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，性别种族大多局限在小范围内。然而，猪柳是一个有趣的例外，其中有性和无性种族广泛分布。我们在6个地区进行了横断面研究，在这些地区发现了牛肝草有性和无性的小种。结果表明，3个地区有性种群中存在着形态特征和生境偏好差异较大的隐种。为了研究这些隐种是否在每个地区或不同地区分化，我们对不同样品进行了ITS测序。获得的结果似乎支持每个区域的分化，但树的分辨率不足以得出结论。AFLP分析正在进行中。ITS树显示大多数无性三倍体聚集在一起，表明它们的单系性。另一方面，雌雄居群分化为至少6个隐种。这些发现表明，当种群在空间异质性环境下快速分化时，有性生殖是有利的。

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.