The true evolutionary position of eukaryotes is hence the subject of continuing debates and it has still not been widely agreed if eukaryotes represent a separate domain Williams et al. Alphaproteobacteria is known to be the ancestor of mitochondria Roger et al. However, our understanding of the archaeal lineage that gave rise to the eukaryotic nuclear genome is still insufficient. Asgard archaea, which were recently identified based on metagenome-assembled sequences Spang et al.
The aim of this opinion manuscript is to debate this newly introduced term. Polyphyletic groups are not taxonomically desirable, and traditionally, characters shared by members of such a group represent homoplasies analogies , i.
A historical error occurred when Hennig , defined two groups, monophyletic and paraphyletic, based on the inclusion of all descendants of a given ancestor. If all the descendants of a given ancestor belonged to one group, it was regarded as a monophyletic group, and if this was not the case, it was regarded as a paraphyletic group Hennig, , Missing from such definition was the distinction between a group with a single ancestor and a group that includes all the descendants of an ancestor, which were both defined as monophyletic by Hennig , A—C Schematic overview of apomorphic characters and states.
Synapomorphy is shared by all the members of a group descending from a single ancestor. Plesiomorphy is ancestral and as such not present in all the members of a group. Monophyletic groups are characterized by apomorphies: synapomorphies in holophyletic or plesiomorphies in paraphyletic groups. Topology of the cladograms shown in A—C is the same, but the distributions of characters and their states are different.
Case A shows a paraphyletic group from which a holophyletic descendant is excluded. Case B shows a paraphyletic group with two holophyletic descendants excluded. Case C shows two paraphyletic groups and a holophyletic group. LECA is the last eukaryotic common ancestor, which originated via a polyphyletic event: symbiogenesis of an archaeon A which gave rise to nuclei, and Bacteria B , specifically Alphaproteobacteria, which gave rise to mitochondria.
Cyanobacteria C are a group of bacteria from which the primary plastid D originated. The dotted lines represent groups with uncertain positions within Eukaryotes. Well-known examples of holophyletic groups are mammals descendants of Therapsida , snakes descendants of earless and legless lizards , birds descendants of Dinosauria , modern amphibians, tetrapods land vertebrates, descendants of fish , jawed vertebrates, bilaterians bilaterally symmetric animals , animals, and eukaryotes Pough et al.
Examples of paraphyletic groups are reptiles or amniotes whose descendants are mammals and birds , amphibians a group including Lissamphibia and extinct amphibians whose descendants are reptiles , sarcopterygians whose descendants are tetrapods , fish Pisces as they include all vertebrates excluding those inhabiting land , jawless fish lampreys, hagfish, and extinct groups related to them, whose descendants are also jawed fish , bryophytes in wider sense as land plants are their descendants , streptophytes stonewort and relatives, if plants are excluded , archaeplastids as secondary plastids of SAR and euglenoids are not considered to be archaeplastid members anymore , cyanobacteria because plastids are regarded as organelles, not cyanobacteria anymore , prokaryotes because eukaryotes are excluded , Archaea because the nucleus is not regarded to be an archeon anymore , and Bacteria because mitochondria are not regarded as Alphaproteobacteria anymore; Pough et al.
If we ignore the presence of mitochondria and existence of lateral gene transfer from bacteria to the eukaryotic host, the origin of the eukaryotic nucleus could be compared to the origin of mammals and birds within amniotes, as described in Fournier and Poole However, the origin of eukaryotes is not comparable to the origin of these groups, and the bacterial contribution to eukaryogenesis should not be neglected.
Eukaryotes are of polyphyletic origin, as their ancestor, LECA, sits on both branches of life—the archaeal Asgard and the bacterial branch Alphaproteobacteria. Well-established examples of natural polyphyletic events include lateral gene transfer LGT in prokaryotes Nelson-Sathi et al.
Genes can also be of polyphyletic origin; those genes are known as chimeric genes e. A lineage of polyphyletic origin should not be united with any of its ancestors in an attempt to form a higher monophyletic group, as it will not result in such.
Even though eukaryotes are a monophyletic and holophyletic group by definition, they are of polyphyletic origin because of the very nature of their ancestor's, LECA's origin. Today, the polyphyletic origin of eukaryotes is a well-supported scientific theory. Eukaryotic syn apomorphies are the traits of eukaryotic complexity: nuclei, mitochondria, Golgi apparatus, endoplasmic reticulum, and sexual reproduction Koonin, ; Koumandou et al.
Eukaryogenesis is not a unique example of polyphyletic origin of a monophyletic group. Other such events are widely dispersed in the tree of life. Known examples are hybrid species, which originated via hybridization of two species, usually but not always from the same genus Seehausen, ; Grant and Grant, ; Meier et al. Homo sapiens is an example of such species. It is a hybrid between H. The Jutland bow-winged grasshopper Chorthippus jutlandica is a unique species which originated from the hybridization of C.
