Department of Structural and Functional Biology, University of Naples Federico II, Via Cinthia MSA, Naples, Italy.
BMC Evol Biol. 2010 Sep 20;10:289. doi: 10.1186/1471-2148-10-289.
Many small vertebrates on islands grow larger, mature later, lay smaller clutches/litters, and are less sexually dimorphic and aggressive than their mainland relatives. This set of observations is referred to as the 'Island Syndrome'. The syndrome is linked to high population density on islands. We predicted that when population density is low and/or fluctuating insular vertebrates may evolve correlated trait shifts running opposite to the Island Syndrome, which we collectively refer to as the 'reversed island syndrome' (RIS) hypothesis. On the proximate level, we hypothesized that RIS is caused by increased activity levels in melanocortin receptors. Melanocortins are postranslational products of the proopiomelanocortin gene, which controls pleiotropically pigmentation, aggressiveness, sexual activity, and food intake in vertebrates.
We tested the RIS hypothesis performing a number of behavioral, genetic, and ontogenetic tests on a blue colored insular variant of the Italian Wall lizard Podarcis sicula, living on a small island off the Southern Italian coast. The population density of this blue-colored variant was generally low and highly fluctuating from one year to the next.In keeping with our predictions, insular lizards were more aggressive and sexually dimorphic than their mainland relatives. Insular males had wide, peramorphic heads. The growth rate of insular females was slower than growth rates of mainland individuals of both sexes, and of insular males. Consequently, size and shape dimorphism are higher on the Island. As predicted, melanocortin receptors were much more active in individuals of the insular population. Insular lizards have a higher food intake rate than mainland individuals, which is consistent with the increased activity of melanocortin receptors. This may be adaptive in an unpredictable environment such as Licosa Island. Insular lizards of both sexes spent less time basking than their mainland relatives. We suspect this is a by-product (spandrel) of the positive selection for increased activity of melanocortins receptors.
We contend that when population density is either low or fluctuating annually as a result of environmental unpredictability, it may be advantageous to individuals to behave more aggressively, to raise their rate of food intake, and allocate more energy into reproduction.
与大陆亲缘种相比，许多岛屿小型脊椎动物体型更大、成熟更晚、产卵/产仔数更少、性二型性和攻击性更低。这组观察结果被称为“岛屿综合征”。该综合征与岛屿上的高种群密度有关。我们预测，当种群密度较低且/或波动时，岛屿脊椎动物可能会进化出相关的特征变化，与岛屿综合征相反，我们将其统称为“反向岛屿综合征”（RIS）假说。在近似水平上，我们假设 RIS 是由黑素皮质素受体活性水平增加引起的。黑素皮质素是前阿黑皮素原基因的翻译后产物，该基因在脊椎动物中控制着多效性的色素沉着、攻击性、性活动和食物摄入。
我们通过对意大利南部海岸小岛屿上的一种蓝颜色的意大利壁蜥 Podarcis sicula 进行一系列行为、遗传和个体发生学测试，对 RIS 假说进行了测试。这种蓝颜色变体的种群密度通常较低，且每年波动很大。与我们的预测一致，岛屿蜥蜴比它们的大陆亲缘种更具攻击性和性二型性。岛屿雄性具有宽阔的、完全变形的头部。岛屿雌性的生长速度比两性的大陆个体以及岛屿雄性的生长速度都慢，因此，岛屿上的大小和形状二型性更高。如预测的那样，黑素皮质素受体在岛屿种群个体中更为活跃。岛屿蜥蜴的食物摄入量高于大陆个体，这与黑素皮质素受体活性的增加是一致的。在像利科萨岛这样不可预测的环境中，这可能是适应性的。两性岛屿蜥蜴比它们的大陆亲缘种花更少的时间晒太阳。我们怀疑这是黑素皮质素受体活性增加的副产品（spandrel）。
我们认为，当由于环境的不可预测性导致种群密度较低或每年波动时，个体表现出更强的攻击性、提高食物摄入量并将更多的能量分配到繁殖中可能是有利的。
