EFFECTS OF LIFE-HISTORY, DOMESTICATION AND AGRONOMIC SELECTION ON PLANT DEFENSE AGAINST INSECTS - EVIDENCE FROM MAIZES AND WILD RELATIVES

Plant domestication and agronomic selection for increased yield may ha ve an associated effect of reducing plant defence against herbivorous insects. This hypothesis is based on evidence for a metabolic cost ass ociated with defence, and on evidence that increases in yield generall y come from the re-partitioning of photoassimilates rather than from f undamental increases in photosynthetic rates. We propose that for plan ts in which domestication and crop development constitute strong selec tion for increased growth and reproduction, reallocation of resources may result in lower defence against insects. We examine this hypothesi s by means of comparative studies of growth, reproduction and resistan ce in a complex of maizes and closely related wild taxa, the teosintes . The results of these studies are consistent with assumptions of diff erential investment in growth and reproduction between wild and domest icated plants. A wild perennial grew slowest and had lowest grain prod uction, while a modern cultivar grew fastest and had the highest grain yield. A wild annual and a land-race cultivar were intermediate. Dama ge from a diverse assemblage of folivorous insects, and from a special ist stemboring lepidopteran larva, fit the defence predictions closely . A gradient of attack levels suggests that the wild perennial is most defended, followed in descending order by the wild annual, the land-r ace cultivar and the modern high-yielding variety. Alternative hypothe ses for this pattern are consistent with some, but not all, of our dat a.