진화론을 이해하는 데 어려움을 겪고 있는 네펜데스 속과 멋진 포충낭은 오늘날 100종 이상이 발견되었는 데도 이들 사이에 크게 관계성이 없다는 것이다. 그리고 그 속에는 명백한 조상이나 과도기적인 종들이 존재하지 않는다. 우리는 본질적으로 완성된 제품을 가지고 있다. 네펜데스 속은 우리에게 속의 유래를 알 수 있는 힌트를 주는 먼 조상을 가지고 있다. 그러나 그 친척들은 그들만의 특별한 적응 능력을 개발하기 위해 6천만년 전에 방향을 틀었다. 육식 능력이 있는 Triphyophyllum 과 육식 능력이 없는Ancistrocladus, Habropetalum 과 Dioncophyllum, are jungle lianas 는 네펜데스가 포충낭이 없는 작은 모습일 때와 매우 비슷하다. 네펜데스 속과는 다르게 Triphyophyllum 의 육식 능력은 끈끈이 주걱과 유사하다. 네펜데스 속과 머나먼 친척인 Drosophyllum은 끈끈이 주걱과 섞이게 된 것이다. 재밌는 부분은 Triphyophyllum 에서 네펜데스 속의 특징들을 제거하다 보면 결국 Drosophyllum 속에 도달하게 된다. 또한 네펜데스 속의 더욱 먼 친척은 드로세라 속, 즉 진짜 끈끈이 주걱이고 탁 닫는 트랩을 가진 육식인 dionaea 속과 aldrovanda 속이 있다. 이러한 특징들은 네펜데스 속과 묶어주는 역할을 하지만 굳이 육식일 필요는 없다. 우린 이들이 DNA 염기서열에 관계가 있다는 것을 알고 있다. 이 계통 분석을 위해 우리는 엽록체 유전자를 함유하고 있는 엽록체 trnK 유전자를 사용하고 있다. 혹시 이해할 수 없다면 Drosera 계통 발생론과 중요한 점은 DNA 염기서열 의 다름이다. 이것은 여전히 엽록체 계열이고 핵 서열이 아니다. 그래서 한편으론 우리는 DNA 염기서열의 다른 점을 비교할 수 있었지만 다른 한편으론 우리는 아주 적은 종의 Drosera 의 matK 염기서열, 적은 종의 Nepenthes 의 rbcL 염기서열만을 가지고 있다. Drosera 속의 계통 발생론에서 찾아낸 것은 Drosera arcturi가 다르게 발전한 Drosera 종들과 밀접한 관계가 있다는 것이었다. . Drosera arcturi는 원시적인 Drosera 속의 특징을 많이 가지고 있어 원시 끈끈이 주걱이 어떻게 생겼는지 상상할 수 있다. 네펜데스 계통 발생론엔 그러한게 우리에겐 아직 없다. |
이 트리에 사용된 데이터는 NCBI에 있는 엽록체 trnK 개재 배열 유전자, matK 염기서열이다. 이 그림에서 주의해야 할 점은 전이된 matK 위유전자 염기서열은 제외되었다는 점이다. 조사는 NCBI blast 에서 이행됐고 그래프는 PHYLIP 트리 만들기 프로그램으로 만들어졌다. 선의 길이는 유전적 거리를 뜻한다. Plumbago auriculata 는 외집단으로서 사용된다. 종별 계통 발생론은 추가적인 정보와 종의 발견으로 언제든 바뀔 수 있다. |
네펜데스 속은 침엽수와 같은 계통 발생 트리를 가지고 있다. 또한 Triphyophyllum 속과 그와 관련된 속들은 Ancistrocladus 속과 관련이 있다고 알 수 있다. 즉, 우리는 한종의 긴 줄을 볼 수 있고, 많은 종들이 이어져 나오는 것을 볼 수 있다. 네펜데스 속의 경우엔 100가지가 넘고 Ancistrocladus 속은 20종이 넘고 Triphyophyllum 속은 단형속의 3개 계통 분기만 속한다. 이것이 뜻하는 건 네펜데스나 원시 네펜데스 종들이 단 한 종만 존재한다는게 아니란 것이다. 이것은 100가지의 원시 네펜데스가 우리가 알고 있는 현재의 하나의 종만 살아남았을 경우도 있다는 것이다. Drosophyllum 속의 다른 2종의 분기도 위의 경우와 같습니다. 이건 물론 우리가 아직 발견하지 못하거나 테스트하지 못한 네펜데스 속에 있을 거라고 추정하고 있다. 그리고 우리가 실제로 추적하고 있는 엽록체 혈통은 오직 역사속에서 살아남은 엽록체 혈통 뿐입니다. 교배와 배수체로 인해 많은 핵 융합과 그 혈통이 그대로 보존되어 왔을 것이다. 네펜데스 속에서 가장 기초가 되는 종은 Nepenthes distillatoria 이다. 이 종은 다른 네펜데스 종들과 원시적인 특징이 연결되어 있는 곳이 없다. 그리고 N. pervillei, N. madagascariensis, N. masoalensis, and N. khasiana,도 다른 종들과 원시적인 연결점이 없다. 이러한 잘 알려진 종들은 matK 염기 서열 변화로 인해 실제 조상에서부터 멀어져 왔을거라고 추측되고 다른 면에서 보면 무엇 때문에 이 네펜데스 속들을 만들었는가 이고 그저 보는 것 만으론"조상" 이라고 생각할 만한 요소가 없다. 우리가 만약 네펜데스 속의 화석을 찾아낸다면 그 화석은 기초적인 종을 알게 되고 그것은 완전히 현대적일 것이다. 이러한 기초 종들에 더해서 기초에 가까운 N. danseri, N. neoguineensis, N. tomoriana and N. paniculata plus N. ampullaria and N. bicalcarata 들은 다른 네펜데스 속들과 한가지 특성이 완전히 다른 종들이다. 이들은 총상꽃차례같은 차례 보다 완전한 원추꽃차례이고 이 속 들에서 더 일반적인 화서다. Ancistrocladus, Triphyophyllum 와 그에 연관된 종들은 또한 원추 꽃차례이고 이것 때문에 우리는 원시적인 네펜데스 속은 원추 꽃차례라고 추정할 수 있다. 우린 네펜데스 속의 생김새는 상관 없이 원시 네펜데스 종에 대해 조금 더 알고 있다. 모든 네펜데스 종들은 8배체(8개의 복제된 염색체) 로 실험됐고 그것은 2n=80 이지만 세포학적으로는 2배체로 나타난다. Lowrey (1991)는 일반적으로 유전자 복제 레벨이 실험된 동질 효소들이 이배체이고 각각의 뜻은 없어지거나 작동하지 않는 동질 효소들을 복제제가 대체하는 것을 확인했다. 완전한 레벨의 유전자 배수체들을 확인하려면 조금 더 깊숙한 연구가 필요하다. 배수체가 되는 방법엔 여러 가지가 있다. 이 그룹에서 가장 흔한 방법은 불임을 낳는 종들을 생산하는 이종 교배된 종들 간의 교배를 통해서이다. 내가 대부분의 불임 종들이라고 말한 이유는 이수체의 생식 세포를 생산하게 되는 아주 적은 가능성이 제대로 교배만 된다면 더 안정적이고 생식력 높은 자손들을 만들기 때문이다. 콜히친은 높은 확률로 이런 자연적인 현상을 유도하는 데 사용된다. 이런 두배 증가가 세번 일어나야 하는데 두번은 다른 두개의 4배체를 얻는데 사용되고 이들의 교배가 배수가 되어 8배체가 됩니다. 이러한 높은 수준의 유전자 변형 정도를 주는 것은 결과적으로 그들에게 새로운 형태의 유전적 변이를 허용하고 새로운 형태의 기능을 개발할 수 있게 한다. 이러한 높은 배수체는 식물의 생리학적 비용을 요구하는데, 이것이 시간이 지날 수록 자주 염색체가 사라지고, 추가된 유전자가 사라지는 이유다. 배수체를 만들어내는 건 쉽지만, 여기서 이점을 가져오는 결과를 얻는 건 시간이 걸린다. 매우 긴 시간이. 그리고 네펜데스 속이 Drosophyllum/Triphyophyllum 속 라인에서 빠져나오는 데에선 Drosophyllum 이 2배체가 되는 사건 이후 부터 일 것이다. 이 모든 네펜데스 종들과 잡종들이 성공적으로 번식하는 것과 더불어 유전적으로 자라난 후손들을 양성하고, 최근에 진화된 유전자를 상대적으로 안정된 게놈으로 그려 낸다. 