The Developmental Stream of Cottonwoods Affects Ramet Growth and Resistance to Galling Aphids

Abstract

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. Ecological Society of America is collaborating with JSTOR to digitize, preserve and extend access to Ecology. Abstract. We define the "developmental stream" of a plant as the gradient of increasingly mature phenotypes its shoots express over its lifetime. This pattern has important implications for studies of morphology, physiology, reproduction, and interactions with other organisms. Using cottonwoods, we test two major hypotheses about their developmental streams. First, within a tree, the developmental stage expressed by a shoot is dependent on its growth distance from the ground. Second, the distribution and fitness of Pemphigus betae, an aphid that is sensitive to host maturity, are positively related to increasingly mature shoot phenotypes within a tree. We show that the developmental variation in plant morphology and reproductive output within a tree are as great as that found among different-aged trees within a clone. In addition, we show that within-tree patterns of variation are stable from year to year and that P. betae responds to this variation by preferentially colonizing branches expressing more mature phenotypes. This pattern of branch selection is adaptive; aphids transferred to branches with shoots expressing mature phenotypes have expected fitnesses >80% higher than those transferred to branches expressing juvenile phenotypes.These findings have several general implications. First, a within-tree series of developmental stages provides a heritable mechanism for creating mosaics of variability in host quality within plants that create host selection problems for herbivores. Second, herbivores sensitive to developmental changes in their hosts will be concentrated in areas of high quality where they will be subject to the detrimental effects of competition, predation, and parasitism. Third, simulation models of plant growth based on the behavior of repeating plant parts (e.g., shoots) must be sensitive to developmental changes in these parts during plant development. Fourth, the developmental streams of plants may affect the evolution of such herbivore traits as territoriality, deme formation, and virulence. Fifth, the use of clonally propagated stock for the study of the genetic components of resistance to herbivory must be carried out with an understanding of the effects of host development on herbivores. Developmental processes in plants (ordered, directional changes in shoot behavior) have many causes, most of which are generalizable into two broad categories: ontogenetic and physiological mechanisms (Brink 1962, Fortanier and Jonkers 1976, Hood andLibby 1978, Huchison et al. 1990, Poethig 1990, Lawson and Poethig 1995). Ontogenetic changes arise from modificat...

Separating ontogenetic and environmental determination of resistance to herbivory in cottonwood

“…Our results support the conclusions of Boege and Marquis (2005) that a new synthesis of plant defense theory is necessary; only when we examine ontogentic-based resistance to P. betae across different zones of an individual tree do we get a clear picture of the selection pressures that have favored the evolution of aphid behaviors to be highly selective in colonizing the most susceptible tree genotypes and mature zone of these trees (Whitham 1989, Kearsley and Whitham 1998. ACKNOWLEDGMENTS Thanks to K. C. Larson for assistance in the field.…”

“…Regardless of the underlying mechanism, the same general outcome occurs: a gradient of age-related changes in ramet characters. This gradient, termed a developmental stream, occurs in cottonwoods and many other plant species (Lillie 1927, O'Rourke 1949, Kearsley and Whitham 1998. The developmental stream produces a pattern in which shoot age is a function of its degree of separation from the root crown.…”

Heteroblasty inEucalyptus globulus(Myricales: Myricaceae) Affects Ovipositonal and Settling Preferences ofCtenarytaina eucalyptiandC. spatulata(Homoptera: Psyllidae)

“…Developmental resistance to arthropod herbivores has been studied in relatively few systems (Frankie and Morgan 1984, Kearsley and Whitham 1989, 1997, Waltz and Whitham 1997, Karban and Thaler 1999. Although there were some differences in leaf size and shape of the aphid resistant versus susceptible leaves of Populus angustifolia L., Kearsley and Whitham (1997) did not consider this host heteroblastic. In addition, Karban and Thaler (1999) found no difference in mite populations on the juvenile and adult leaves of cotton, although both differed from the cotyledons.…”

The biological improbability of a clone

“…In nature, there are numerous clonal or largely asexual animals and plants (Gill et al, 1995 and references therein). Recently studied examples include : cottonwoods, Populus salicaceae (Kearsley & Whitham 1998) ; dandelions, Taraxacum officinale (King & Schaal, 1990) ; grasses, e.g. clonal prairie grass, Spartina pectinata (Davelos et al, 1996) and Red mangrove trees, Rhizophora mangle (Klekowski, 1998).…”