Une grande diversité de formes foliaires caractérise le monde végétal. Cette diversité s'étend des feuilles simples avec des marges lisses aux feuilles composées, avec des marges disséquées. Cependant, les dentelures des marges de ces feuilles simples ou composées se développent en suivant un mécanisme similaire. Ce mécanisme repose sur l'action des gènes NO APICAUX MERISTEM/ CUP-SHAPED COTYLEDONS (NAM/CUC) ainsi que sur la voie auxinique. Chez Arabidopsis, qui possède des feuilles simples, un équilibre entre les expressions de CUC2 et de son répresseur, miR164, est nécessaire au bon développement des dents. Nous avons montré qu'un autre membre de la famille CUC, CUC3, contribue également au développement de ces dents chez Arabidopsis. Bien que son action soit principalement dépendante de CUC2, il agit également plus tard au cours du développement foliaire. En outre, nous avons démontré qu'une boucle de rétro-contrôle entre CUC2 et la voie auxinique permet le développement de dents avec plus ou moins marquées. Nous avons également montré qu'un modèle d'expression temporelle existe entre l'auxine et le module CUC2-miR164. En outre, la production de plantes transgéniques de Cardamine hirsuta, un proche parent d' Arabidopsis, qui possède des feuilles composées, a mis en évidence l'importance des éléments cis-régulateurs dans le promoteur de CUC1 de Cardamine hirsuta. En effet, la divergence de ces éléments cis-régulateurs entre les promoteurs de CUC1 de Cardamine hirsuta et d' Arabidopsis pourrait expliquer que CUC1 soit fortement exprimé dans les feuilles de Cardamine hirsuta alors qu'il est faiblement exprimé dans celles d' Arabidopsis. / A wide diversity of leaf shapes characterises the plant world. This diversity ranges from simple leaves with smooth margins to compound leaves with dissected margins. However, all serrations of simple or compound leaf margins are developed using a similar mechanism. This mechanism includes the action of the NO APICAL MERISTEM/CUP-SHAPED COTYLEDON (NAM/CUC) genes as well as the auxin pathway. In Arabidopsis simple leaves, a balanced expression of CUC2 and its repressor miR164 is controlling the serrations development. We have shown that another member of the CUC family, CUC3, also contributes to the serration development in Arabidopsis simple leaves. While its action is mainly dependent of the one of CUC2, it also acts later during leaf development. Additionally, we have demonstrated that a feed-back loop was regulating the CUC2 and auxin pathways, in order to form leaves with more or less incisions. We also shown that a temporal expression pattern was established between the auxin and the CUC2-miR164 module. Moreover, generation of transgenic Cardamine hirsuta plants, a close relative of Arabidopsis, that possesses compound leaves, has enlighten the importance of cis-regulatory elements in the promoter of CUC1 from Cardamine hirsuta. Indeed, the divergence of cis-regulatory elements between promoters of CUC1 from Cardamine hirsuta and Arabidopsis could explain that CUC1 is expressed strongly in Cardamine hirsuta leaves whereas it is weakly expressed in Arabidopsis leaves.

<b>Cardamine hirsuta</b> L. is an emerging model system in developmental genetics, where natural genetic variation within <b>C. hirsuta</b> provides the means to investigate the genetic basis of morphological traits. This thesis investigates the geographical structure and genealogical history of genetic variation within <b>C. hirsuta</b> and identifies its closest relatives. This will enable the accurate selection of species for comparison with <b>C. hirsuta</b> when making interpretations of evolutionary processes, and provide a better understanding of morphological character evolution in <b>C. hirsuta</b>. The phylogeographic history of <b>C. hirsuta</b> was reconstructed using multiple chloroplast and nuclear markers and widespread accession sampling from across its native range. A distinct group was identified within <b>C. hirsuta</b>, restricted to the high mountains of East Africa. Climate suitability modelling showed that Pleistocene glacial dynamics have had a strong effect on the distribution of genetic variation within <b>C. hirsuta</b>. The phylogeographical data generated here was used to investigate the origin of <b>C. hirsuta</b> in the Azores, an oceanic archipelago. The Azores are dominated by an endemic chloroplast haplotype which is associated with an endemic phenotype. Thus, <b>C. hirsuta</b> appears to have diversified <b>in situ</b> in the Azores. Phylogenetic analyses of Cardamine, restricted to diploid species to remove the confounding effects of polyploids, found that <b>C. hirsuta</b> is most closely related to <b>C. oligosperma</b>, a western North American species. Multiple loci and extensive intraspecific sampling were brought to bear to demonstrate that <b>C. hirsuta</b> and <b>C. oligosperma</b> are reciprocally monophyletic. <b>Cardamine pattersonii</b>, a restricted endemic from north-west Oregon is likely to be an allopolyploid, with <b>C. oligosperma</b> as the maternal parent and possibly <b>C. nuttallii</b> as the paternal parent.

