Evolution of asexual reproduction in the New World tropics

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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.