Fruit evolution and adaptation to Mediterranean habitats in Raphanus (Brassicaceae)

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It is widely accepted that past changes in environmental conditions plus whole-genome duplications have driven radiation of Brassicaceae, resulting in novel adaptive morphological and physiological traits. Here we suggest taking an interdisciplinary approach to investigate the adaptive value of the divergence in fruit and diaspore architecture in the genus Raphanus in respect to spatial dispersal patterns, germination synchronization, and patterns of genetic diversity within and among populations. Raphanus is a Mediterranean genus comprising two closely related allopatric wild annuals, R. raphanistrum and R. pugioniformis and the cultivated R. sativus. The first two are native to the East Mediterranean and differ in fruit morphology and dispersal ability, suggesting that single-seeded units of R. raphanistrum and the heavy multi-seeded diaspores of R. pugioniformis are adaptive in their specific habitats. Accordingly, we hypothesize that in their native range , long-distance wind dispersal in R. raphanistrum, can explain its wide distribution along similar homogenous habitats, while short- distance dispersal in R. pugioniformis ensures remaining in safe sites for seedling establishment in heterogeneous mountainous environments. Thus, Raphanus provides an opportunity to investigate and develop a model of evolutionary processes in fruit architecture and dispersal mechanisms in a small genus. We suggest conducting a detailed population survey of each of the two wild species in Israel and the Palestinian Authority, e.g. spatial arrangements within populations, distribution patterns , and assessment of biotic, edaphic and climatic factors. Dispersal over long and short distances will be tested in confined sites. Furthermore, fruit and seed morphology, and fruit anatomy will be investigated, and short- and long- term germination trials, as well as longevity tests will be set on seeds enclosed in the pericarp vs. bare seeds. Population-based threshold models will be used to explain eco-physiological regulation of seed dormancy /germination by the environment. In this respect we hypothesize that germination of R. raphanistrum occurs upon long imbibition, while pericarp-mediated mechanical dormancy of R. pugioniformis is released only after decomposition of the pericarp by microbial activity or abrasion. With the use of microsatellites we aim also to understand how the differences in habitat and dispersal strategy affect genetic variation within and among populations of these species. The results of our project will provide insight into adaptive processes in annual Brassicaceae and we expect that this interdisciplinary study will stimulate Israeli and Palestinian students to join this research area.