Phylogenetic Significance of Interspecific Hybridization in Jatropha (Euphorbiaceae)

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Abstract
Jatropha is a morphologically diverse genus of 160-175 species of trees, shrubs, rhizomatous subshrubs, and geophytes, distributed primarily in the seasonally dry tropics. The results of attempted interspecific hybridization of 20 species in eight of the ten sections support the previously suggested phylogenetic relationships of various taxa; decrease in ability to cross corresponds with evolutionary advancement, which is indicated by morphological reductions including dioecy, drastic shifts in growth habit, and/or changes in ploidy level. Except for the dioecious species, these plants have an autoxenogamous breeding system and moderate to well-defined interspecific differences in floral mechanisms. Related species show largely a high degree of unilateral compatibility (genetic incongruity) and may be separated by preferential fertilization, rather than incompatibility, whereas more phylogenetically distant taxa are separated by actual incompatibility barriers. Geographical isolation notwithstanding, related species are capable of gene exchange within wide limits under artificial conditions; phylogenetic relationships may be inferred by crossability of the taxa.