이러한 네펜데스 게놈의 세부 사항들은 현재 종들의 복제들로 어떻게든 시간이 지나면 필연적으로 완성 될 것이다. During the period between the split with its relatives and the radiation of the known Nepenthes species, something else genetic happened besides polyploidy. Nepenthes switched from having hermaphrodite flowers to being dioecious. That is its relatives have flowers containing both male and female parts. Nepenthes plants produce either male or female flowers but not both. And the flowers are very male or very female; there is no vestige left of the other sexual parts. This is not common among flowering plants. About 10% of flowering plant species are gender dimorphic and but less than 1% are are fully dioecious (See Barrett 2002 and Wikipedia Plant Sexuality for a review). A little is known about how to evolutionarily get from from having both sex flowers to single sex flowers. Miller and Venable (2000, 2002) found an example of such a transition in the genus Lycium (Boxthorn, Solanaceae) facilitated by polyploidy. Most Lycium species are self-incompatible (the female part of the flower kills the pollen with the same genotype as its own to avoid self fertilization, see Wikipedia Self-incompatibility for more information) but some species are gender dimorphic; in this case having either female flowers or hermaphrodite flowers. Miller and Venable found the gender dimorphic species are polyploid and hypothesized that doubling the chromosomes disrupted the self-incompatibility system and created a situation were a male sterile mutation was able to spread throughout the population producing essentially female plants and both-sex plants. This is an example of how our genetic system can cause seemingly bizarre things to happen evolutionarily (McDonald 1974). At the time of the self-incompatibility disruption there is a window of opportunity for sex-specific sterility genes to invade and/or persist in the population leading to sexual dimorphism. Until there has been time for the gene frequencies to reach an equilibrium, male sterile (i.e. female) plants would experience much less inbreeding depression and thus have higher fecundity and produce more healthier offspring than plants that are hermaphrodite and self pollinate. This would be the first step in the evolution of dioecious plants. The next step would be for the hermaphrodite flowers to become female sterile (i.e. male) and thus not put the energy into seeds that will not contribute much to the next generation. Of course the species will commit suicide if all the plants are male sterile or female sterile so the system needed to evolve with a proper balance and controls for there to be Nepenthes today. Could the Nepenthes ancestor have been self-incompatible? Self-incompatibility is very common in flowering plants with some estimates on the order of 50% of species. Current dogma says evolution of self-incompatibility is very rare while the disruption of self-incompatibility is relatively common. Self-incompatibility continues to be common because self-incompatible species can maintain more genetic variation and are more likely to produce new species. Since many Drosera species in Australia are self-incompatible it is likely the ancestor of this whole group was self-incompatible. Self-incompatibility was lost in "simple" ways multiple times in Nepenthes relatives, something uncommon happened with Nepenthes. So a lot had to happen genetically to Nepenthes after its split with its ancestors. And we have not even gotten to the most obvious feature of the genus, its pitchers. (This will be covered separately.) All these evolutionary changes take time. Lots of time. How much time are we talking about? To put dates on the Nepenthes phylogeny we need fossils. For these plants the relevant fossils are mostly pollen. The Wikipedia Droseridites page has a good summary of the pollen data associated with Nepenthes. It is apparent that in the 35 to 65 million year old time frame it is difficult to link the pollen fossils to existing genera so we end up with, "well, it is most like Nepenthes but it could be Drosera". For all we know, the fossils could all be representatives of totally extinct lineages. What they do indicate is proto-Nepenthes-like plants were abundant along the northern Tethys Sea in what is now Europe and southwestern Asia. However there are candidates in other locations as well. An additional fossil of interest in this time frame is a winged fruit similar to