The evolution of morphological complexity in the Dictyostelids

生物学家想了解像我们这样复杂的多细胞生物是如何从它们简单的单细胞祖先进化而来的。我们在理论上知道这是如何发生的：早期生物体基因的自发突变导致其后代发育程序的微小变化。这有时会导致一个更成功地繁殖的改进的成年人，因此逐渐取代了早期的形式。然而，要真正理解这一过程并证明它确实发生了，我们必须追溯哪些基因发生了突变，以及这种突变如何改变基因功能。我们还需要知道哪些发育机制受到突变基因的调控，以及改变的发育机制最终如何产生改进的成体形式。由于不可能获得像我们这样高度进化的动物的详细信息，我们在社会阿米巴中研究这个问题。这些生物在进食时以单细胞形式存在。然而，当饥饿时，它们聚集在一起并形成多细胞子实体，其中一部分细胞作为孢子保存下来。其他细胞被牺牲以形成有助于孢子传播的结构。这种生活方式依赖于细胞的相互协作和专业化。在进化过程中，社会性变形虫已经从形成具有10-100个细胞和仅两种细胞类型的结构的基础物种发展到形成具有超过100.000个细胞和多达五种细胞类型的大型复杂结构的物种。其中一个物种D.discoideum被许多实验室用作模型系统，以了解细胞如何移动，进食，繁殖和相互交流。因为它的基因组已经测序，我们可以获得控制这些过程的所有基因。盘状杜父菌（D.discoideum）使用环腺苷酸（cAMP）作为细胞间通讯的主要信号分子。它作为一种化学引诱物将饥饿的细胞聚集在一起。然后，它继续引导细胞连贯地移动并建立子实体。cAMP还决定哪些细胞应该分化成孢子。盘状阿米巴是已知的75种社会性阿米巴之一。这些物种在其果实结构的大小和形状上表现出很大的差异。为了了解这些物种是如何逐渐变得莫尔复杂和彼此不同的，我们首先需要知道它们是如何相互联系的，以及它们与它们的祖先--孤独的变形虫--的关系。在之前的研究中，我们使用DNA数据构建了一个社会性变形虫的家谱，现在我们知道有四个主要的社会性变形虫群体。D.discoideum属于最进化的组4。我们还发现，cAMP信号传导所必需的许多基因存在于所有四组中。然而，在物种之间，这些基因活跃的发育阶段存在差异。此外，一些基因已经复制并开始承担新的角色。在这个项目中，我们将重建在社会变形虫进化过程中发生的所有形状和大小变化的顺序。我们将通过测量大量决定所有75个物种的典型大小和形状的特征，并将这些特征绘制在家谱上来做到这一点。这将使我们能够得出结论，哪个角色首先出现，以及它如何逐渐变成更大或有时更小的复杂性。我们还将绘制特定cAMP信号传导基因的存在到家谱和这些基因的变化。这使我们能够得出结论，基因中的特定变化是否伴随着特定的性格变化。通过操纵所讨论的基因并观察其对物种特征的影响，我们将能够证明特定的遗传变化是特定特征变化的实际原因。通过这种方式，我们将能够解开进化中用来产生物种多样性和复杂性的遗传机制。

Biologists want to understand how complex multicellular organisms like ourselves have evolved from their simple single-celled ancestors. We know in theory how this happened: Spontaneous mutations in the genes of earlier organisms caused small changes in the developmental program of their offspring. This sometimes resulted in an improved adult that more successfully reproduced, and therefore gradually replaced the earlier form. However, to really understand this process and prove that it actually occurred, we have to trace back which genes were mutated and how this mutation changed gene function. We also need to know which developmental mechanisms were regulated by the mutated genes and how the altered developmental mechanism eventually produced the improved adult form. Because it is not possible to obtain such detailed information for highly evolved animals like ourselves, we investigate this problem in the social amoebas. These organisms live as single cells when they are feeding. However, when starved, they come together and form a multicellular fruiting body, in which a proportion of cells is preserved as spores. The other cells are sacrificed to form a structure that aids spore dispersal. This life style depends on mutual collaboration and specialization of cells. In the course of evolution the social amoebae have progressed from basal species that formed structures with 10-100 cells and only two cell-types, to species that form large complex structures with over 100.000 cells and up to five cell types. One species, D.discoideum, is used by many laboratories as a model system to understand how cells move, eat, propagate and communicate with each other. Because its genome has been sequenced, we have access to all the genes that control these processes. D.discoideum uses cyclic AMP (cAMP) as the major signal molecule for cell-cell communication. It acts as a chemoattractant to bring starving cells together. It then continues to guide cells to move coherently and build a fruiting body. cAMP also decides which cells should differentiate into spores. D.discoideum is one of 75 known social amoeba species. These species display large differences in the size and shape of their fruiting structures. To understand how these species gradually became mor complex and different from each other, we first need to know how they are related to each other and to their ancestors, the solitary amoebas. In previous research we used DNA data to construct a family tree of the social amoebas and we now know that there are four major groups of social amoebas. D.discoideum belongs to the most evolved group 4. We also found that many of the genes that are necessary for cAMP signalling are present in all four groups. However, between species, there are differences in the stage of development at which these genes are active. In addition, some genes have duplicated and started to assume novel roles. In this project we will reconstruct in what order all changes in shape and size occurred during social amoeba evolution. We will do this by measuring a large number of characters that determine the typical size and shape of all 75 species and by plotting these characters on the family tree. This will allow us to conclude which character was there first and how it gradually changed into greater or sometimes lesser complexity. We will also plot the presence of specific cAMP signalling genes to the family tree and the changes in these genes. This allows us to conclude whether a specific change in a gene was accompanied by a specific change in character. By manipulating the gene in question and observing its effect on species character we will be able to prove that a particular genetic change was the actual cause for a specific change in character. In this manner we will be able to unravel the genetic mechanisms that have been used by evolution to generate species diversity and complexity.