Rank influences human sex differences in dyadic cooperation

In marked contrast, around 16,000 species within the 650–700 genera of legumes have been identified as forming nitrogen-fixing symbioses with rhizobia. If such diversity is taken as a measure of evolutionary success or fitness, the legume family within the Fabales has been considerably more successful than any other family of nodulating plants in the closely-related orders. Why is it that nitrogen-fixing nodules on legumes should appear to confer so much benefit compared with the non-legume nodulating plants? One clear difference is the different bacteria that infect them. Possibly Frankia spp. are less efficient at fixing nitrogen than rhizobia. However, the wide diversity of rhizobia that can nodulate plants suggests that successful nitrogen-fixing symbioses can be established by very diverse bacteria, even between different divisions of the bacteria. It is not apparent why Frankia spp. did not evolve a symbiosis that was as mutually beneficial (based on relative evolutionary diversity) as that seen with rhizobial-legume symbioses. The difference in the structures of legume nodules compared with non-legume nodules may give an insight into why legume nodules are so successful. Nodules on nonlegumes are somewhat similar to short modified lateral roots with a central vasculature. In contrast, legume nodules have a peripheral vasculature. A clear advantage of a peripheral vasculature is that oxygen will be available for energy production by mitochondria associated with the vasculature, and so ATP generated by respiration would be readily available to drive the energy demands of the vasculature. This would not normally be a significant problem in roots, but in a nodule with a central vasculature, the haemoglobin in the cells surrounding the vasculature would tend to bind most of the available oxygen. I propose that a physiological limitation on nodules with a central vasculature could be the relative difficulty of respiration, and hence ATP synthesis in the cells of the vasculature. The identification of a gene (COCHLEATA in pea and NOOT in M. truncatula) that normally represses root identity in nodules may be the first step toward identifying how legumes differentiate a peripheral vasculature. Other reasons for the relative efficiency of legume nodules could be related to the genome duplications that occurred in some of the legume families. These duplications could have enabled the acquisition of additional evolved functions in duplicated genes, to help develop the highly efficient nitrogen-fixing nodules in current legumes.