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Polyacetylenes from Bidens

University of British Columbia

The Hawaiian species of Bidens are morphologically and ecologically diverse taxa which have evolved from a single ancestral species. Adaptive radiation has occurred without the evolution of physiological or genetic interspecific isolating mechanisms since all species are interfertile and genetic distances among populations, based on isozyme loci, show little correlation with morphological differences or taxonomic classification. This disparity between the evolution of morphological and biochemical characters makes it of interest to determine whether or not there has been divergence in secondary metabolites in these species. Leaves and roots of 19 species and six subspecies of Hawaiian Bidens were examined for polyacetylenes. Eleven C₁₃ hydrocarbons, aromatic and thiophenyl derivatives, one C₁₄ tetrahydropyran and three C₁₇ hydrocarbons were isolated and identified. All can be derived from oleic acid. Polyacetylenes were not detected in the leaves of 13 taxa although they are found in the roots of all species. The occurrence of 2-(2-phenylethyne-1-yl)-5 acetoxymethyl thiophene in Bidens has not been previously reported. Most taxa could be distinguished by their complement of leaf and root acetylenes and no variation was found within taxa except in B. torta. There appears to be no taxonomically significant pattern to the distribution of polyacetylenes above the species level in this group. The complexity of polyacetylene inheritance was assessed using experimentally produced interspecific hybrids. Crosses between species which do not produce leaf acetylenes resulted in F₁, individuals without acetylenes. Crosses between species which produce leaf acetylenes and those which do not yielded hybrids with acetylenes not always identical to parental arrays. Progeny from parents with different sets of acetylenes expressed a combination of the major compounds found in both parents. In all cases, nonparental acetylenes in the F₁, generation were biosynthetically closely related to compounds found in the parents. Polyacetylene synthesis was not segregated in the F₂ individuals from Type B crosses. De novo biosynthesis of polyacetylenes in Bidens leaves was investigated in pulse-chase studies. ¹⁴C-labelled acetylenes were recovered from three species of Bidens administered ¹⁴C0₂ and subsequently allowed to metabolize in ¹⁴C0₂ for 12, 24 and 168 hours. Radioactive C₁₃ ene-tetrayne-ene was also isolated from the roots of all plants, indicating that translocation of ¹⁴C-labelled precursors from aerial tissues occurred. Phenylheptatriyne (PHT) was detected in two day old seedlings of B. alba, suggesting that polyacetylene biosynthesis begins during germination or soon thereafter. Quantities in the leaves continue to increase up to and beyond 24 days while amounts in the hypocotyls peak at seven days. Relative PHT values in the roots are 100 times higher than those in the aerial tissues for the first 24 days, but there is also a gradual decline in these levels beginning at two weeks and continuing beyond the experimental period. Phenylheptatriyne is absent from the roots of mature B. alba. Many polyacetylenes are toxic to biological systems in the presence of UV-A radiation. These in vitro effects have led to speculation about the putative functions of polyacetylenes in the organisms which produce them. Nineteen species of phylloplane yeasts and yeast-like fungi were isolated from species of Hawaiian Bidens with and without leaf acetylenes. Although all these organisms, members of the Sporobolomycetaceae, Cryptococcaceae and Fungi Imperfecti, were photosensitive to some polyacetylenes and resistant to others, there was no correlation between the presence or absence of leaf polyacetylenes and the distribution of these saprophytes among species of Bidens. Nevertheless, it is significant that the only pathogenic species isolated in this study, ColIetolrichum gloeosporiodes , did not colonize Bidens leaves containing C₁₃ aromatic acetylenes to which it is extremely photosensitive in vitro.

Text

2010-06-23

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http://hdl.handle.net/2429/25932

Botany

University of British Columbia

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