Triphyophyllum named Dioncophyllites amurensis Fedotov. It was found in central Asia. Manchester, et al. (2009) document a number of east Asian endemic plants with fossil records along the northern Tethys. Similar studies should also be done for Africa. If we assume Nepenthes and the related genera Drosophyllum, Triphyophyllum, Ancistrocladus, Habropetalum and Dioncophyllum or their progenitors originated along the Tethys Sea and the locations of these plants today are the results of migrations from the Tethys, the pompom phylogenies make sense. We basically lost all the Tethys ancestors from climate change as the continents migrated and the Tethys Sea disappeared. This would mean Nepenthes as we know it evolved in what is now Europe and/or southwest Asia and at one point a fully modern Nepenthes arrived in the region of what is today the Indian Ocean. Similarly the progenitor or progenitors of Ancistrocladus and Triphyophyllum / Habropetalum / Dioncophyllum migrated to Africa. Nepenthes pompomed some time after arriving at its new home. Ancistrocladus did the same to become quite widespread across Africa and southeast Asia including areas where Nepenthes is found but not necessarily immediately after leaving Europe. An alternative hypothesis, Nepenthes out-of-Africa, proposes that Nepenthes took the India taxi from southern Africa to Asia with the basal species getting off the taxi as Madagascar and the Seychelles split from India. For this to work fully modern Nepenthes would have had to exist over 100 million years ago. If they had existed at that time in that location we would expect to find Nepenthes in South America and Africa today along with other plants that were in that area at that time. Even if we assume they were there and are now lost or not found yet, a date this old is not concordant with the fossil dates or with the pompom configuration of the phylogenetic tree. The out-of-Europe hypothesis makes the European Drosophyllum lusitanicum quite interesting. It would be very easy to confuse all the other species we have been discussing if you ignore the pitchers on Nepenthes and the occasional sundew-like leaves on Triphyophyllum. The plants are all more or less tropical lianas with similar leaves, stems, and flowers (if you squint). Without Triphyophyllum and its sundew-like leaves, the presence of the sundew-like Drosophyllum in this clade would stand out as being most bizarre. A sundew in the middle of clade of lianas? But unlike the surviving lianas, Drosophyllum did not migrate out of Europe. Europe is no longer a liana kind of place. Drosophyllum apparently adapted to the changed climate in Europe by losing all the no longer useful liana characters and taking the form of a full-on sundew. |
In the taxonomy literature for the Caryophyllales carnivores there are references to cyme, panicle, raceme, and probably other inflorescence forms. Although these terms may have specific definitions that some taxonomists and plant anatomists might agree to (Wikipedia) the plants could not care less about definitions and do whatever works for them. It is not clear Nepenthes inflorescences fit any of the strict definitions exactly but panicle is closest. Raceme appears to be used most for inflorescences typical for Drosera where the flowers open in a wave going up the stem—never mind Nepenthes does that too but it is more subtle. Evolutionarily, changes in inflorescence structure and the pattern of flower opening generally involve simple changes in gene regulatory sites coordinating when, where, and how much hormone genes are expressed.
