Morek Witold, Surmacz Bartłomiej, López-López Alejandro, Michalczyk Łukasz
Department of Invertebrate Evolution, Institute of Zoology and Biomedical Research, Faculty of Biology, Jagiellonian University, Kraków, Poland.
Mol Ecol. 2021 Jul;30(14):3590-3609. doi: 10.1111/mec.15951. Epub 2021 Jun 7.
There is ample evidence that macroscopic animals form geographic clusters termed as zoogeographic realms, whereas distributions of species of microscopic animals are still poorly understood. The common view has been that micrometazoans, thanks to their putatively excellent dispersal abilities, are subject to the "Everything is everywhere but environment selects" hypothesis (EiE). One of such groups, <1 mm in length, are limnoterrestrial water bears (Tardigrada), which can additionally enter cryptobiosis that should further enhance their potential for long distance dispersion (e.g., by wind). However, an increasing number of studies, including the most recent phylogeny of the eutardigrade genus Milnesium, seem to question the general applicability of the EiE hypothesis to tardigrade species. Nevertheless, all Milnesium phylogenies published to date were based on a limited number of populations, which are likely to falsely suggest limited geographic ranges. Thus, in order to test the EiE hypothesis more confidently, we considerably enlarged the Milnesium data set both taxonomically and geographically, and analysed it in tandem with climate type and reproductive mode. Additionally, we time-calibrated our phylogeny to align it with major geological events. Our results show that, although cases of long distance dispersal are present, they seem to be rare and mostly ancient. Overall, Milnesium species are restricted to single zoogeographic realms, which suggests that these tardigrades have limited dispersal abilities. Finally, our results also suggest that the breakdown of Gondwana may have influenced the evolutionary history of Milnesium. In conclusion, phylogenetic relationships within the genus seem to be determined mainly by paleogeography.
有充分证据表明，宏观动物形成了被称为动物地理区的地理集群，而微观动物物种的分布仍知之甚少。普遍观点认为，微型后生动物由于其假定的出色扩散能力，符合“万物皆无处不在，环境选择之”假说（EiE）。其中一类体长小于1毫米的动物是淡水陆生水熊虫（缓步动物门），它们还能进入隐生状态，这应该会进一步增强其长距离扩散的潜力（例如通过风力）。然而，越来越多的研究，包括最近对真缓步虫属米尔恩斯缓步虫的系统发育研究，似乎对EiE假说对缓步虫物种的普遍适用性提出了质疑。尽管如此，迄今为止发表的所有米尔恩斯缓步虫系统发育研究都是基于数量有限的种群，这可能会错误地表明其地理分布范围有限。因此，为了更有信心地检验EiE假说，我们在分类学和地理范围上大幅扩充了米尔恩斯缓步虫的数据集，并结合气候类型和繁殖方式进行分析。此外，我们对系统发育进行了时间校准，使其与主要地质事件相匹配。我们的结果表明，虽然存在长距离扩散的情况，但似乎很少见且大多发生在远古时期。总体而言，米尔恩斯缓步虫物种局限于单个动物地理区，这表明这些缓步虫的扩散能力有限。最后，我们的结果还表明，冈瓦纳大陆的解体可能影响了米尔恩斯缓步虫的进化历史。总之，该属内的系统发育关系似乎主要由古地理学决定。
