Which adaptation is indicative of an evolutionarily advanced plant

Local adaptation and ecological differentiation under selection, migration, and drift in Arabidopsis lyrata. Evolution 72 , — Genomic patterns of local adaptation under gene flow in Arabidopsis lyrata. Serpentine: The evolution and ecology of a model system. University of California Press. Evidence for extensive parallelism but divergent genomic architecture of adaptation along altitudinal and latitudinal gradients in Populus trichocarpa. Plant ionomics: from elemental profiling to environmental adaptation.

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In the phylogeny here, taxon A is the cousin of taxa B, C, and D — not their ancestor. This is true even if the organisms shown on the phylogeny are extinct. For example, Tiktaalik shown on the phylogeny below is an extinct, fish-like organism that is closely related to the ancestor of modern amphibians, mammals, and lizards.

Though Tiktaalik is extinct, it is not an ancestral form and so is shown at a tip of the phylogeny, not as a branch or node. The actual ancestor of Tiktaalik , as well as that of modern amphibians, mammals, and lizards, is shown on the phylogeny below. To learn more phylogenetics , visit our tutorial on the topic. On the phylogeny below, the earliest and most recent branching points are labeled. Usually phylogenies are presented so that the taxa with the longest branches appear at the bottom or left-hand side of the phylogeny as is the case in the phylogeny above.

These clades are connected to the phylogeny by the deepest branching point and did diverge from others on the phylogeny first. This means that the characteristics we associate with these long-branched taxa today may not have evolved until substantially after they were a distinct lineage.

For more on this, see the misconception below. Branch length usually does not mean anything at all and is just a function of the order of branching on the tree. However, advanced students may be interested to know that in the specialized phylogenies where the branch length does mean something, a longer branch usually indicates either a longer time period since that taxon split from the rest of the organisms on the tree or more evolutionary change in a lineage!

On longer branches, the protein collagen seems to have experienced more evolutionary change than it did along shorter branches.

The phylogeny on the right shows the same relationships, but branch length is not meaningful in this phylogeny. Notice the lack of scale bar and how all the taxa line up in this phylogeny. The misconception that a taxon on a short branch has undergone little evolutionary change probably arises in part because of how phylogenies are built. Sometimes a particular outgroup is selected because it is thought to have characteristics in common with the ancestor of the clade of interest.

The outgroup is generally positioned near the bottom or left-hand side of a phylogeny and is shown without any of its own close relatives — which causes the outgroup to have a long branch.

This means that organisms thought to have characteristics in common with the ancestor of a clade are often seen with long branches on phylogenies. It may help to remember that often, long branches can be made to appear shorter simply by including more taxa in the phylogeny. For example, the phylogeny on the left below focuses on the relationships among reptiles, and consequently, the mammals are shown as having a long branch. However, if we simply add more details about relationships among mammals as shown on the right below , no taxon on the phylogeny has a particularly long branch.

Both phylogenies are correct; the one on the right simply shows more detail regarding mammalian relationships. Incorporate evolution throughout your teaching. Dealing with objections to evolution. Subscribe to our newsletter. Email Facebook Twitter. Evolutionary theory implies that life evolved and continues to evolve randomly, or by chance. Evolution results in progress; organisms are always getting better through evolution.

Individual organisms can evolve during a single lifespan. Evolution only occurs slowly and gradually. Because evolution is slow, humans cannot influence it. Genetic drift only occurs in small populations. Humans are not currently evolving. Species are distinct natural entities, with a clear definition, that can be easily recognized by anyone. Misconceptions about natural selection and adaptation Natural selection involves organisms trying to adapt. Natural selection gives organisms what they need.

Natural selection acts for the good of the species. Natural selection is about survival of the very fittest individuals in a population. Natural selection produces organisms perfectly suited to their environments. All traits of organisms are adaptations. Misconceptions about evolutionary trees Taxa that are adjacent on the tips of phylogeny are more closely related to one another than they are to taxa on more distant tips of the phylogeny.

