Hosts provide the main environmental traits parasites have to deal with, resulting in covariation between both associates at both micro‐ and macro‐evolutionary scales; phylogenetic analyses of highly host‐specific parasites have shown that parasite and host phylogeny might be highly congruent, and adaptation of a host species to new environments may lead to concordant changes of their parasites. Procamallanus (Spirocamallanus) neocaballeroi is a highly host‐specific parasitic nematode of the Neotropical freshwater fish genus Astyanax in Mexico. One of the host species of the nematode is the emblematic Mexican tetra, A. mexicanus, which exhibits two contrasting phenotypes, a cave‐dwelling morph (with troglomorphic features), and the surface‐dwelling morph; other congeneric species inhabit rivers and lakes, and some of them occur in sympatry, displaying trophic specializations. Here, we explored the hypothesis that contrasting environments (surface rivers vs cave rivers), and host morphological divergence (sympatric ecomorphs in a lacustrine environment) might result in the divergence of their parasites, even though the hosts maintain a cohesive genetic structure as the same species. To test the hypothesis, several populations of Astyanax spp. were sampled to search for P. (S.) neocaballeroi. The nematode was found in 10 of the 52 sampled sites; two localities corresponded to cave populations. The phylogenetic analysis based on COI sequences yielded three major lineages for P. (S.) neocaballeroi. We found no concordance between the three lineages and the habitat where they occur in Astyanax mexicanus, even considering those living in drastic environmental conditions (caves), or between these lineages and lacustrine ecomorphs of Astyanax aeneus and A. caballeroi occurring in sympatry. Instead, genetic lineages of the nematode exhibit a clearer pattern of host species association and geographical distribution; our results showed that P. (S.) neocaballeroi is experiencing an incipient divergence although the morphological study of lineages shows no conspicuous differences.

中文翻译：

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早期的遗传差异还是神秘的物种形成？Procamallanus (Nematoda) 在淡水鱼 ( Astyanax )

寄主提供了寄生虫必须处理的主要环境特征，导致两个关联体在微观和宏观进化尺度上的协变；高度宿主特异性寄生虫的系统发育分析表明，寄生虫和宿主系统发育可能高度一致，宿主物种对新环境的适应可能导致其寄生虫的一致变化。Procamallanus (Spirocamallanus) neocaballeroi 是墨西哥新热带淡水鱼属 Astyanax 的一种高度宿主特异性寄生线虫。线虫的宿主物种之一是象征性的墨西哥四线虫，A. mexicanus，它表现出两种截然不同的表型，洞穴栖息形态（具有穴居形态特征）和表面栖息形态；其他同类物种栖息在河流和湖泊中，其中一些共生，显示营养专长。在这里，我们探讨了这样的假设，即对比环境（地表河流与洞穴河流）和宿主形态差异（湖泊环境中的同域生态形态）可能导致其寄生虫的差异，即使宿主保持有凝聚力的遗传结构相同物种。为了检验这个假设，几个 Astyanax 种群。被采样以搜索 P. (S.) neocaballeroi。在 52 个采样点中的 10 个中发现了线虫；两个地点对应于洞穴种群。基于 COI 序列的系统发育分析产生了 P. (S.) neocaballeroi 的三个主要谱系。我们发现这三个谱系与它们出现在 Astyanax mexicanus 的栖息地之间没有一致性，即使考虑到那些生活在极端环境条件（洞穴）中的谱系，或在这些谱系和 Astyanax aeneus 和 A. caballeroi 的湖泊生态形态之间发生。相反，线虫的遗传谱系表现出更清晰的宿主物种关联和地理分布模式；我们的结果表明 P. (S.) neocaballeroi 正在经历初期的分化，尽管谱系的形态学研究没有显示出明显的差异。