Johri MM Hormonal regulation in green plant lineage families. Transactions of the Royal Society of Edinburgh. Kutschera U Acid growth and plant development.
Evolutionary developmental biology of plants
Origin of vascular plant cellulose synthase? Abstract Plant function and evolutionary biology. Shopping Cart: empty. Search our journals. Previous Next Contents Vol 36 8. Other pattern-matching programs may be helpful in this regard Yan et al. To summarize, sequence data are increasingly available to permit—even demand—phylogenetic analyses. However, identifying the genes that underlie diversification of plant form is proving to be difficult from sequence analysis alone. Differences among both plant and animal species are likely to be due to regulatory evolution Doebley and Lukens, ; Lee et al.
While analyses of coding sequences are necessary for EDG, they are insufficient. One way to determine whether a gene could have a different developmental role in different species is to examine its expression pattern. If the expression pattern varies, then it suggests that perhaps the gene is being used in a different way to produce different morphologies.
Here the distinction between biochemical function and developmental role becomes particularly critical. Expression studies cannot illuminate biochemical function, nor can they determine whether it is conserved or modified.
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However, for EDG, a change in developmental role is as interesting as a change in biochemical function. Comparative gene expression data can help test developmental hypotheses. For example, Vollbrecht et al. They then examined the expression of orthologous genes in closely related species Miscanthus and Sorghum , which developed short inflorescence branches at different times during development.
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In all cases, onset of ra1 expression correlated with formation of short branches, consistent with the developmental hypothesis. However, when multiple disparate species of grasses were investigated, gene expression was restricted to the apical flower only in species with two- or three-flowered spikelets in which floral maturation proceeded from the top down. In species with multiple flowers per spikelet and maturation from the bottom up, LHS1 was expressed in multiple flowers and not restricted solely to the apical one.
Thus the protein appears to have changed its developmental role during evolution. For example, Lee et al. In Arabidopsis, CRC is required for development of nectaries.
Lee et al. CRC orthologues were also expressed in extrafloral nectaries in Capparis flexuosa Capparaceae and in Gossypium hirsutum Malvaceae , consistent with the hypothesis that the genes are indeed necessary for nectary development. This hypothesis was then tested and supported by viral-induced gene silencing, and by transformation experiments in Arabidopsis.
In contrast, CRC was not expressed in nectaries in Aquilegia , a basal eudicot. This suggests that the deployment of CRC for specification of nectary tissue originated at the origin of the core eudicots. The most precise data on gene expression comes from in situ hybridization data rather than from reverse transcription—PCR RT—PCR , by providing information on the cellular location of gene expression.
Despite the power of in situ hybridization, relatively few laboratories are using it in a comparative context. There are several reasons for this. First, appreciable background information is necessary to interpret the results. Also, the developmental morphology of the plants needs to be understood in enough detail to interpret thin sections from all developmental stages. The second reason for the paucity of in situ data has to do with challenges of the technique combined with the sociology of science. It requires someone who is familiar enough with anatomy and morphology to carry out careful dissections, fixation, and sectioning—all the skills of a classical morphologist.
It also requires someone who is able to work with sequence data and molecular evolution—the skills of a molecular evolutionist. Finally, it requires the patience and ability to work with multistep laboratory procedures, including working with RNA—the skills of a molecular biologist. Few students and post-docs have the time or interest in developing this full range of skills.
Immunolocalization, which assesses the cellular location of proteins rather than RNA, is much less technically challenging than in situ hybridization, and is considerably faster. Polyclonal antibodies are often quite tolerant of modest differences in protein sequence, permitting the same antibodies to be used on multiple species, often as part of the same experiment. Monoclonal antibodies, because they are based on a single epitope, are likely to be too species specific, making them less suitable for cross-species comparisons.
Immunolocalization was used productively to examine the localization of photosynthetic enzymes in independently derived C 4 grasses Sinha and Kellogg, These authors found that the two carboxylases, Rubisco and phosphoenolpyruvate carboxylase, were expressed in bundle sheath and mesophyll, respectively, in all C 4 lineages, but that pryvuate orthophosphate dikinase PPDK , light-harvesting chlorophyll a , b -binding protein LHCP , and the two malic enzymes NAD-ME and NADP-ME were expressed in different tissues in representatives of different C 4 lineages.
More recently, Bharathan et al.
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These two studies together showed that the enormous diversity of leaf morphology seen in seed plants can be explained in part by variations in the localization of KN1-like and PHAN-like proteins. Despite the appeal of immunolocalization, production of antibodies to plant proteins is difficult, time-consuming, and often unsuccessful. The field of EDG would certainly be advanced by the production of antibodies to as many plant proteins as possible, but this would require a community-wide effort and dedicated funding.
