CONTEXT-DEPENDENCY RULES THE DAY IN THE GRASS-ENDOPHYTE SYMBIOSIS

A review of “Ecology and Evolution of the Grass-Endophyte Symbiosis” by Gregory P. Cheplick and Stanley H. Faeth (2009, Oxford University Press, hardback, 256 pages, ISBN13: 978-019-530808-2, ISBN10: 0-19-530808-5, $75.00) In the Introduction to their book on the grass-endophyte symbiosis, Cheplick and Faeth state that “A general theme will emerge that much of the variation in the nature of the interaction is due to environmental and genetic contingency.” Indeed, one of the striking features of the grass-endophyte symbiosis that emerges in this thorough review of the empirical literature is that contingency and variability run rampant, and generalizations are nonexistent. This theme of contingency is one example of how the book frames the empirical literature on the grass-endophyte symbiosis in an appropriately broad and contemporary framework of basic ecological and evolutionary ideas. In introducing the theme, they highlight parallels with other putative mutualisms, such as mycorrhizal interactions, and draw inspiration from general theory on contingency in putative mutualisms (e.g., Ewald 1987, Bronstein 1994). Overall, the book effectively argues that explorations of the wide variability in ecological outcomes in grass-endophyte symbioses are likely to yield fruits in multiple subfields of ecology and evolution. The contingency theme, with respect to effects of endophytes on hosts, is continued in Chapter 2, where the continuum of positive to negative effects that fungal endophytes can have on host plants is presented in detail. In this chapter, it becomes apparent that the authors not only highlight most or all of the important work already conducted on the symbiosis, such as key studies documenting contingency of ecological outcomes on drought stress, but that the review also aims to highlight important gaps in the literature. For example, the authors point out that another continuum present in the symbiosis is the habitation of a range of light conditions from sunny, open fields to dark forest understories, and that this continuum has yet to be thoroughly explored. In fact, an important argument made in the book is that grassendophyte symbioses may be particularly useful for answering big questions in ecology and evolution, precisely because they span so many different continua of variables that may influence ecological and evolutionary outcomes. It has recently been argued that comparative studies along environmental gradients provide powerful tests of hypotheses in community ecology (McGill et al. 2006); grass-endophyte symbioses seem to present the opportunity to use gradients of biotic and abiotic factors in a similar manner to test hypotheses about how traits of species interactions influence ecological and evolutionary outcomes. Besides the abiotic gradients discussed above, other gradients in the symbiosis highlighted by the authors include the degree of horizontal versus vertical transmission of the endophyte, sexual versus asexual reproduction of both host and endophyte, and infection frequency, all of which have been observed to vary within and among different species involved in these symbioses. Although such factors are not factorially distributed among different examples of the symbioses, their autocorrelation may provide some interesting clues to the combinations of traits favored by natural selection. One example is the fascinating pattern that asexual endophytes (e.g., Neotyphodium spp.) are vertically transmitted by hosts that retain sexual