- 26.1C: Evolution of Angiosperms
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- Ecology And Evolution Of Flowers Oxford Biology
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Aizen and Diego P. Eckert, Karen E. Samis, and Sara Dart The evolution of separate sexes: a focus on the ecological context — Tia-Lynn Ashman Effects of colonization and metapopulation dynamics on the evolution of plant sexual systems — John R. Pannell Floral design and the evolution of asymmetrical mating systems — Spencer C.
Barrett and Kathryn A. Conner Geographical context of floral evolution: towards an improved research programme in floral diversification — Carlos M. Johnson Floral characters and species diversification — Kathleen M. Kay, Claudia Voelckel, Ji Y. Yang, Kristina M. Hufford, Debora D.
Kaska, and Scott A. Hodges Floral biology of hybrid zones — Diane R. Campbell and George Aldridge. Click on the cover image above to read some pages of this book! The reproductive organs and mating biology of angiosperms exhibit greater variety than those of any other group of organisms.
Flowers and inflorescences are also the most diverse structures produced by angiosperms, and floral traits provide some of the most compelling examples of evolution by natural selection.
Given that flowering plants include roughly , species, their reproductive diversity will not be explained easily by continued accumulation of case studies of individual species. Instead a more strategic approach is now required, which seeks to identify general principles concerning the role of ecological function in the evolution of reproductive diversity.
The Ecology and Evolution of Flowers uses this approach to expose new insights into the functional basis of floral diversity, and presents the very latest theoretical and empirical research on floral evolution. Floral biology is a dynamic and growing area and this book, written by the leading internationally recognized researchers in this field, reviews current progress in understanding the evolution and function of flowers. In maize , a mutation in only one gene called abphyl abnormal phyllotaxy was enough to change the phyllotaxy of the leaves.
It implies that sometimes, mutational tweaking of a single locus on the genome is enough to generate diversity. The abphyl gene was later on shown to encode a cytokinin response regulator protein. Once the leaf primordial cells are established from the SAM cells, the new axes for leaf growth are defined, one important and more studied among them being the abaxial-adaxial lower-upper surface axes. The genes involved in defining this, and the other axes seem to be more or less conserved among higher plants.
These proteins deviate some cells in the leaf primordium from the default abaxial state, and make them adaxial. It is believed that in early plants with leaves, the leaves just had one type of surface - the abaxial one. This is the underside of today's leaves.
26.1C: Evolution of Angiosperms
The definition of the adaxial identity occurred some million years after the abaxial identity was established. How the infinite variety of plant leaves is generated is a subject of intense research. Some common themes have emerged. One of the most significant is the involvement of KNOX genes in generating compound leaves , as in tomato see above. But this again is not universal. For example, pea uses a different mechanism for doing the same thing. There also exist different morphogen gradients in a developing leaf which define the leaf's axis.
Changes in these morphogen gradients may also affect the leaf form. Another very important class of regulators of leaf development are the microRNAs , whose role in this process has just begun to be documented. The coming years should see a rapid development in comparative studies on leaf development, with many EST sequences involved in the process coming online. Molecular genetics has also shed light on the relation between radial symmetry characteristic of stems and dorsiventral symmetry typical for leaves.
James stated that "it is now widely accepted that In fact, it is simply the timing of the KNOX gene expression! The flowering plants have long been assumed to have evolved from within the gymnosperms ; according to the traditional morphological view, they are closely allied to the gnetales.
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However, recent molecular evidence is at odds to this hypothesis,   and further suggests that gnetales are more closely related to some gymnosperm groups than angiosperms,  and that gymnosperms form a distinct clade to the angiosperms,. The main function of a flower is reproduction , which, before the evolution of the flower and angiosperms , was the job of microsporophylls and megasporophylls. A flower can be considered a powerful evolutionary innovation , because its presence allowed the plant world to access new means and mechanisms for reproduction. It seems that on the level of the organ, the leaf may be the ancestor of the flower, or at least some floral organs.
