Developmental instability in a stem-mining sawfly: can fluctuating asymmetry detect plant host stress in a model system?

Abstract

Fluctuating asymmetry (FA) may be a sensitive indicator of the stress experienced by organisms during their development. Its use in this manner is an intuitively appealing, frequently proposed, and potentially powerful tool but remains controversial partially because its underlying premise rarely has been critically tested. Such tests should include direct comparisons among individuals for which levels of FA, stress and fitness have been unambiguously quantified. We assessed the use of FA as a bioindicator of the stress experienced during egg-to-adult development by the stem-mining sawfly, Cephus cinctus Norton. Sawflies were reared in a common garden from seven different wheat cultivars, which were selected to represent a gradient of stem solidness, a key factor imposing stress on sawflies during development. In this model system, stress was quantified by the weight of emergent adults. Fitness was quantified by counting the number of eggs in dissected females, which emerge with their full lifetime complement. FA was measured for wing length, three wing cells, and three wing veins using image analyses. The greatest amount of stress was induced by solid-stemmed cultivars from which the adults were significantly smaller than those developing in hollow-stemmed hosts. In turn, adult weight was positively correlated with fitness. The net effect was a 25-fold variation in sawfly fitness, which gave a reasonable expectation that FA levels would differ across cultivars. However, FA levels of all the traits were similar among cultivars and there was no negative relationship between FA and fitness. These results: (1) document the failure of FA as an indicator of stress in this model system, (2) identify adult weight as a satisfactory indicator of plant-induced stress and sawfly fitness, and (3) add to the growing body of literature questioning the value of FA as a biomonitor tool of developmental stress.

Fluctuating Asymmetry: Methods, Theory, and Applications

“…Presumed stressors often have no effect on asymmetry, even while decreasing growth and increasing mortality [200]. Moreover, fluctuating asymmetry can be a poor indicator of both stress and individual fitness [201]. Such findings, however, do not invalidate fluctuating asymmetry as an indicator of stress.…”

“…The promise of an easily measured component of fitness has not borne out. The initial optimism has been replaced with skepticism, and the belief that fluctuating asymmetry is a weak measure of developmental instability, and consequently a poor measure of individual fitness [238,239,201,240]. The main problem is that with only two sides to a bilaterally symmetrical trait, only one degree of freedom is available to estimate a variance and a sample size n = 2 to estimate a mean absolute deviation.…”

Size polymorphism and fluctuating asymmetry of Artemia (Branchiopoda: Anostraca) populations from the Crimea

“…Phenomenon of FA in different groups of organisms is well known, including in different crustacean orders (Gomelyuk and Ozolinsh, 1986;Garmew et al, 1994;Shadrin and Popova, 1994;Stige et al, 2006;Ho et al, 2009;Maia et al, 2009); increased level of environmental stress was shown lead to increased levels of FA within populations. However, on other hand there is the growing body of literature questioning the value of FA as a biomonitor tool of developmental stress (Floate and Fox, 2000;Cárcamo et al, 2008).…”

Body size, symmetry and abundance of Euxesta stigmatias (Loew) and Euxesta sororcula (Wiedemann) (Diptera: Ulidiidae) in a natural reserve and in a guava orchard in Uberlândia, MG, Brazil

“…Thus, AF also provides a measure of how much a particular organism was able to buffer its development against stressful conditions. Because AF is inversely correlated with developmental homeostasis, it is often used to estimate the stability of development (Fuller & Houle 2002, Cárcamo et al 2008.…”