Variation and evolution of sexual and asexual races in Mexican Stevia

墨西哥甜菊有99种，其中42种为无性小种。其中，有性种族多局限于小范围内。然而，S. origanoides是一个有趣的例外，其中有性和无性种族广泛分布。本文对六个分布区进行了样带研究，发现了有性和无性小种的分布区。结果表明，三个地区的有性种群中均存在形态特征和生境偏好不同的隐种。为了研究这些隐种是否在不同地区或不同地区有差异，我们对不同的S. oritanoides。所获得的结果似乎支持在每个区域的分化，但树的分辨率不足以成为结论。AFLP分析正在进行中。ITS分析表明，绝大多数无性三倍体聚在一起，表明它们属于单系。另一方面，S.类折纸植物分化为至少六种隐蔽物种。这些研究结果表明，有性生殖是有利的，当人口迅速分化的空间异质性环境下。

Mexica Stevia is represented by 99 species among which asexual races are found in 42 species. Among them, sexual races are mostly restricted to small area. However, S. origanoides is an interesting exception in which sexual and asexual races are widely distributed. We carried out transect studies in six area where sexual and asexual races of S. origanoides are found. As a result, we found that sexual populations in three area include cryptic species that are differentiated in morphological traits and habitat preferences. To examine whether these cryptic species differentiated in each area or in defferent area, we sequenced ITS for various samples of S. oritanoides. The results obtained seem to support the differentiation in each area, but resolution of the tree was not sufficient to be conclusive. Now, AFLP analysis is on going. The ITS tree showed that most of asexual triploids clustered together, indicating their monophyly. On the other hand, sexual populations of S. origanoides are differentiated to at least six cryptic species. These findings suggest that sexual reproduction is advantageous when populations are rapidly differentiating under spatially heterogeneous environments.