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The data used for this tree are chloroplast trnK gene intron, matK sequences from Heubl, Meimberg, and associates (see references) greater than 2000 bases long and deposited with NCBI. Care was taken to exclude the translocated matK pseudogene sequences in the figure. Analysis was via NCBI Blast, graphed via PHYLIP drawtree unrooted tree drawing program and detailed manually in a drawing program. The lengths of the lines are an indication of genetic distance. Triphyophyllum peltatum, Drosophyllum lusitanicum, and a number of Ancistrocladus species were used as the outgroup. When two samples of the same species were included in the analysis, the one with the best quality sequence was selected for the figure. Some samples may be misidentified as to species. "N. alata" was not included because the source location information given in Meimberg et al. (2001) was impossible for that species. There were no clarifications or corrections in later publications where the same data were used. Although the probability is small that "N. thorelii" is really N. thorelii, the plant used for the study could be a related species although it could be a hybrid outside its species group. It is left it in for the sake of discussion. Specific details of the phylogeny could change with additional species or analysis with a different program. The data used for the tree are raw sequence data not cleaned to removed artifacts. The tree contains four species marked as "not accepted" in the Carnivorous Plant Names Database:
In four cases direct examination of the sequence data do not support the assertion in the database. In one it is too close to call. The classification problems could be inadequate description of the species. Floristic region data used are from the Carnivorous Plant Names Database and are grouped by geographic association. Floristic regions may not be fully concordant with geographic features or locations of the same or similar names. |
The dioecious nature of Nepenthes puts some fairly strong constraints on the migration of species to new locations. One seed of a plant with flowers that can self pollinate is enough to found a whole new branch of the species at some distant location. But one seed of Nepenthes is not enough. The plant will be either male or female. If it is male there is no hope of producing seeds on its own. If female it might produce seeds without pollination but they should be clones and thus females as well. Since Nepenthes plants are potentially immortal they can hang out for a long time and maybe a second seed of the opposite sex will happen along. Maybe very very rarely a plant will produce flowers of both sexes. It is of course possible two or more seeds will hitchhike to the same place at the same time. On the other hand since all Nepenthes species can interbreed and in fact function genetically like one hyper-variable species, one seed can bring in a lot of new variation if it establishes a plant among a population of another species. This can lead to the establishment of new taxonomic characters in that population leading humans to name that population as a new species. This has undoubtedly happened many times in Nepenthes so a phylogenetic tree produced from nuclear genes would probably lay out differently from the tree above which is based on chloroplast genes. Meimberg et al. (2006) did a limited study of nuclear genes and did in fact find some species that showed different associations. I do not feel confident enough in the public data to reproduce it here. The chloroplast phylogenetic tree for Nepenthes is consistent with an initial population of Nepenthes in India and/or southwest Asia that gave rise to the species now in Madagascar and the Seychelles via long distance dispersal. The species now in Sri Lanka and Assam could be more direct remnants of that ancestor with the Sri Lanka population isolated very early. It would be incorrect to say any of the exiting species today is the ancestor although with the current analysis N. khasiana is the last species to branch off before the ancestor infested the Malay Peninsula and Borneo founding the bulk of the genus. It appears there were two or more introductions into the Malay/Borneo region or if it was one introduction there was a split into at least four branches soon after. One branch was probably on the west side of Borneo and the others on the coast and in the islands on