Invasion facilitates hybridization with introgression in the Rattus rattus species complex

Biological invasions result in novel species interactions, which can have significant evolutionary impacts on both native and invading taxa. One evolutionary concern with invasions is hybridization among lineages that were previously isolated, but make secondary contact in their invaded range(s). Black rats, consisting of several morphologically very similar but genetically distinct taxa that collectively have invaded six continents, are arguably the most successful mammalian invaders on the planet. We used mitochondrial cytochrome b sequences, two nuclear gene sequences (Atp5a1 and DHFR), and nine microsatellite loci to examine the distribution of three invasive black rat lineages (R. tanezumi, R. rattus I, and R. rattus IV) in the U.S. and Asia, and determine the extent of hybridization among these taxa. Our analyses revealed two mitochondrial lineages that have spread to multiple continents, including a previously undiscovered population of R. tanezumi in the southeastern U.S., whereas the third lineage (R. rattus IV) appears to be confined to Southeast Asia. Analyses of nuclear DNA (both sequences and microsatellites) suggested significant hybridization is occurring among R. tanezumi and R. rattus I in the U.S., and also suggest hybridization between R. tanezumi and R. rattus IV in Asia, although further sampling of the latter species pair in Asia is required. Furthermore, microsatellite analyses suggest unidirectional introgression from both R. rattus I and R. rattus IV into R. tanezumi. Within the U.S., introgression appears to be occurring to such a pronounced extent that we were unable to detect any nuclear genetic signal for R. tanezumi, and a similar pattern was detected in Asia.

生物入侵会催生全新的物种互作关系，对本地类群与入侵类群均能产生显著的进化影响。入侵过程中一个值得关注的进化问题是：原本彼此隔离的演化支系在入侵区域发生二次接触后，会产生杂交事件。黑家鼠类群包含多个形态高度相似但遗传特征迥异的类群，整体已入侵六大洲，堪称全球范围内最为成功的哺乳动物入侵物种。本研究借助线粒体细胞色素b（mitochondrial cytochrome b）序列、2个核基因序列（Atp5a1与DHFR）以及9个微卫星位点，对美国与亚洲境内的3个入侵性黑家鼠支系——罗赛鼠（R. tanezumi）、家鼠I型（R. rattus I）以及家鼠IV型（R. rattus IV）——的分布情况展开调查，并解析这些类群间的杂交程度。分析结果显示，有2个线粒体支系已扩散至多洲区域，其中包括美国东南部此前未被记录的罗赛鼠种群；而第三个支系（家鼠IV型）似乎仅分布于东南亚地区。核DNA（包括序列数据与微卫星数据）分析表明，美国境内的罗赛鼠与家鼠I型之间正发生显著的杂交事件；同时研究也发现亚洲地区的罗赛鼠与家鼠IV型之间存在杂交现象，但针对后者这一类群组合的亚洲采样工作仍需进一步开展。此外，微卫星分析结果显示，家鼠I型与家鼠IV型均存在向罗赛鼠的单向基因渐渗现象。在美国境内，基因渐渗的程度极为显著，以至于我们无法检测到罗赛鼠的核遗传信号；而在亚洲地区也观测到了类似的模式。