Disruptive selection and the genetic basis for repeated evolution of nonparasitism in lampreys

我建议的研究将使用分子遗传和发育技术来研究七鳃鳗物种对中产生和维持截然不同的摄食类型的机制。七鳃鳗是古老的鳗鱼状脊椎动物；它们被称为“无颚鱼”，因为它们有一个带齿的吸吮盘，而不是铰接的颚。所有的七鳃鳗在生命的最初几年都是失明的幼虫，生活在河流和小溪的淤泥底部，以微生物为食。在变态之后，它们可能寄生在鱼身上，也可能在成年后不进食就繁殖并死亡（即非寄生）。在全世界已知的38个物种中，大约有一半是非寄生的，人们相信这些非寄生物种是从寄生祖先进化而来的。这种情况在许多不同的七鳃鳗群体中独立发生，大多数七鳃鳗物种可以分成成对，其中幼虫难以区分，但两种摄食方式在变态时分道扬镳。传统上，寄生和非寄生七鳃鳗被认为是不同的物种，但最近的研究表明，它们可能是同一物种的不同摄食类型。然而，如果这两种类型都是由一个共同的基因库产生的，为什么在自然界中没有观察到这两种摄食类型的中间七鳃鳗？这是我的研究将试图回答的问题。例如，通过实地调查和DNA亲子分析，我将确定是否存在防止杂交的生态机制（例如，两种不同类型的鱼是否在不同的时间或地点产卵？）不同种类之间的体型差异会阻止它们杂交吗？也可能存在遗传机制（例如，对“不适合”杂交的选择），即使有显著的基因流动，也能维持两种不同的喂养类型。还有其他几种鱼类也有类似的不同的摄食类型（例如，“侏儒”和普通白鱼），七鳃鳗提供了另一个很好的模型系统来研究允许这种独立、重复和快速形态多样化的过程。对这些物种对的研究将有助于回答一个重要的基本进化问题：物种是如何形成和维持的？

My proposed research will use molecular genetic and developmental techniques to investigate the mechanisms responsible for creating and maintaining dramatically different feeding types in lamprey species pairs.  Lampreys are ancient eel-like vertebrates; they are called "jawless fishes" since they possess a toothed suctorial disc rather than hinged jaws.  All lampreys spend the first years of life as blind larvae in the silt bottoms of rivers and streams and feed on microscopic organisms.  Following metamorphosis, they may feed parasitically on fish or they may reproduce and die without ever feeding as adults (i.e. are nonparasitic).  Approximately half of the 38 described species worldwide are nonparasitic, and it is believed that these nonparasitic species evolved from parasitic ancestors.  This has happened independently within many different lamprey groups, and most lamprey species can be grouped into pairs in which the larvae are indistinguishable but the two feeding types diverge at metamorphosis.  Paired parasitic and nonparasitic lampreys have traditionally been considered distinct species, but recent research indicates that they may be different feeding types of a single species.  However, if both types are produced by a common gene pool, why are lampreys intermediate to the two feeding types not observed in nature?  This is the question that my research will attempt to answer.  For example, using field surveys and DNA parentage analysis, I will determine if there are ecological mechanisms that prevent interbreeding (e.g., do the two different types spawn at different times or places? do size differences between the types prevent them from interbreeding?).  There may also be genetic mechanisms (e.g., selection against "unfit" hybrids) that maintain two distinct feeding types even with significant gene flow.  There are several other fish species with similarly divergent feeding types (e.g., "dwarf" and normal whitefishes), and lampreys provide another good model system to investigate processes that permit this independent, repeated, and rapid morphological diversification.  Research into these species pairs will help answer an important fundamental evolutionary question: how are species formed and maintained?