When we mutate some crucial genes involved in flower development, we end up with a cluster of leaf-like structures. Thus, sometime in history, the developmental program leading to formation of a leaf must have been altered to generate a flower. There probably also exists an overall robust framework within which the floral diversity has been generated.
Ecology And Evolution Of Flowers Oxford Biology
The homologs of this gene are found in angiosperms as diverse as tomato , snapdragon , pea , maize and even gymnosperms. Expression of Arabidopsis thaliana LFY in distant plants like poplar and citrus also results in flower-production in these plants. These genes, in turn, act as direct controllers of flower development. The members of the MADS-box family of transcription factors play a very important and evolutionarily conserved role in flower development.
According to the ABC model of flower development , three zones - A, B and C - are generated within the developing flower primordium, by the action of some transcription factors , that are members of the MADS-box family. Among these, the functions of the B and C domain genes have been evolutionarily more conserved than the A domain gene.
Pollinators can drive evolution of flower traits: study
Many of these genes have arisen through gene duplications of ancestral members of this family. Quite a few of them show redundant functions. The evolution of the MADS-box family has been extensively studied. These genes are present even in pteridophytes , but the spread and diversity is many times higher in angiosperms. It is expressed in today's flowers in the stamens , and the carpel , which are reproductive organs. It's ancestor in gymnosperms also has the same expression pattern. Here, it is expressed in the strobili , an organ that produces pollens or ovules.
Their descendants in the modern angiosperms also are expressed only in the stamens , the male reproductive organ. Thus, the same, then-existing components were used by the plants in a novel manner to generate the first flower. This is a recurring pattern in evolution. How is the enormous diversity in the shape, color and sizes of flowers established?
There is enormous variation in the developmental program in different plants.
For example, monocots possess structures like lodicules and palea, that were believed to be analogous to the dicot petals and carpels respectively. It turns out that this is true, and the variation is due to slight changes in the MADS-box genes and their expression pattern in the monocots.
Another example is that of the toad-flax, Linaria vulgaris , which has two kinds of flower symmetries: radial and bilateral.
Arabidopsis thaliana has a gene called AGAMOUS that plays an important role in defining how many petals and sepals and other organs are generated. Mutations in this gene give rise to the floral meristem obtaining an indeterminate fate, and many floral organs keep on getting produced. We have flowers like roses , carnations and morning glory , for example, that have very dense floral organs. These flowers have been selected by horticulturists since long for increased number of petals.
Researchers have found that the morphology of these flowers is because of strong mutations in the AGAMOUS homolog in these plants, which leads to them making a large number of petals and sepals. Some of these changes also cause changes in expression patterns of the developmental genes, resulting in different phenotypes. The researchers confirmed that the ABC Model of flower development is not conserved across all angiosperms.
Sometimes expression domains change, as in the case of many monocots , and also in some basal angiosperms like Amborella. Different models of flower development like the fading boundaries model , or the overlapping-boundaries model which propose non-rigid domains of expression, may explain these architectures.
Another floral feature that has been a subject of natural selection is flowering time. Some plants flower early in their life cycle, others require a period of vernalization before flowering. This decision is based on factors like temperature , light intensity , presence of pollinators and other environmental signals. Allelic variation in these loci have been associated with flowering time variations between plants. For example, Arabidopsis thaliana ecotypes that grow in the cold temperate regions require prolonged vernalization before they flower, while the tropical varieties and common lab strains, do not.
Many genes in the flowering time pathway are conserved across all plants studied to date. However, this does not mean that the mechanism of action is similarly conserved. For example, the monocot rice accelerates its flowering in short-day conditions, while Arabidopsis thaliana , a eudicot, responds to long-day conditions. The Anthophyte Theory was based on the observation that a gymnospermic family Gnetaceae has a flower-like ovule. It has partially developed vessels as found in the angiosperms , and the megasporangium is covered by three envelopes, like the ovary structure of angiosperm flowers.
However, many other lines of evidence show that gnetophytes are not related to angiosperms. The Mostly Male Theory has a more genetic basis.