Department of Entomology, Federal University of Viçosa, Viçosa, Brazil.
PLoS One. 2013 Apr 23;8(4):e61582. doi: 10.1371/journal.pone.0061582. Print 2013.
Defence from parasites and pathogens involves a cost. Thus, it is expected that organisms use this only at high population densities, where the risk of pathogen transmission may be high, as proposed by the "density-dependent prophylaxis" (DDP) hypothesis. These predictions have been tested in a wide range of insects, both in comparative and experimental studies. We think it pertinent to consider a continuum between solitarious and gregarious living insects, wherein: (1) solitarious insects are those that are constitutively solitary and do not express any phenotypic plasticity, (2) the middle of the continuum is represented by insects that are subject to fluctuations in local density and show a range of facultative and plastic changes; and (3) constitutively gregarious forms live gregariously and show the gregarious phenotype even in the absence of crowding stimuli. We aimed to chart some of the intermediary continuum with an insect that presents solitarious aspects, but that is subject to fluctuations in density. Thus, Anticarsia gemmatalis (Lepidoptera: Noctuidae) larvae reared at higher densities showed changes in coloration, a greater degree of encapsulation, had higher hemocyte densities and were more resistant to Baculovirus anticarsia, but not to Bacillus thuringiensis. Meanwhile, with increased rearing density there was reduced capsule melanization. Hemocyte density was the only variable that did not vary according to larval phenotype. The observed responses were not a continuous function of larval density, but an all-or-nothing response to the presence of a conspecific. As A. gemmatalis is not known for gregarious living, yet shows these density-dependent changes, it thus seems that this plastic phenotypic adjustment may be a broader phenomenon than previously thought.
防御寄生虫和病原体需要付出代价。因此，人们预计生物体只有在种群密度较高的情况下才会使用这种防御机制，因为此时病原体传播的风险可能很高，这一假设被称为“密度依赖预防”（DDP）假说。这些预测已经在广泛的昆虫中得到了检验，包括比较研究和实验研究。我们认为，有必要考虑独居和群居昆虫之间的连续体，其中：（1）独居昆虫是指那些天生独居且不表现出任何表型可塑性的昆虫；（2）连续体的中间部分代表的是那些受局部密度波动影响并表现出一系列适应性和可塑性变化的昆虫；（3）天生群居的形式群居，即使在没有聚集刺激的情况下也表现出群居表型。我们旨在用一种表现出独居特征但受密度波动影响的昆虫来描绘其中一些中间连续体。因此，饲养在较高密度下的 Anticarsia gemmatalis（鳞翅目：夜蛾科）幼虫表现出体色变化、更高程度的包被、更高的血细胞密度和对 Baculovirus anticarsia 的更高抗性，但对 Bacillus thuringiensis 没有抗性。同时，随着饲养密度的增加，胶囊黑化程度降低。血细胞密度是唯一不随幼虫表型变化的变量。观察到的反应不是幼虫密度的连续函数，而是对同种生物存在的全有或全无反应。由于 A. gemmatalis 不以群居生活而闻名，但表现出这些密度依赖性变化，因此这种可塑的表型调整似乎比以前认为的更为普遍。
