Reverse Evolution: Driving Forces Behind the Loss of Acquired Photosynthetic Traits
Reverse Evolution: Driving Forces Behind the Loss of Acquired Photosynthetic Traits
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DOI：
10.1371/journal.pone.0008465
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发表时间：
2009-12-29
期刊：
影响因子：
3.7
通讯作者：
Boenigk, Jens
中科院分区：
文献类型：
--
作者：
de Castro, Francisco;Gaedke, Ursula;Boenigk, Jens
背景：在真核生物的进化中，光合作用的丧失经常发生，甚至比它的获得还要多，它至少独立发生了9次，并产生了古质纲、根霉属、ChroMalveolata和Exavata超群的进化。这种自养能力的二次丧失对于解释真核生物的进化和原生生物的高度多样性至关重要，直到最近，这一点一直被严重低估。方法/主要发现：利用异养和混合营养鞭毛虫和两种猎物--大型细菌和超微细菌的动态模型，我们研究了DOC浓度、混合营养生物的光合作用生长速率和光合作用的外部限制对这两种类型鞭毛虫共存的影响。我们的主要前提是：在高DOC浓度下，大型细菌比小型细菌生长得更快，反之亦然；异养鞭毛虫比捕食小型细菌的混合营养体更有效(两者都得到了经验支持)。我们表明，细菌放牧的差异效率强烈依赖于细胞大小，这是解释混合营养体(结合光合作用和细菌)光合作用丧失导致纯异养谱系的关键因素。进一步，我们展示了在什么条件下异养突变体可以与其混合营养的祖先共存，甚至竞争，这表明细菌象牙和细胞尺寸减小可能是真核生物多样化的主要触发因素。结论/意义：我们的结果表明，如果混合营养突变体的光合作用优势不是太大，(小的)异养突变菌也将在营养贫乏的环境中占据主导地位，并由于其更高的利用超微细菌的效率而容易入侵混合营养菌和细菌的群落。由于碳限制的条件可能在整个地球历史上都很普遍，这样的情景可以解释几乎所有主要藻类从光营养到混合营养再到异质营养的无数转变。我们挑战那些将吞噬作用的进化与富营养化或强烈光限制环境联系在一起的主流概念。
Background: The loss of photosynthesis has occurred often in eukaryotic evolution, even more than its acquisition, which occurred at least nine times independently and which generated the evolution of the supergroups Archaeplastida, Rhizaria, Chromalveolata and Excavata. This secondary loss of autotrophic capability is essential to explain the evolution of eukaryotes and the high diversity of protists, which has been severely underestimated until recently. However, the ecological and evolutionary scenarios behind this evolutionary "step back'' are still largely unknown.Methodology/Principal Findings: Using a dynamic model of heterotrophic and mixotrophic flagellates and two types of prey, large bacteria and ultramicrobacteria, we examine the influence of DOC concentration, mixotroph's photosynthetic growth rate, and external limitations of photosynthesis on the coexistence of both types of flagellates. Our key premises are: large bacteria grow faster than small ones at high DOC concentrations, and vice versa; and heterotrophic flagellates are more efficient than the mixotrophs grazing small bacteria (both empirically supported). We show that differential efficiency in bacteria grazing, which strongly depends on cell size, is a key factor to explain the loss of photosynthesis in mixotrophs (which combine photosynthesis and bacterivory) leading to purely heterotrophic lineages. Further, we show in what conditions an heterotroph mutant can coexist, or even out-compete, its mixotrophic ancestor, suggesting that bacterivory and cell size reduction may have been major triggers for the diversification of eukaryotes.Conclusions/Significance: Our results suggest that, provided the mixotroph's photosynthetic advantage is not too large, the (small) heterotroph will also dominate in nutrient-poor environments and will readily invade a community of mixotrophs and bacteria, due to its higher efficiency exploiting the ultramicrobacteria. As carbon-limited conditions were presumably widespread throughout Earth history, such a scenario may explain the numerous transitions from phototrophy to mixotrophy and further to heterotrophy within virtually all major algal lineages. We challenge prevailing concepts that affiliated the evolution of phagotrophy with eutrophic or strongly light-limited environments only.