Nonetheless, as antibodies become available for developmentally interesting proteins, they could be used much more widely than they are now. A final limitation to gene expression studies is availability of funding. In the USA at least, funding is difficult to obtain for comparative gene expression as a project of its own. Funding can be garnered for alpha taxonomy and molecular phylogenetics, but simply exploring the expression of a developmentally interesting gene in multiple species is seen as something outside the purview of systematics.
Plant evolution and development in a post-genomic context | Nature Reviews Genetics
At the same time, comparative gene expression data can determine when in evolutionary time gene function biochemical or developmental has changed but not how it has changed. The data are thus incomplete from the point of view of a developmental geneticist and not fundable by agencies focused on the identification of gene function. EDG requires information on which genes might affect a given phenotype. The more precisely a set of candidate genes can be specified, the more likely are comparative studies to be able to identify critical changes in those genes.
Until the late s and early s when genes controlling morphology began to be cloned from Arabidopsis , there was no gene list—no one knew how many genes might be involved in controlling morphological development, and gene function and developmental role were known for only a tiny handful of genes. The cloning and functional description of the MIKC-type MADS box genes in Arabidopsis , and generation of the ABC model, provided some early hypotheses about the possible control of floral morphology and hence floral diversity Coen and Meyerowitz, This provided a small but plausible list of candidates and spawned a minor growth industry in exploring the evolution, expression, and function of MADS box genes.
This has led to a number of striking results and emerging hypotheses Theissen et al. Nonetheless, this relatively small number of developmental regulators cannot explain all of angiosperm diversity.
A striking example of the former approach involves the production of floral pigments in species of Antirrhinum Schwinn et al. In this study, three R2R3 Myb genes were cloned and characterized in Antirrhinum majus , which together are responsible for the distribution and intensity of floral colour and patterning. Various accessions of A.
Segregation of the F 2 progeny in each of these crosses showed that different alleles of the three Myb genes could account for many of the differences in floral colour among the species of the genus. In QTL mapping, two plants that differ in one or more interesting traits are crossed, their F 1 progeny self-pollinated or backcrossed to one of the parents, and the resulting segregating population of plants used to construct a genetic map using DNA-based markers.
Members of the population backcross or F 2 are also scored for the phenotype of interest. If variation in the phenotype correlates with allelic variation at a particular marker, it provides evidence for a gene near that marker controlling the particular phenotype. This approach has been used, for example, to determine the number of genes underlying morphological differences between two species of Mimulus Bradshaw et al.
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If the DNA-based markers can also be placed on a map or genome sequence of a model organism, it becomes possible to clone the gene underlying the QTL. To date, only a few genes have been cloned in this way Liu et al. Thus, if the gene underlying the QTL can be identified, a function is likely to be ascribed. QTL studies in non-model species are likely to uncover genes that do not vary in the model.
For example, in studies of foxtail millet and its wild relative, green millet, a major QTL was identified on foxtail chromosome VI controlling axillary branching Doust et al.
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The foxtail genome is apparently rearranged in this region compared with maize and rice, but to the extent that comparisons can be made, neither maize nor rice appears to have known mutants mapping to this region with phenotypes that affect vegetative branching. QTL analyses are statistically powerful, but have low resolution, and it is often difficult to localize the phenotype precisely enough to target a single gene Whitt and Buckler, ; Flint-Garcia et al. Additional precision can be obtained by generating a set of recombinant inbred lines RILs or near isogenic lines NILs , although this requires a major investment of resources Lynch and Walsh, Another approach that is used in some models is association analysis, which in contrast to QTL analysis can have very high resolution, albeit low power Whitt and Buckler, ; Flint-Garcia et al.
In association analysis, many closely related plants are genotyped, using any of several molecular markers. The resulting genotypes are then searched for mutations that correlate with phenotypic variation. This requires a large amount of genotyping for every correlation detected. The advantage, however, is the high precision with which different alleles can be characterized. A final challenge, not really solved for any system other than a handful of models, is how to prove that the different forms of the gene of interest really did cause the morphological transition observed.
If, for example, it was possible to identify a plausible candidate that affected cotyledon number, and it could be demonstrated that different protein structures and different regulatory patterns correlated with the presence of one versus two cotyledons, how could it be proved that modifications in this gene had led to the origin of the monocots? For many evolutionary biologists a strong correlation would be sufficient evidence, but most geneticists would like additional proof, including information on biochemical function.
One way to prove the connection of a genotype with a phenotype is by one or more experiments in which all or part of the locus of interest is placed in a model plant and the resulting phenotype observed.