Doellman Meredith M, Saint Jean Gilbert, Egan Scott P, Powell Thomas H Q, Hood Glen R, Schuler Hannes, Bruzzese Daniel J, Glover Mary M, Smith James J, Yee Wee L, Goughnour Robert, Rull Juan, Aluja Martin, Feder Jeffrey L
Department of Biological Sciences University of Notre Dame Notre Dame Indiana USA.
Present address: Department of Ecology and Evolution University of Chicago Chicago Illinois USA.
Ecol Evol. 2020 Oct 28;10(23):12727-12744. doi: 10.1002/ece3.6667. eCollection 2020 Dec.
An important criterion for understanding speciation is the geographic context of population divergence. Three major modes of allopatric, parapatric, and sympatric speciation define the extent of spatial overlap and gene flow between diverging populations. However, mixed modes of speciation are also possible, whereby populations experience periods of allopatry, parapatry, and/or sympatry at different times as they diverge. Here, we report clinal patterns of variation for 21 nuclear-encoded microsatellites and a wing spot phenotype for cherry-infesting (Diptera: Tephritidae) across North America consistent with these flies having initially diverged in parapatry followed by a period of allopatric differentiation in the early Holocene. However, mitochondrial DNA (mtDNA) displays a different pattern; cherry flies at the ends of the clines in the eastern USA and Pacific Northwest share identical haplotypes, while centrally located populations in the southwestern USA and Mexico possess a different haplotype. We hypothesize that the mitochondrial difference could be due to lineage sorting but more likely reflects a selective sweep of a favorable mtDNA variant or the spread of an endosymbiont. The estimated divergence time for mtDNA suggests possible past allopatry, secondary contact, and subsequent isolation between USA and Mexican fly populations initiated before the Wisconsin glaciation. Thus, the current genetics of cherry flies may involve different mixed modes of divergence occurring in different portions of the fly's range. We discuss the need for additional DNA sequencing and quantification of prezygotic and postzygotic reproductive isolation to verify the multiple mixed-mode hypothesis for cherry flies and draw parallels from other systems to assess the generality that speciation may commonly involve complex biogeographies of varying combinations of allopatric, parapatric, and sympatric divergence.
理解物种形成的一个重要标准是种群分化的地理背景。异地物种形成、邻域物种形成和同域物种形成这三种主要模式定义了分化种群之间空间重叠和基因流动的程度。然而，混合的物种形成模式也是可能的，即种群在分化过程中会在不同时间经历异地、邻域和/或同域阶段。在这里，我们报告了北美地区21个核编码微卫星和一种樱桃实蝇（双翅目：实蝇科）翅斑表型的渐变模式，这与这些果蝇最初在邻域中分化，随后在全新世早期经历一段异地分化期相一致。然而，线粒体DNA（mtDNA）呈现出不同的模式；美国东部和太平洋西北部渐变群两端的樱桃实蝇共享相同的单倍型，而美国西南部和墨西哥中部的种群则拥有不同的单倍型。我们推测线粒体差异可能是由于谱系分选，但更有可能反映了一个有利的mtDNA变体的选择性清除或内共生体的传播。mtDNA的估计分化时间表明，美国和墨西哥果蝇种群之间过去可能存在异地隔离、二次接触以及随后的隔离，这些隔离在威斯康星冰川作用之前就已开始。因此，樱桃实蝇当前的遗传学可能涉及果蝇分布范围不同部分发生的不同混合分化模式。我们讨论了进行额外DNA测序以及对合子前和合子后生殖隔离进行量化的必要性，以验证樱桃实蝇的多重混合模式假说，并借鉴其他系统的情况来评估物种形成可能普遍涉及异地、邻域和同域分化不同组合的复杂生物地理学这一普遍性。
