitivity to carrion scents. If all flowers begin to smell like carrion, carrion insects would not be able to locate carrion and become extinct. Without such pollinators, there is no point in smelling like carrion. So the majority of flowers cannot smell like carrion.
This addresses one of your points. Doubling on carrion insects benefits a plant only if few other flowering plants pursue the same strategy, so an insect mistakes a flower for carrion. Having the specific scent signal, by contrast, makes it possible for a flower to co-exist with many other types of flowers. It is the adaptation selected by long-term advantage for angiosperms as a class rather than individual plants. The cheaters cannot occupy a large niche and they would be replaced. The straight players prevail.
This sounds right to me, but it does not quite explain chosing the fruity scents as a signal to pollinators. Well, at least it answers the original question. It also explains why my idea about excrement origin of fruity smell is likely to be incorrect. It could have worked only when flowering plants began colonizing the planet. The advantage of cheating would be short lived.
IP: Если честно, мне это объяснение совершенно не нравится. Примерная причина такой моей реакции: дело заключается во взаимодействии множества факторов; объяснение пытается сделать ситуацию простой, не обращая внимания на массу специфических эффектов, которые есть и которые могли бы быть. То есть это объяснение, на мой взгляд, обладает исключительно риторической убедительностью - тем, кто любит аргументы такого сорта, оно кажется пригодным. Для меня аргументами будут те, что я пытался назвать - указание на группы, которые тогда ели пыльцу голосеменных и покрытосеменных (предположим. жуки и скорпионницы). наличие среди них форм. которые питаются падалью. Выяснение, что за падаль, как находят - по современных формам. Реконструкция следствий и проверка на имеющемся материале цветков. Вот из чего-то такого вырастет эволюционный сценарий, который будет мне казаться убедительным - пока не появится другой сценарий, опирающийся на иные факты.
S: OK, another friend pointed me to a paper
http://onlinelibrary.wiley.com/doi/10.1111/j.1461-0248.2010.01451.x/full
on comparative analysis of floral scents in angio- and gymno- sperms and insects. It's a crossover between mine and your ideas - and I think I got my answer.
One component of the fruity scent (monoterpenes) had been produced by gymnosperms before the flowering plants, while another component (benzenoids) had been produced by insect and non-insect arthropods THEMSELVES.
It turns out that insect grazing on gymnosperms & their gymnosperm hosts do share a lot of scent molecules with flowering plants. I reinvented the bicycle: the question has been asked before, and the dilemma that bothered me has been also recognized. There is significant overlap between floral and insects scent molecules, so either the plants mimicked insect scents to attract pollinators or the herbivorous insects used scent molecules extracted from plants to meet their own needs, and they were subsequently recruted by flowering plants through this pre-existing dependence. I was right (there were pre-existing source of flowery/fruity scents) but you were also right (the origin of these scents is interaction of gymnosperms and their herbivores).
...Two evolutionary scenarios may potentially account for associations between floral and insects scent molecules, namely coevolution and one-sided evolution. Coevolution between pollinators and flowers makes the assumption that (1) ‘floral’ scent compounds have primarily evolved in the pollinators, and (2) scent production in plants and pollinators evolved at approximately the same time. However, none of the compounds was produced exclusively by pollinator groups. Additionally, the insects’ use of ‘floral’ aromatics precedes the evolution of flowers, because aromatics are widespread in both early (e.g. Hemiptera) and late (e.g. Lepidoptera) diverging insect orders. Some floral aromatics are even used in non-insect arthropods.
....As an alternative, one-sided evolution predicts that one partner has evolved similarities to a pre-existing pattern in the other partner. One-sided evolution can principally occur in insects and plants, however a scenario where insects evolve onto patterns of plants seems unlikely as it assumes, similar to coevolution, that ‘floral’ compounds have evolved primarily in pollinator insects, which is not supported by my dataset. In the reverse scenario, floral signals evolved to match pre-existing sensory signals of their pollinators during the relatively recent radiation of angiosperms. This ‘pre-existing bias’ hypothesis thus assumes that (1) floral scent is associated with insect volatiles in both pollinator and non-pollinator groups, since their commonness should not be influenced by an association with flowers, and (2) insect volatiles should be evolutionarily older than floral scent. In support of the pre-existing bias hypothesis, floral aromatics were correlated with aromatics from pollinators and herbivores and marginally correlated with insects with no association to plants. Additionally, ‘floral’ aromatics appear to have originated in arthropods well before the angiosperm radiation.
...Pre-existing bias may be an important selective force for the evolution of mutualisms. Invoking pre-existing bias as the driver of floral scent evolution suggests that plants exploited sensory or neuronal preferences of their pollinators outside the context of sexual selection by mimicking volatiles with established receptors and perceptual preferences. In a mutualistic relationship, the operators (in this case, the pollinators) are selected to respond to a mimetic signal, even if it is not identical to the model. Therefore, this scenario does not predict an exact one-to-one match between insect and floral volatile chemistry. Instead, it suggests a loose overlap with floral scents that contain both insect-like and plant-specific components. This pattern is prevalent in floral bouquets.
PS: I've learned my lesson, and I understand the problem better. Floral scents is the result of convergence of two interdependent biochemistries that have been mimicking and exploiting each other from the time immemorial. This interdependency started long before the angisperms and their pollinators evolved.
That we would eventually enjoy the fragrance of flowers had been preordained when the first arthropods started to graze on the first land plants.
Blessed is the one who grants sweet fragrance to fruit.
בָּרוּךְ אַתָּה יְיָ אֱלֹהֵֽינוּ מֶֽלֶךְ הָעוֹלָם,
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