Mitochondrial genotype influences the response to cold stress in the European green crab Carcinus maenas

Hybrid zones provide natural experiments in recombination within and between genomes that may have strong effects on organismal fitness. On the East Coast of North America, two distinct lineages of the European green crab (Carcinus maenas) have been introduced in the last two centuries. These two lineages with putatively different adaptive properties have hybridized along the coast of the eastern Gulf of Maine, producing new nuclear and mitochondrial combinations that show clinal variation correlated with water temperature. To test the hypothesis that mitochondrial or nuclear genes have effects on thermal tolerance, we first measured the response to cold stress in crabs collected throughout the hybrid zone, then sequenced the mitochondrial CO1 gene and two nuclear SNPs representative of nuclear genetic lineage. Mitochondrial haplotype had a strong association with the ability of crabs to right themselves at 4.5 &[deg]C that was sex specific: haplotypes originally from northern Europe gave male crabs an advantage while there was no haplotype effect on righting in female crabs. By contrast, the two nuclear SNPs that were significant outliers in a comparison between northern and southern C. maenas populations had no effect on righting response at low temperature. These results add C. maenas to the short list of ectotherms in which mitochondrial variation has been shown to affect thermal tolerance, and suggests that natural selection is shaping the structure of the hybrid zone across the Gulf of Maine. Our limited genomic sampling does not eliminate the strong possibility that mitonuclear coadaptation may play a role in the differences in thermal phenotypes documented here. Linkage between mitochondrial genotype and thermal tolerance suggests a role for local adaptation in promoting the spread of invasive populations of C. maenas around the world.

杂交带为基因组内部及基因组间的重组提供了天然实验体系，此类重组可对生物体适合度产生显著影响。在北美东海岸，欧洲绿蟹（*Carcinus maenas*）的两个独立支系在过去两个世纪中被相继引入。这两个具有推测性不同适应性特征的支系，在缅因湾东部沿岸发生杂交，产生了新的核基因与线粒体基因组合，这些组合呈现出与水温相关的渐变变异。为验证"线粒体基因或核基因对热耐受性存在影响"这一假说，我们首先对采集自整个杂交带的欧洲绿蟹开展冷应激响应测定，随后对线粒体CO1基因以及两个代表核遗传支系的核单核苷酸多态性（Single Nucleotide Polymorphism, SNP）进行了测序。线粒体单倍型与绿蟹在4.5℃条件下的翻正能力存在显著关联，且该关联具有性别特异性：源自北欧的单倍型赋予雄性绿蟹生存优势，而雌性绿蟹的翻正能力则未受单倍型影响。与之形成对照的是，在欧洲绿蟹南北种群比对中呈现显著异常值的两个核SNP，并未对低温下的翻正响应产生任何影响。本研究结果将欧洲绿蟹纳入了为数不多的已证实线粒体变异影响热耐受性的外温动物（ectotherm）之列，并表明自然选择正在塑造缅因湾杂交带的种群结构。我们有限的基因组采样并未排除核-线粒体共适应（mitonuclear coadaptation）可能在本研究记录的热表型差异中发挥作用的极大可能性。线粒体基因型与热耐受性之间的关联提示，局部适应或许在推动欧洲绿蟹全球入侵种群扩散的过程中扮演了重要角色。