The Genetic Control and Evolutionary Significance of a Barrier to Interspecific Hybridization

10年的研究已经证明，地蟋蟀，异种的fasciatus和A. socius，通过同种精子优先性在生殖上是隔离的。换句话说，这两个物种在自然界中很少产生杂交后代，因为其中一个物种的精子在另一个物种雌性的生殖道中表现不佳。这是为数不多的情况之一，生物学家有一个清楚的认识特征，生殖隔离两个密切相关的物种。将通过数量性状位点（QTL）定位研究来评估这一生殖屏障的遗传控制。这是一项相对较新的技术，允许生物学家列举和定位控制复杂性状的基因（或qtl），例如同种精子优先权。绘制的种群将足够大，可以检测到对同种精子优先权影响很小的qtl。因此，这项研究将能够清楚地评估对生殖隔离的发展有大影响的基因和小影响的基因的作用。一旦确定了qtl，将对每个qtl的影响进行个别检查。通过精心设计的遗传杂交，筋膜库蚊的单个qtl将被引入到社会库蚊中，反之亦然。这样产生的蟋蟀将使QTL能够从其通常的遗传背景中分离出来进行研究，从而直接测试QTL在控制同种精子优先权方面的作用。最后，最近的研究表明，同种精子优先权进化得很快，可能经常是在不同种群之间出现的第一个生殖障碍。因此，了解这种屏障如何影响密切相关物种之间的重叠区域是很重要的。同种精子优先级对重叠区域的影响将通过遗传和人口统计学建模方法进行分析。物种形成是新物种形成的过程，也是导致地球上生命多样性的过程。生物学家对物种形成的理解相对较差。这是人类保护物种多样性努力中的一个重大缺陷，除非新物种继续产生，否则保护物种多样性的努力不可能成功。事实证明，了解近亲物种之间生殖障碍的遗传基础特别困难。这一困难可归因于两个因素：1)生殖隔离的多面性使得很难确定将两个物种隔离的特征，2)直到最近才有必要分析复杂特征的工具。就筋膜库蠓和社会库蠓而言，经过十年的艰苦研究，克服了第一个困难。第二个困难已经被遗传学的最新进展所克服，这些进展导致了几乎可以在任何生物体中绘制的标记的发展。绘制的标记可以监测基因组不同区域对生殖屏障的影响。将目前对异卵母细胞系统的理解与遗传学的这些新发展相结合，将为生殖障碍的遗传学提供前所未有的见解。特别是，拟议的研究将允许确定筋膜库蚊和社会库蚊之间的生殖隔离是由少数主要影响基因还是由许多小影响基因控制。这种区别是一场持续了一个多世纪的争论的中心，对于理解种群的空间分离是否是物种形成所必需的，以及物种形成所需的时间是长还是短，至关重要。最后，对同卵精子优先权遗传控制的研究归结为对生育问题遗传控制的研究。因此，这项研究提供的对不孕症的见解将与包括人类在内的许多生物相关。

Howard 9726502 Ten years of research have demonstrated that the ground crickets, Allonemobius fasciatus and A. socius, are reproductively isolated by conspecific sperm precedence. In other words, the two species produce few hybrids in nature because sperm from one species operates poorly in the reproductive tract of females from the other species. This is one of the very few situations in which biologists have a clear understanding of the trait that reproductively isolates two closely related species. The genetic control of this reproductive barrier will be assessed through a quantitative trait loci (QTL) mapping study. This is a relatively new technique that allows biologists to enumerate and locate the genes (or QTLs) that control a complex trait, such as conspecific sperm precedence. The mapping populations will be large enough to detect QTLs that have even a small effect on conspecific sperm precedence. Thus the study will allow a clear evaluation of the role of genes with large effects and the role of genes with small effects on the development of reproductive isolation. Once QTLs have been identified, the effects of each will be examined on an individual basis. Through carefully designed genetic crosses, single QTLs from A. fasciatus will be introduced into A. socius and vice-versa. The crickets so produced will allow a QTL to be studied in isolation from its usual genetic background, and thereby directly test the role of the QTL in controlling conspecific sperm precedence. Finally, recent studies indicate that conspecific sperm precedence evolves quickly and may often be the first reproductive barrier to arise between diverging populations. Therefore, it is important to understand how such a barrier influences zones of overlap between closely related species. The effect of conspecific sperm precedence on zones of overlap will be analyzed through a genetic and demographic modeling approach. Speciation, the process by which new species are formed and ther efore the process responsible for the diversity of life on earth, is relatively poorly understood by biologists. This is a critical shortcoming in humanity's efforts to preserve species diversity, which cannot succeed unless new species continue to be generated. Reaching an understanding of the genetic basis of reproductive barriers between closely related species has proven particularly difficult. The difficulty can be attributed to two factors: 1) the multi-faceted nature of reproductive isolation makes it hard to identify the traits that isolate two species, and 2) the tools necessary to dissect complex traits were not, until recently, available. In the case of A. fasciatus and A. socius the first difficulty was overcome by ten years of painstaking research. The second difficulty has been overcome by recent advances in genetics that have led to the development of markers that can be mapped in virtually any organism. Mapped markers allow the effects of different regions of the genome on a reproductive barrier to be monitored. Combining the current understanding of the Allonemobius system with these new developments in genetics will provide unprecedented insight into the genetics of reproductive barriers. In particular, the proposed research will allow a determination of whether reproductive isolation between A. fasciatus and A. socius is controlled by a few genes of major effect or by many genes of small effect. This distinction is at the center of a debate that has raged for more than a century and is critical for understanding whether spatial separation of populations is necessary for speciation and whether the time required for speciation is large or small. Finally, the study of the genetic control of conspecific sperm precedence boils down to the study of the genetic control of fertility problems. Thus, the insights into infertility provided by this research will be relevant to many organisms, including humans.