Molecular studies of southern hemisphere disjunction in three plant genera: Eucryphia, Griselinia and Coriaria
thesisposted on 15.12.2014, 10:33 by Martha Rebecca Jane. Clokie
Three plant genera with South American-Australasian disjunctive distributions were studied: Eucryphia, Griselinia and Coriaria . The aims of the project were: a) to assess the utility of different chloroplast and nuclear DNA sequences for phylogenetic reconstruction and b) by means of molecular clocks to establish whether or not this distribution is due to vicariance (continental drift) or more recent dispersal events. DNA was extracted, and regions of interest were amplified and sequenced. These were rpoA, trnL-F and trnU-K from the chloroplast genome, and rDNA 5.8S and intergenic spacers 1 and 2, Adh and two loci of the G3pdh gene from the nuclear genome. In each of the three genera, a single trans-Antarctic disjunction was indicated. Molecular phylogenies were produced using parsimony. Individual base pair variation was analysed in detail, and graphs were drawn to highlight variable regions. Sequences providing the most resolution were trnL-F, ITS and the two loci of the G3pdh. Robust molecular phylogenies were produced which are consistent with morphological, fossil and biogeographical evidence. Constancy of mutation rate across the different branches of the trees was checked using the Tajima test. Where constancy was established, the rate of evolution for the different gene regions was used to estimate the divergence times within each genera. Calibrations were performed using fossil data for Eucryphia and Coriaria average mutation rates derived from Eucryphia were applied to the Griselinia data set, owing to the absence of fossils. New Zealand became separated from Antarctica (and therefore South America) 95 - 80 million years ago, whereas average divergence times for Coriaria and Griselinia were found to be around 3 and 50 million years respectively. Similiarly, with respect to Eucryphia, Australia split from Antarctica 40 million years ago but the divergence time is 23.23 million years. The disjunction therefore must have arisen by dispersal which may have occurred either in the water by rafting or floating, or in the air in wind currents ( Eucryphia has winged seeds) or by means of birds (Coriaria and Griselinia have fleshy diaspores). A review of similar disjunctions in other plants and animals revealed that vicariance explanations were more likely to apply to groups at the taxonomic ranks of genus and above, whereas dispersal explanations were more likely to apply at the level of genus and below. The generic level is consequently of great interest.