Department of Botany, University of Wisconsin-Madison, 430 Lincoln Drive, Madison, WI 53706, USA.
Proc Biol Sci. 2012 Sep 7;279(1742):3377-83. doi: 10.1098/rspb.2012.0498. Epub 2012 Jun 13.
A ring species arises when a parental population expands around an area of unsuitable habitat in such a way that when the two fronts meet they behave as distinct species while still being connected through a series of intergrading populations. Ring species offer great possibilities for studying the forces causing species divergence (e.g. the nature of pre-zygotic or post-zygotic reproductive isolation) or helping to maintain species integrity (e.g. reinforcement). Yet, ring species are extremely rare, and have only been documented convincingly in animals. Here, we present phylogenetic analyses of two nuclear gene regions from the Caribbean slipper spurge (Euphorbia tithymaloides) species complex that provide evidence that this group forms a ring species. These data show that the species complex originated in the area where Mexico and Guatemala meet, and expanded around the Caribbean basin along two distinct fronts: one eastward through the Yucatan Peninsula and into the Greater Antilles (GA); one southeastward through northern South America and then northward to the Lesser Antilles and eastern GA. The two terminal forms co-occur in the Virgin Islands and appear to be morphologically and ecologically distinct. Thus, our results suggest that Euphorbia tithymaloides is the first compelling example of a ring species in plants.
当一个亲代种群在不适宜的栖息地周围扩张时，就会产生一个环种，当两个前沿相遇时，它们表现为不同的物种，而仍然通过一系列逐渐过渡的种群连接。环种为研究导致物种分化的力量（例如，合子前或合子后生殖隔离的性质）或帮助维持物种完整性（例如，强化）提供了很大的可能性。然而，环种极为罕见，仅在动物中得到了令人信服的记录。在这里，我们展示了来自加勒比拖鞋黄麻（Euphorbia tithymaloides）物种复合体的两个核基因区域的系统发育分析，这些分析提供了证据表明该群体形成了一个环种。这些数据表明，该物种复合体起源于墨西哥和危地马拉交界的地区，并沿着两个不同的前沿向加勒比盆地扩张：一个向东通过尤卡坦半岛并进入大安的列斯群岛（GA）；一个向东南通过南美洲北部，然后向北到小安的列斯群岛和东部 GA。两个终端形式在维尔京群岛共存，并且在形态和生态上似乎明显不同。因此，我们的结果表明，Euphorbia tithymaloides 是植物中环种的第一个令人信服的例子。