Taxa that appear near the top or right-hand side of a phylogeny are more advanced than other organisms on the tree. Taxa that are nearer the bottom or left-hand side of a phylogeny represent the ancestors of the other organisms on the tree.

Taxa that are nearer the bottom or left-hand side of a phylogeny evolved earlier than other taxa on the tree. A long branch on a phylogeny indicates that the taxon has changed little since it diverged from other taxa. Misconceptions about population genetics Each trait is influenced by one Mendelian locus. Each locus has only two alleles.

Misconceptions about evolution and the nature of science Evolution is not science because it is not observable or testable. Evolutionary theory is invalid because it is incomplete and cannot give a total explanation for the biodiversity we see around us. Gaps in the fossil record disprove evolution. Evolution is a theory in crisis and is collapsing as scientists lose confidence in it. Misconceptions about the implications of evolution Evolution leads to immoral behavior.

If students are taught that they are animals, they will behave like animals. Misconceptions about evolution and religion Evolution and religion are incompatible.

Evolution is itself religious, so requiring teachers to teach evolution violates the first amendment. Most of evolutionary biology deals with how life changed after its origin.

Regardless of how life started, afterwards it branched and diversified, and most studies of evolution are focused on those processes.

For example, consider the process of natural selection , which results in adaptations — features of organisms that appear to suit the environment in which the organisms live e. The process of mutation , which generates genetic variation , is random, but selection is non-random.

Selection favored variants that were better able to survive and reproduce e. Over many generations of random mutation and non-random selection, complex adaptations evolved. To learn more about random mutation , visit our article on DNA and mutations. Hence, evolutionary change is not always necessary for species to persist. Many taxa like some mosses, fungi, sharks, opossums, and crayfish have changed little physically over great expanses of time. Mutation, migration , and genetic drift may cause populations to evolve in ways that are actually harmful overall or make them less suitable for their environments.

Climates change, rivers shift course, new competitors invade — and an organism with traits that are beneficial in one situation may be poorly equipped for survival when the environment changes. It is tempting to see evolution as a grand progressive ladder with Homo sapiens emerging at the top. But evolution produces a tree, not a ladder — and we are just one of many twigs on the tree. Populations, not individual organisms, evolve.

Changes in an individual over the course of its lifetime may be developmental e. New gene variants i. We have many examples of slow and steady evolution — for example, the gradual evolution of whales from their land-dwelling, mammalian ancestors, as documented in the fossil record. But we also know of many cases in which evolution has occurred rapidly.

In the most advanced forms found exclusively among legumes Fabales and Gunnera [ 4 ], symbiotic bacteria are delimited from the host cell cytoplasm only by a plant-derived membrane in the mature stage of the symbioses.

In the respective legumes, they develop into bacteroids contained in organelle-like symbiosomes, where nitrogen fixation takes place for a recent review, see [ 5 ]. Bacterial endosymbioses in both legumes and actinorhizal plants are typically associated with the formation of novel plant organs, so-called nodules, which are root-derived in the majority of cases [ 6 ].

Nitrogen-fixing root nodule symbiosis RNS occurs in two major forms. Actinorhiza hosts belong to three eurosid orders Figure 1 and nodulate with Gram-positive actinobacteria of the genus Frankia [ 7 ]. Legumes, on the contrary, enter specific interactions with members of a diverse group of Gram-negative bacteria, termed rhizobia. For almost a century, the extreme diversity in organ structure, infection mechanisms, and bacterial symbionts among nodulating plants obscured the fact that the nodulating clade is monophyletic, which was revealed by molecular phylogeny relatively recently [ 8 ].

The restriction of endosymbiotic root nodulation to a monophyletic group of four angiosperm orders Figure 1 is coincident with a patchy occurrence within this clade. These observations led to the hypothesis that a genetic change acquired by a common ancestor may predispose members of this lineage to evolve nodulation endosymbiosis [ 8 ].

SYMRK regions encoding putative kinase domains exhibit conserved intron positions and phases. Bars illustrate the exon-intron and predicted protein domain structure of representative SYMRK candidates.