east side of Borneo. If this event happened 20 to 35 million years ago simple within-landmass dispersal would be sufficient to stage the species near where their future homes would appear. If it happened later, longer distance dispersal over a few hundred km of ocean could have established major branches of the eastern population in Sulawesi, the Maluku (Moluccan) Islands, and the Philippines when they were closer to Borneo than they are today. So how could all this movement happen? Nepenthes seeds are typically wind dispersed. If you have ever handled the seeds or tried to make seed packets you will know they get everywhere. Typical strong storm winds will probably move seeds on the order of a km. However it is quite reasonable that major tropical storms will disperse seeds over distances on the order of hundreds of km. Can this account for what we can deduce from the phylogeny? Yes it does for clusters of species on large landmasses like the initial migration around proto-Borneo and the more recent invasion of high elevations. But it does not work for the very long distances seen in the phylogeny. During periods of low sea level such as during the recent Pleistocene ice ages it is quite possible Nepenthes species spread across the land bridges. This may account for the distribution of a few species but it is not a general feature of Nepenthes range expansion and evolution. N. albomarginata, N. rafflesiana, N. eymae, N. tentaculata, and N. gymnamphora might fit this scenario but they may have already been in place before the land bridges. The most likely agent for long distance dispersal of Nepenthes is migrating shorebirds and seabirds. During their migration the birds will stop in coastal marshes to recharge before moving on. Some islands also have large rookeries of seabirds. It is hard to explain the presence of Nepenthes on Madagascar, the Seychelles, New Guinea, New Caledonia, and other islands such as Palau in Micronesia any other way. What is most astonishing is the range over which N. mirabilis is found. Most Nepenthes species have very small ranges. A range map for N. mirabilis would include almost everywhere other Nepenthes are found in south east Asia and then include an area well beyond that. Charles Clark reports that even though this species is widespread it tends to live in coastal swamps where few if any other Nepenthes species are found. It would be interesting to know if N. mirabilis has special adaptations to facilitate migration or if just being in coastal swamps is enough. The dioecious nature of Nepenthes limits migration. Does having separate sexes also explain the large number of species in a relatively small area? It does. The impossibility of self pollination in Nepenthes allows small populations to maintain large amounts of genetic variation relative to species that can inbreed. With this variation the populations can develop characters that taxonomists would use to define species. So if the explosion of species in Sumatra of the clade with N. lavicola at the base resulted from the rise of the Barisan Mountains, both isolation between populations on new mountains and the generation of new habitats gave great opportunity for Nepenthes to create new forms. A selfing species would be less likely to be able to maintain that much variation in characters. That is not to say it could not happen. With selfing species we might refer to the variant populations as "ecotypes". With Nepenthes each ecotype or population could have enough distinguishing taxonomic characters to be referred to as separate species. This results in each mountain having its own species of Nepenthes while a patchy but widespread self-compatible Drosera species such as D. ultramafica growing in association with Nepenthes on mountain tops shows little variation between distant locations. -- John Brittnacher |
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출처 : http://www.carnivorousplants.org/cp/EvolutionNepenthes.php
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