North American rice rats (Oryzomys) inhabit the coastal marshes and inland wetlands of the southeastern United States and parts of Mexico. The phylogeographic origins of rice rat populations in southern Illinois, the northern-most extent of their range in North America, has not previously been examined. Here, we present evidence, based on mitochondrial DNA sequencing, that rice rats in Southern Illinois belong to the western species, Oryzomys texensis and not Oryzomys palustris palustris as current range maps and previous morphological studies indicate. The pattern also points to a potential hybrid zone between the two species located further east than current distribution maps would suggest. Finally, we present a new estimate of the divergence time between O. palustris and O. texensis.

北美稻鼠（Oryzomys）栖息在美国东南部和墨西哥部分地区的沿海沼泽和内陆湿地。伊利诺伊州南部稻鼠种群的系统发育地理学起源是北美洲稻鼠分布范围的最北端，此前尚未对此进行过研究。在这里，我们基于线粒体 DNA 测序提供证据，表明伊利诺伊州南部的稻鼠属于西部物种，即德克萨斯稻鼠(Oryzomys texensis)，而不是当前分布图和之前的形态学研究表明的沼泽稻鼠(Oryzomys palustris palustris)。该模式还表明，这两个物种之间存在潜在的杂交区，其位置比目前的分布图显示的更远。最后，我们提出了O. palustris和O. texensis之间分歧时间的新估计。