Positions of introns are indicated by black arrowheads. Names refer to species sampled and are shaded according to their root endosymbiotic capabilities: black, endosymbiosis with Frankia bacteria Actinorhiza and AM formation; gray, endosymbiosis with rhizobia and AM formation; white, AM formation only. Dashed frames have no phylogenetic implications.

The cladogram depicts relationships of angiosperm orders as deduced by molecular markers [ 53 , 54 ]. The four orders containing nodulating taxa are shaded light gray. Squares at the tips of branches indicate the presence of taxa with particular root endosymbiotic phenotypes colour code is as for sampled plants. Filled and white wedges indicate branches where taxa on order and family level have been omitted, respectively.

Popular species designations refer to Alder, Alnus glutinosa ; Poplar, Po. The molecular adaptations underlying the evolution of plant-bacterial endosymbioses are still a mystery, despite a substantial biotechnological interest in understanding the genetic differences between nodulating and non-nodulating plants. While the molecular communication between legumes and rhizobia has been studied in some detail, important clues are expected from the genetic analysis of the yet underexplored Actinorhiza.

Bacterial signalling molecules and corresponding plant receptors involved in RNS are known only for the legume—rhizobium interaction. Frankia signals may be biochemically distinct from rhizobial chito-oligosaccharide nodulation factors [ 9 ], which would suggest an independent mechanism of host—symbiont recognition in Actinorhiza.

Phenotypic analysis of legume mutants has revealed a genetic link between RNS and Arbuscular Mycorrhiza AM , which is a phosphate-scavenging association between plant roots and fungi belonging to the phylum Glomeromycota [ 10 ]. AM is widespread among land plants, where forms of AM are found in representatives of all major lineages. Fossil evidence for ancient AM-like associations [ 11 ] suggests a role of this symbiosis in the colonization of land about million years ago.

However, the molecular steps involved are not clear. To gain insight into the evolution of nitrogen-fixing root nodulation, we analysed common symbiosis genes across angiosperm lineages with different symbiotic abilities. We discovered exceptional diversification among genes encoding the symbiosis receptor kinase SYMRK in different species Figure 1. While putative SYMRK kinase domains are conserved and contain characteristic sequence motifs discriminating them from related kinases Figure S1 , the predicted extracellular portion of SYMRK occurs in at least three versions of domain composition Figure 1 and Table 1.

The longest SYMRK version is present in all tested eurosids, including nodulating and non-nodulating lineages. Outside of the eurosid clade, which encompasses all nodulating groups, one or more exons are absent from SYMRK coding sequences Figure 1 and Table 1. The presumed ability of its diverged extracellular domain to perceive symbiosis-related signals [ 16 ] renders it a prime target for investigating the molecular adaptations underlying the evolution of RNS.

Furthermore, also non-nodulating members of this monophyletic clade may carry nodulation-competent versions of SYMRK. Nonsilenced control roots of the same plants and roots transformed with a binary vector lacking the silencing cassette transgenic control roots showed wild type—like nodules with lobed structure typical for Datisca Figure 2 A and 2 B.

In conjunction with the well-documented role of legume SYMRK in the interaction with rhizobia [ 16 , 23 ], SYMRK thus represents a common genetic requirement for the two types of bacterial root endosymbiosis. Co-transformed roots express DsRED1 as visible marker. C—H AM phenotype of D. C and D Wild-type and E and F transgenic control roots are well colonized and show arbuscules in inner cortical cells. Such features were not seen in Datisca wild-type or transgenic control roots and are reminiscent of those observed on L.

Roots were inoculated simultaneously with Frankia bacteria and G. A—D L. A and B Transgenic control roots devoid of intraradical hyphae or arbuscules, with aborted fungal infection structures within epidermal cells B and arrow in A. C and D Transgenic control roots showing no nodules. Nodules exhibit pink coloration in white light, indicating the presence of symbiosis-specific leghemoglobins I, O, U, and AA and DsRED fluorescence in inner nodule tissue indicating the presence of M.