Dispersal is a key trait across biology. Within plants, a variety of explosive seed dispersal mechanisms are seen. Whilst ecological and mechanical studies have described this important evolutionary adaptation in many species, a genetic and developmental understanding of explosive seed dispersal is lacking. In this thesis, the morphology and development of the explosive seed pods of Cardamine hirsuta – a member of the Brassicaceae – are characterised in detail, with reference to its close relative, the model organism A. thaliana. Comparison of fruit morphology between these two species and across other Brassicacean species generated hypotheses regarding the function and polarity of morphological features. In order to identify genes that are necessary for C. hirsuta fruit development, a genetic screen was conducted and a range of mutants identified and subsequently characterised. Analysis of the indehiscent valveless (val) mutant revealed a loss of valve tissue and an expansion of valve margin identity in the silique. Mapping and sequencing identified a mutation in the MADS-box gene FRUITFULL (FUL), which results in a truncated protein, as the likely cause of the val phenotype. Consideration of ful mutants in C. hirsuta and A. thaliana allowed comparison of the genetic patterning of the fruit dehiscence zone in these two species. The genetic interactions between fruit mutants characterised in this thesis and mutants in shoot patterning genes revealed common regulatory networks underlying leaf and fruit development in C. hirsuta. Together, comparison of wild-type and mutant C. hirsuta siliques with those of A. thaliana and other Brassicacean species suggests that specialised cell layers within the valve silique region are of key importance to C. hirsuta’s explosive dehiscence mechanism.

A major goal in biology is to understand the genetic basis of morphological variation at different evolutionary scales, for example between and within species. Here I investigate this issue by using plant leaves as an example. Previously comparative studies between the simple leaf model plant Arabidopsis thaliana and its dissected leaf relative Cardamine hirsuta have shown that inter-specific differences in leaf shape mostly result from variation in local tissue growth and patterning (Vlad et al., 2014; Hay et al., 2006; Barkoulas et al., 2008). Here, I aim to elucidate the genetic basis of natural variation in leaf form within species, by using divergent strains of C. hirsuta. I present evidence that variation in six strains collected from geographically diverse locations results from different rates of progression of an age-dependent leaf development programme in a phenomenon known as heteroblasty. By using Quantitative trait loci (QTL) mapping with a recombinant inbred line (RIL) population derived from a cross between the Oxford and Azores strains, I detected six QTL that influence leaflet production on multiple leaves. A QTL located on the 4th linkage group was validated and selected for further analysis. Characterisation of QTL effect indicated that the QTL influences leaf form by altering the rate of heteroblastic development. Subsequently I fine mapped this QTL to a DNA segment of 48 kb containing the gene SQUAMOSA PROMOTER PROTEIN BINDING LIKE 9 (ChSPL9), a previously characterised regulator of age dependent development. The parental alleles of ChSPL9 show variation in their sequence and were transformed into A. thaliana to evaluate whether they contribute to the QTL effect. Resultant phenotypes mirrored the QTL effect suggesting that ChSPL9 does indeed contribute to this QTL effect. These results indicate that age-dependent leaf shape progression underlies variation in leaflet number within species and more broadly suggest that in the case of plant leaves different processes might underlie morphological variation between and within species.