CRAWFORD Daniel.j. Department of Plant Biology, Ohio State University Professor, 植物学部, 教授
OYAMA Ken Centro de Ecologia, UNAM Associate Professor, 生態学センター, 準教授
MURAKAMI Noriaki Botanical Gardens, University of Tokyo Assistant lectuer, 理学部, 助手 (60192770)
SOEJIMA Akiko Department of Biology, Osaka Prefecture University Assistant lectuer, 総合科学部, 助手 (00244674)
WATANABE Kuniaki Department of Biology, Kobe University Professor, 理学部, 教授 (80031376)
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.