Domestic wheat is a hybrid between species belonging to the genera Triticum and Aegilops Ozkan et al. There are even examples of one of the ancestral species being extinct, but its mitochondrial genome still being present, which is called a ghost lineage Recuero et al. There is no example of a natural monophyletic group that could be composed of any of the aforementioned species and one of its parents, as is the case with Eukaryotes, Asgard, and Eukaryomorpha.
Lichens not only gave rise to the concept of symbiosis de Bary, , but they are also the classical example of organisms that originated by symbiogenesis Lutzoni and Miadlikowska, Symbiosis is species-specific Lindsay, , co-dependent, and the symbionts usually cannot survive outside the lichen.
Lichens are an example of a polyphyletic group with multiple polyphyletic origins. The case of Archaeplastida primary photosynthetic eukaryotes is an interesting one and should be addressed in a separate essay. The supergroup originated via plastidogenesis, an anastomosis between cyanobacteria and eukaryotes; and has since contributed to many anastomoses secondary endosymbioses in the eukaryotic tree McFadden, Prokaryotes are not a monophyletic group.
However, a much more widely accepted view is that the prokaryotic group actually share a common ancestor, from which Archaea and Bacteria descend, and excludes a group derived from the archaeal branch although not necessarily from within Archaea proper : Eukarya. Prokaryotes lack a distinct cell nucleus and their DNA is not organized into chromosomes. They also lack the internal structures bound by membranes called organelles, such as mitochondria.
Bacteria have prokaryotic cells. Snails, frogs and food, do not. With an accout for my. Groups that do include all the descendants of the most recent common ancestor are said to be monophyletic. A paraphyletic group is a monophyletic group from which one or more of the clades is excluded to form a separate group as in the paradigmatic example of reptiles and birds, shown in the picture.
These terms were developed during the debates of the s and 70s accompanying the rise of cladistics a clade is a term for a monophyletic group. Before that period the distinction between mono- and polyphyletic groups was based on the inclusion or exclusion of the most recent common ancestor.
It was shown, however, that the inclusion of ancestors in the classification leads to unavoidable logical inconsistencies, and, in some schools of taxonomy, the phylogenetic pattern is described exclusively in terms of nested patterns of the sister group relationships between the known representatives of taxa without referring to the ancestor-descendant relationships.
Many of the older classifications contain paraphyletic groups, especially the traditional 2—6 kingdom systems and the classic division of the vertebrates. Paraphyletic groups are often erected on the basis of sym -plesiomorphies ancestral similarities instead of syn apomorphies derived similarities. Examples of well-known paraphyletic groups includes:.
The term paraphyly may be used in any system in which genetic descent modeled by trees is useful. Brueckner, J. Bacterial genes outnumber archaeal genes in eukaryotic genomes. Genome Biol. Costello, M. Can we name Earth's species before they go extinct? Science , — Die Erscheinung der Symbiose. Strassburg: Verlag von Karl J. Google Scholar. Doolittle, W. Cold Spring Harbor Perspect. Evolution: two domains of life or three?.
Envall, M. On the difference between mono-, holo-, and paraphyletic groups: a consistent distinction of process and pattern. Esser, C. Fournier, G. A briefly argued case that Asgard archaea are part of the eukaryote tree. Garg, S. Mitochondria, the cell cycle, and the origin of sex via a syncytial eukaryote common ancestor.
Gottsberger, B. PubMed Abstract Google Scholar. Gould, S. Bacterial vesicle secretion and the evolutionary origin of the eukaryotic endomembrane system. Trends Microbiol.
Grant, P. Hennig, W. Berlin: Deutcher Zentralverlag. Phylogenetic Systematics. Urbana: University of Illinois Press. Imachi, H. Isolation of an archaeon at the prokaryote-eukaryote interface. Koonin, E. The origin and early evolution of eukaryotes in the light of phylogenomics. Koumandou, V. Molecular paleontology and complexity in the last eukaryotic common ancestor. Ku, C. Endosymbiotic origin and differential loss of eukaryotic genes. Lindsay, W. London: Lovell Reeve and Co.
Lutzoni, F. Lynch, M. Reply to Lane and Martin: Mitochondria do not boost the bioenergetic capacity of eukaryotic cells. MacLeod, F. Asgard archaea: Diversity, function, and evolutionary implications in a range of microbiomes.
AIMS Microbiol. Margulis, L. Symbiosis as a Source of Evolutionary Innovation. Speciation and Morphogenesis. Cambridge MIT Press. Martin, W. A briefly argued case that mitochondria and plastids are descendants of endosymbionts, but that the nuclear compartment is not.
McFadden, G. Primary and secondary endosymbiosis and the origin of plastids. McInerney, J.
0コメント