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The currently favoured model of the evolution of C photosynthesis relies heavily on the interpretation of the broad phenotypic range of naturally growing C -C intermediates as proxies for evolutionary intermediate steps. On the other hand, C -C intermediates had earlier been interpreted as hybrids or hybrid derivates. By first comparing experimentally generated with naturally growing C -C intermediates, and second summarising either direct or circumstantial evidence for hybridisation in lineages comprising C , C and C -C intermediates, we conclude that a possible hybrid origin of C -C intermediates deserves careful examination. While we acknowledge that the current model of C photosynthesis evolution is clearly the best available, C -C intermediates of hybrid origin, if existing, should not be used for further analysis of this model. However, experimental C × C hybrids potentially are excellent systems to analyse the genetic differences between C and C species and, also using segregating progeny, to study the relationship between individual photosynthetic traits and environmental factors.

Sesuvium sesuvioides represents a young C4 lineage with C4-like metabolism: CO2 compensation points range between C4 and C3–C4 intermediate values, and Rubisco was detected in bundle sheath and mesophyll.

Background and aims Internal seed morphological traits such as embryo characteristics and nutritive tissue can vary considerably within a plant lineage. These traits play a prominent role in germination processes and the success of seedling establishment, and are therefore under high selective pressure, especially in environments hostile to seedlings such as arid, saline or highly dynamic habitats. We investigated the relationships of seed internal morphology and germination characteristics of 84 species of Amaranthaceae s.l., a family with numerous lineages that have adapted to stressful growing conditions. Methods We used seed cross sections to assess embryo type, embryo to seed surface ratio and radicle to cotyledon length ratio. Furthermore, seed mass, mean time to germination, habitat preferences and further plant traits such as C3 or C4 photosynthesis and life form were compiled for each species. Data were analyzed using phylogenetic comparative methods. Key results We found embryo type (λ = 1), log seed mass (λ = 0.86) and embryo to seed size ratio (λ = 0.78) to be evolutionarily stable with an annular embryo as ancestral in the family. Linked to shifts to the three derived embryos types (spiral, horseshoe-shaped and curved) is an increase in root to cotyledon length ratio and a reduction of nutritive tissue. We observed stabilizing selection towards seeds with relatively larger embryos with longer radicles and less nutritive tissue that are able to germinate faster, especially in lineages with C4 photosynthesis and/or salt tolerance. Conclusions We conclude that the evolutionary shift of nutrient storage from perisperm to embryo provides an ecological advantage in extreme environments, because it enables faster germination and seedling establishment. Furthermore, the evolutionary shift towards higher root to cotyledon length ratio especially in small-seeded Amaranthaceae growing in saline habitats can provide an ecological advantage for fast seedling establishment.

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