Feder Jeffrey L, Chilcote Charley A, Bush Guy L
Department of Zoology, Michigan State University, E. Lansing, MI, 48824.
Evolution. 1990 May;44(3):570-594. doi: 10.1111/j.1558-5646.1990.tb05939.x.
True fruit flies in the Rhagoletis pomonella sibling species group are at the center of a long-standing debate concerning modes of speciation. The allopatric separation of populations is widely thought to be a prerequisite for speciation in sexually reproducing animals. However, speciation in the R. pomonella group appears to have occurred sympatrically as a consequence of these flies shifting and adapting to new host plants. The sympatric shift of R. pomonella from its native host hawthorn to introduced domestic apple, which occurred approximately 150 years ago, provides a test of whether host specialization is sufficient to allow populations to differentiate in the absence of geographic barriers to gene flow. We report the results of a geographic study of allozyme variation for hawthorn and apple infesting populations of R. pomonella across the eastern United States and Canada. Six loci consistently show significant allele frequency differences at paired apple and hawthorn sites. These six loci map to three different regions of the genome, and linkage disequilibrium exists between non-allelic genes within each of these regions. Allele frequencies for five of the six loci displaying host associated differences also co-vary significantly with latitude. Inter-host divergence is, therefore, superimposed on north-south clinal patterns of intra-host variation such that the magnitude of genetic divergence between hawthorn and apple flies is a function of latitude. The findings suggest that partially reproductively isolated "host races" can evolve in sympatry as a consequence of R. pomonella infesting new host plants. Host recognition and host associated developmental traits are discussed as important factors differentiating apple and hawthorn flies.
苹果实蝇（Rhagoletis pomonella）所属的同域物种组中的真正果蝇，是一场关于物种形成模式的长期争论的核心。人们普遍认为，种群的异域隔离是有性繁殖动物物种形成的先决条件。然而，苹果实蝇组中的物种形成似乎是在同域发生的，原因是这些果蝇转移并适应了新的寄主植物。大约150年前，苹果实蝇从其原生寄主山楂转移到引入的家苹果上，这为检验在没有基因流动的地理障碍的情况下，寄主专一化是否足以使种群分化提供了一个测试。我们报告了一项关于美国东部和加拿大苹果实蝇侵染山楂和苹果种群的等位酶变异的地理研究结果。在成对的苹果和山楂地点，六个基因座始终显示出显著的等位基因频率差异。这六个基因座映射到基因组的三个不同区域，并且在每个区域内的非等位基因之间存在连锁不平衡。显示出寄主相关差异的六个基因座中的五个基因座的等位基因频率也与纬度显著共变。因此，寄主间的分化叠加在寄主内变异的南北渐变模式上，使得山楂果蝇和苹果果蝇之间的遗传分化程度是纬度的函数。这些发现表明，由于苹果实蝇侵染新的寄主植物，部分生殖隔离的“寄主族”可以在同域进化。寄主识别和与寄主相关的发育特征被讨论为区分苹果果蝇和山楂果蝇的重要因素。
