Phenotypic Variation across Chromosomal Hybrid Zones of the Common Shrew (Sorex araneus) Indicates Reduced Gene Flow

Sorex araneus, the Common shrew, is a species with more than 70 karyotypic races, many of which form parapatric hybrid zones, making it a model for studying chromosomal speciation. Hybrids between races have reduced fitness, but microsatellite markers have demonstrated considerable gene flow between them, calling into question whether the chromosomal barriers actually do contribute to genetic divergence. We studied phenotypic clines across two hybrid zones with especially complex heterozygotes. Hybrids between the Novosibirsk and Tomsk races produce chains of nine and three chromosomes at meiosis, and hybrids between the Moscow and Seliger races produce chains of eleven. Our goal was to determine whether phenotypes show evidence of reduced gene flow at hybrid zones. We used maximum likelihood to fit tanh cline models to geometric shape data and found that phenotypic clines in skulls and mandibles across these zones had similar centers and widths as chromosomal clines. The amount of phenotypic differentiation across the zones is greater than expected if it were dissipating due to unrestricted gene flow given the amount of time since contact, but it is less than expected to have accumulated from drift during allopatric separation in glacial refugia. Only if heritability is very low, Ne very high, and the time spent in allopatry very short, will the differences we observe be large enough to match the expectation of drift. Our results therefore suggest that phenotypic differentiation has been lost through gene flow since post-glacial secondary contact, but not as quickly as would be expected if there was free gene flow across the hybrid zones. The chromosomal tension zones are confirmed to be partial barriers that prevent differentiated races from becoming phenotypically homogenous.

普通鼩鼱（Sorex araneus）是拥有70余种核型宗（karyotypic race）的物种，其中多数类群形成邻域杂交带（parapatric hybrid zone），使其成为研究染色体物种形成的经典模型生物。不同核型宗间的杂交后代适合度降低，但微卫星标记（microsatellite marker）研究证实宗间存在显著基因流，这使得“染色体屏障是否真的能推动遗传分化”这一问题受到质疑。我们针对两个杂合子结构尤为复杂的杂交带开展了表型梯度（phenotypic cline）研究：新西伯利亚宗与托木斯克宗的杂交后代在减数分裂过程中会形成9条与3条染色体组成的链，而莫斯科宗与塞利格宗的杂交后代则会形成11条染色体组成的链。本研究旨在探究杂交带处的表型是否存在基因流受限的迹象。我们采用最大似然法（maximum likelihood）将双曲正切（tanh）梯度模型拟合至几何形态学数据，结果发现，两处杂交带中颅骨与下颌骨的表型梯度，其中心位置与宽度均与染色体梯度相近。杂交带间的表型分化程度，相较于接触后随时间推移因无限制基因流而消散的预期水平更高，但又低于冰川避难所中异域隔离期间由遗传漂变积累的预期水平。仅当遗传力极低、有效种群数量（Ne）极高且异域隔离时长极短时，我们观测到的分化程度才足以匹配遗传漂变的预期结果。因此，本研究结果表明，自冰期后次生接触以来，表型分化已通过基因流发生消散，但消散速度并未达到杂交带间存在完全自由基因流时的预期水平。本研究证实，染色体张力带属于部分屏障，可阻止已分化的核型宗在表型上趋于同质化。