Datisca wild-type roots of the same plants used for hairy root induction and independent transgenic control roots formed AM, with dense arbuscular colonization of inner cortical cells Figure 2 C— 2 F. Occasional infection attempts occurred but typically were aborted in the outer cell layers Figure 2 G and 2 H.

We conclude that similar to the situation in legumes, SYMRK of the actinorhizal plant Datisca is involved in both bacterial and fungal endosymbioses. The specific symbiont of Lotus is Mesorhizobium loti , whereas Medicago truncatula Medicago interacts with Sinorhizobium meliloti. Dmi2 5P plants form no infection threads or nodules upon inoculation with either rhizobial strain. Transgenic roots of these plants, and of wild-type control plants carrying LjSYMRK , formed infection threads and indeterminate, pink nodules typical for Medicago [ 24 ] with S.

Medicago dmi2 5P mutants are also impaired in AM. No arbuscules were observed within 2 wk of co-cultivation, with fungal infection being aborted at the root surface or after entry into epidermal cells Figure S2 and Table 2. Upon inoculation with Glomus , symrk roots form no AM, and fungal infections are typically associated with aberrant hyphal swellings and are aborted after entry into epidermal cells Figure 3 A and 3 B, and Table 2. Interaction with M.

In conclusion, consistent with a role of SYMRK in the predisposition to evolve RNS, we could not detect a functional diversification of the eurosid SYMRK version linked to features differentiating actinorhizal or legume nodulation, or to the specific recognition of bacterial symbionts.

Thus, other factors, such as nod factor receptor kinases [ 22 , 27 , 28 ] or yet-unknown additional components, are likely accountable for the fine-tuning of recognition specificity in plant—bacterial endosymbioses within the eurosids. SYMRK from the non-nodulating eudicots Papaver rhoeas poppy and Lycopersicon esculentum tomato represent intermediate length and domain composition Figure 1 and Table 1.

AM formation was fully restored in these roots, whereas nodulation with M. In rare cases, infection threads and small round nodules were observed, which contained bacterial colonies Figure S3 Y and S3 Z. We show here that this endosymbiosis gene is also required for nodulation in the actinorhizal plant Datisca. SYMRK , which is likewise essential for Actinorhiza formation of the tree species Casuarina glauca Fagales [ 29 ], represents the first known plant gene required for Actinorhiza, indicating a shared genetic basis of the two different types of RNS.

A future task will be to determine whether further endosymbiosis genes acting in concert with SYMRK in legumes are also required for Actinorhiza. This is consistent with the observation that loss-of-function mutations in the rice version of the legume symbiosis gene CCaMK results in loss of AM symbiosis [ 30 ]. Our survey of SYMRK sequences across angiosperms revealed at least three structurally distinct versions, and we could show that this polymorphism is functionally related to the root symbiotic capabilities of host plants.

An attractive hypothesis is that SYMRK sequence divergence was a critical step in mediating the recruitment of the otherwise conserved common symbiosis pathway from the pre-existing AM genetic program. Recruitment was proposed to account for the genetic link of AM and nodulation in legumes [ 17 , 18 ] and would make root—bacterial endosymbiosis as a whole a fascinating example for novel traits evolving on the basis of pre-existing genetic patterns. A common feature associated with endosymbiotic bacterial infection in both actinorhizal [ 33 ] and legume hosts [ 34 ] is the formation of intracellular pre-infection threads PITs in host cells.

These cytoplasmic structures resemble the pre-penetration apparatus PPA preceding fungal infection during AM formation [ 35 ]. Forming in anticipation of bacterial symbionts, PITs are thought to coordinate the uptake of bacteria and determine the spatial progression of infection through the host cell [ 33 , 34 ]. A similar role in guiding fungal transition through host cells in AM has been demonstrated for PPAs [ 35 ]. These developmental similarities in AM, Actinorhiza, and legume-rhizobium infection may reflect a common genetic program for endosymbiosis establishment and symbiont uptake in all three types of interactions.

It is therefore possible that a recruitment of AM symbiosis genes during the evolution of RNS facilitated the induction of intracellular accommodation structures in response to bacteria.