Craddock Elysse M
School of Natural and Social Sciences, Purchase College, State University of New York, 735 Anderson Hill Road, Purchase, NY, 10577-1400, USA.
Biol Direct. 2016 Sep 6;11(1):44. doi: 10.1186/s13062-016-0146-1.
Species-rich adaptive radiations arising from rare plant and animal colonizers are common on remote volcanic archipelagoes. However, they present a paradox. The severe genetic bottleneck of founder events and effects of inbreeding depression, coupled with the inherently stressful volcanic environment, would seem to predict reduced evolutionary potential and increased risk of extinction, rather than rapid adaptive divergence and speciation. Significantly, eukaryotic genomes harbor many families of transposable elements (TEs) that are mobilized by genome shock; these elements may be the primary drivers of genetic reorganization and speciation on volcanic islands.
Here I propose that a central factor in the spectacular radiation and diversification of the endemic Hawaiian Drosophila and other terrestrial lineages on the Hawaiian and other oceanic islands has been repeated bursts of transposition of multiple TEs induced by the unique ecological features of volcanic habitats. Founder individuals and populations on remote volcanic islands experience significant levels of physiological and genomic stress as a consequence of both biotic and abiotic factors. This results in disruption of the usual epigenetic suppression of TEs, unleashing them to proliferate and spread, which in turn gives rise to novel genetic variation and remodels genomic regulatory circuits, facilitating rapid morphological, ecological and behavioral change, and adaptive radiation.
To obtain empirical support for the hypothesis, test organisms should be exposed to prolonged heat stress, high levels of carbon dioxide and other volcanic gases, along with inbreeding. Data from subsequent whole genome sequencing and bioinformatics screening for TE numbers and locations would then be compared with initial pre-exposure TE information for the test strains, a labor-intensive project. Several predicted outcomes arising from the hypothesis are discussed. Currently available data are consistent with the proposed concept of stress-induced TE mobilization as a trigger of evolutionary diversification and speciation on volcanic islands.
The main implication is that both TEs and species should proliferate at a much higher rate on volcanic islands than elsewhere. Second, the evolvability of a lineage may correlate with the abundance and distribution of TEs in the genome. Successful colonizers of volcanic habitats with high genomic proportions of TEs may be best poised to found a speciose lineage that gives rise to a dramatic adaptive radiation. Colonizers that are depauperate in TEs are likely to be evolutionarily constrained and diversify little, if at all.
This article was reviewed by Dr. James Shapiro and Dr. Wolfgang Miller (nominated by Editorial Board member Dr. I. King Jordan).
由稀有动植物殖民者产生的物种丰富的适应性辐射在偏远的火山群岛上很常见。然而，它们呈现出一个悖论。奠基者事件造成的严重遗传瓶颈以及近亲繁殖衰退的影响，再加上火山环境固有的压力，似乎预示着进化潜力降低和灭绝风险增加，而不是快速的适应性分化和物种形成。值得注意的是，真核生物基因组包含许多可通过基因组冲击而被激活的转座元件（TEs）家族；这些元件可能是火山岛上基因重组和物种形成的主要驱动因素。
在此我提出，夏威夷特有的果蝇以及夏威夷和其他海洋岛屿上的其他陆生物种惊人的辐射和多样化的一个核心因素，是由火山栖息地独特的生态特征诱导的多个TEs的多次转座爆发。偏远火山岛上的奠基个体和种群由于生物和非生物因素，经历了显著水平的生理和基因组压力。这导致了对TEs通常的表观遗传抑制被破坏，使其得以增殖和扩散，进而产生新的遗传变异并重塑基因组调控回路，促进快速的形态、生态和行为变化以及适应性辐射。
为了获得对该假说的实证支持，应使受试生物暴露于长期热应激、高浓度二氧化碳和其他火山气体中，同时进行近亲繁殖。随后对受试菌株进行全基因组测序以及对TEs数量和位置进行生物信息学筛选所得到的数据，将与受试菌株暴露前的初始TE信息进行比较，这是一个劳动密集型项目。文中讨论了该假说产生的几个预测结果。目前可得的数据与所提出的应激诱导TEs激活作为火山岛进化多样化和物种形成触发因素的概念一致。
主要意义在于，TEs和物种在火山岛上的增殖速度应比其他地方高得多。其次，一个谱系的进化能力可能与基因组中TEs的丰度和分布相关。具有高基因组比例TEs的火山栖息地的成功殖民者可能最有条件建立一个产生显著适应性辐射的物种丰富的谱系。TEs匮乏的殖民者可能在进化上受到限制，几乎不会多样化。
本文由詹姆斯·夏皮罗博士和沃尔夫冈·米勒博士评审（由编辑委员会成员I. 金·乔丹博士提名）。
