Tansley Review No. 75 Arabidopsis- its use in the genetic and molecular analysis of plant morphogenesis

Demographic history of european populations of Arabidopsis thaliana

The model plant species Arabidopsis thaliana is successful at colonizing land that has recently undergone human-mediated disturbance. To investigate the prehistoric spread of A. thaliana, we applied approximate Bayesian computation and explicit spatial modeling to 76 European accessions sequenced at 876 nuclear loci. We find evidence that a major migration wave occurred from east to west, affecting most of the sampled individuals. The longitudinal gradient appears to result from the plant having spread in Europe from the east approximately 10,000 years ago, with a rate of westward spread of approximately 0.9 km/year. This wave-of-advance model is consistent with a natural colonization from an eastern glacial refugium that overwhelmed ancient western lineages. However, the speed and time frame of the model also suggest that the migration of A. thaliana into Europe may have accompanied the spread of agriculture during the Neolithic transition.

Comparative studies of evolutionary responses to light environments in Arabidopsis

In this article, we compare the reaction norms to foliage shade (changes in light quality, spatially fine-grained environmental variation) and photoperiod (day length, spatially coarse-grained environmental variation) in several haplotypes of Arabidopsis thaliana from Scandinavia. We found that both across-environment means and phenotypic plasticities evolved continuously and very rapidly within this group. Both character means and trait plasticities were highly integrated, in part as predicted by the adaptive plasticity hypothesis for response to foliage shade (the so-called shade-avoidance syndrome). We found that a significant but small fraction of the variance in across-treatment trait means and plasticities in response to one environmental factor is explained by variation of the same traits in response to the other factor. Genetic relatedness based on chloroplast DNA sequence variation among haplotypes was not associated with variation in across-treatment character means or their plasticities, suggesting that evolution of these characters has occurred on a local geographic scale via reticulation (outcrossing) among maternal lines rather than by the differential survival of selfing lineages.

Ecology and evolutionary biology of Arabidopsis

Arabidopsis thaliana is now widely used as a model system in molecular and developmental biology, as well as in physiology and cell biology. However, ecologists and evolutionary biologists have turned their attention to the mouse ear cress only much more recently and almost reluctantly. The reason for this is the perception that A. thaliana is not particularly interesting ecologically and that it represents an oddity from an evolutionary standpoint. While there is some truth in both these attitudes, similar criticisms apply to other model systems such as the fruit fly Drosophila melanogaster, which has been extensively studied from an organismal perspective. Furthermore, the shortcomings of A. thaliana in terms of its restricted ecological niche are counterbalanced by the wealth of information on the molecular and developmental biology of this species, which makes possible to address evolutionary questions that can rarely be pursued in other species. This chapter reviews the history of the use of A. thaliana in organismal biology and discusses some of the recent work and future perspectives of research on a variety of field including life history evolution, phenotypic plasticity, natural selection and quantitative genetics. I suggest that the future of both molecular and especially organismal biology lies into expanding our knowledge from limited and idiosyncratic model systems to their phylogenetic neighborhood, which is bound to be more varied and biologically interesting.

Tansley Review No. 74 Control of organogenesis at the shoot apex

The processes involved in the formation of primordia on the shoor apex are those controlling (1) growth rate, (2) division plane. (3) surface microstructure, and (4) extensibility of the surface. Changes in growth rate and division planes may accompany primordium formation but are considered as probably not in themselves being causal. Changes in surface microstructure may be necessary to delimit the position and area occupied by an incipient primordium. However, attention is directed to changes in surface extensibility as perhaps being the overriding factor in primordium formation. Nevertheless, the position and form of the primordia will also depend on growth rate, division plane, and surface microstructurc being permissive. The relative importance of these four sets of processes may differ from species to species and from one stage of development to another. Chemical and metabolic changes within the apex may first be necessary to determine whether the surface can extend sufficiently for any primordia to form at all, but their positions and time of initiation may depend more on the other factors. The surface microstructure may become more important when patterning is detailed and precise as it is in the developing flower, whereas a less precise mechanism dependent on localized induction of synthesis of a morphogen (auxin?) may provide sufficient information to determine the general position and liming of primordium initiation in vegetative apices. In determining the pattern of primordia on the apex, primordial area at initiation is important and reasons for believing that auxin may be involved in determining this are summarised. The different developmental pathways of primordia seem to diverge from the moment of initiation. Developmental fate of primordia is determined by the hamcotic genes which may in fact be heterochronic genes. How these regulatory genes control the processes involved in differentiation of different types of primordia is so far unknown. Contents Summary 1 I. Introduction 2 II. The mechanism of primordium formation: what causes an outgrowth of the apical surface? 2 III. The positions of successive primordia: what determines their size and localization, so giving rise to pattern? 11 IV. What determines the developmental pathways of the primordia once initiated? 13 V. Conclusions 15 VI. Acknowledgements 16 VII. References 16.