Evolution of asexual reproduction in the New World tropics

有性繁殖在与病原体的共同进化中是有利的（红皇后假说）。这一假说预测，在病原体压力高的热带地区，有性繁殖占主导地位。另一方面，Yahara（1990）声称，无性种族几乎都是多倍体，多倍体在竞争环境中具有优势，而有性二倍体可以在压力（竞争较少）的环境中生存。这一假说预测在热带地区无性繁殖可能占主导地位。为了验证这些假设，我们进行了实地调查，以检查甜叶菊和膜胚菊在墨西哥和委内瑞拉的有性和无性形式的分布。甜菊糖在S.pilosa和s.p upatoria中发现有性和无性两种。在S.perfoliata, s.f。perfoliata, S.trifida S.velutinella,我们发现只有性植物,而S.serrata S.iltisiana, S.hirsuta, S.jorullensis, S.suaveolens, S.tomentosa, S.incognita, S.caracasana, S.origanioides S.aff。alatipes S.aff。cordifolia、s.s ickida、s.s elatifolia、s.s stricta均为无性。因此，无性植物在墨西哥亚热带地区占主导地位。在S.eupatoria，我们发现有性和无性植物共存的地方。在那个地方（沼泽），有性植物比无性植物要少，前者在湿地的中心部分，后者在边缘部分。所有这些发现都支持Yahara的假设，而不是红皇后的假设。在委内瑞拉收集到的黑叶子膜（Hymenoasplenium obtusifolium）的材料似乎不支持这两种假设，但我们的分析仍然是初步的。

It has been claimed that sexual reproduction is advantageous in the coevolution with pathogen (Red Queen Hypothesis). This hypothesis predicts that sexual reproduction is dominating in tropical area where pathogen pressure is high. On the other hand, Yahara (1990) claimed that asexual races are almost always polyploids and those are advantageous in competitive environments while sexual diploids can survive under stress (less competitive) environments. This hypothesis predicts that asexual reproduction can be dominating in tropical area. To test these hypotheses, we carried out field surveys to examine distributions of sexual and asexual forms of Stevia and Hymenoasplenium in Mexico and Venezuela.For Stevia, both sexual and asexual races were found in S.pilosa and S.eupatoria. In S.perfoliata, S.aff.perfoliata, S.trifida, and S.velutinella, we found only sexual plants, while S.serrata, S.iltisiana, S.hirsuta, S.jorullensis, S.suaveolens, S.tomentosa, S.incognita, S.caracasana, S.origanioides, S.aff.alatipes, S.aff.cordifolia, S.viscida, S.elatior, S.latifolia, and S.stricta were all asexual. Thus, asexual plants are dominating in subtropical Mexico.In S.eupatoria, we found a place where both sexual and asexual plants co-occurred. In that place (a marsh), sexual plants are lower than asexual plants and the former are found in the central part of the wetland with the latter in the marginal part.All of those findings support Yahara's hypothesis rather than the Red Queen hypothesis.Materials of Hymenoasplenium obtusifolium collected in Venezuela seem to support neither of the two hypothesis, but our analyzes are still preliminary.