Summary• Some green orchids obtain carbon (C) from their mycorrhizal fungi and photosynthesis. This mixotrophy may represent an evolutionary step towards mycoheterotrophic plants fully feeding on fungal C. Here, we report on nonphotosynthetic individuals (albinos) of the green Cephalanthera damasonium that likely represent another evolutionary step.• Albino and green individuals from a French population were compared for morphology and fertility, photosynthetic abilities, fungal partners (using microscopy and molecular tools), and nutrient sources (as characterized by 15 N and 13 C abundances).• Albinos did not differ significantly from green individuals in morphology and fertility, but tended to be smaller. They harboured similar fungi, with Thelephoraceae and Cortinariaceae as mycorrhizal partners and few rhizoctonias. Albinos were nonphotosynthetic, fully mycoheterotrophic. Green individuals carried out photosynthesis at compensation point and received almost 50% of their C from fungi. Orchid fungi also colonized surrounding tree roots, likely to be the ultimate C source.• Transition to mycoheterotrophy may require several simultaneous adaptations; albinos, by lacking some of them, may have reduced ecological success. This may limit the appearance of cheaters in mycorrhizal networks.

Although a budding yeast culture can be propagated eternally, individual yeast cells age and eventually die. The detailed knowledge of this unicellular eukaryotic species as well as the powerful tools developed to study its physiology makes budding yeast an ideal model organism to study the mechanisms involved in aging. Considering both detrimental and positive aspects of age, we review changes occurring during aging both at the whole-cell level and at the intracellular level. The possible mechanisms allowing old cells to produce rejuvenated progeny are described in terms of accumulation and inheritance of aging factors. Based on the dynamic changes associated with age, we distinguish different stages of age: early age, during which changes do not impair cell growth; intermediate age, during which aging factors start to accumulate; and late age, which corresponds to the last divisions before death. For each aging factor, we examine its asymmetric segregation and whether it plays a causal role in aging. Using the example of caloric restriction, we describe how the aging process can be modulated at different levels and how changes in different organelles might interplay with each other. Finally, we discuss the beneficial aspects that might be associated with age.

The maintenance of cooperation in populations where public goods are equally accessible to all but inflict a fitness cost on individual producers is a long-standing puzzle of evolutionary biology. An example of such a scenario is the secretion of siderophores by bacteria into their environment to fetch soluble iron. In a planktonic culture, these molecules diffuse rapidly, such that the same concentration is experienced by all bacteria. However, on solid substrates, bacteria form dense and packed colonies that may alter the diffusion dynamics through cell-cell contact interactions. In Pseudomonas aeruginosa microcolonies growing on solid substrate, we found that the concentration of pyoverdine, a secreted iron chelator, is heterogeneous, with a maximum at the center of the colony. We quantitatively explain the formation of this gradient by local exchange between contacting cells rather than by global diffusion of pyoverdine. In addition, we show that this local trafficking modulates the growth rate of individual cells. Taken together, these data provide a physical basis that explains the stability of public goods production in packed colonies.biofilm | evolution | noise | variability | ecology I ron is required for many enzymatic processes, and is therefore an essential nutrient. To overcome the low abundance of free iron in aerobic environments, bacteria, as well as other microorganisms, synthesize and secrete iron-chelating molecules called siderophores (1). Once released in the extracellular environment, siderophores diffuse and complex with iron. Because other bacteria can import them, siderophores can be regarded as public goods (2) and siderophore secretion is often cited as a model system for studying the stability of cooperation (3, 4). In this context, nonproducing mutants, which can enjoy the benefit of siderophores without bearing the cost of their production, have a selective advantage over the WT-producing strain and could, in principle, displace them on evolutionary time scales.This argument usually assumes that public good molecules are readily available to all, as is the case in planktonic cultures, where molecules diffuse freely and rapidly between cells. In liquid conditions, mutants that do not produce siderophores have, in fact, been shown to outcompete WT strains (5-8). However, the limited spatial dispersal of public goods can challenge this picture and has been proposed as a general mechanism for explaining the maintenance of cooperation (8-12). When dispersal is limited, public good molecules tend to stay in the vicinity of the producing subpopulations, allowing them to benefit preferentially from their own production, and thus to balance the advantage of opportunistic nonproducing strains. In many ecological situations, bacteria form complex, tightly packed, and spatially structured colonies, such as biofilms, where public good dispersal may be severely reduced compared with planktonic cultures. This raises the questions of how public good molecules circulate between cells in these natur...