Allozyme diversity was analyzed to determine the genetic structure of populations of Trochodendron aralioides Siebold & Zucc., a primitive angiosperm that is insect-pollinated, has wind-dispersed seeds and grows only in eastern Asia. Nineteen populations were analyzed, including 16 from Taiwan, two from the main islands of Japan and one from Iriomote Island in the Ryukyu Islands. Genetic variability (0.133) at the species level was more similar to that of a narrow geographic species than an endemic species. Based on genotype frequencies, mating within populations was non-random (FIS=0.065) and there was significant genetic differentiation (FST=0.164) among populations. Genetic diversity was largely within, rather than among, populations (Hs=0.132, 85.14%). Bisexual flowers of T. aralioides exhibit synchronized dimorphism. The protogynous and protandrous morphs are self-incompatible, which probably promotes outcrossing and contributes to the high levels of heterozygosity within populations. Using a maximum likelihood tree, populations of T. aralioides were separated into two distinct clusters: Japan and Taiwan/ Iriomote Island. Populations of T. aralioides on Iriomote Island and in northern Taiwan exhibited a high degree of genetic similarity. The isolation by distance model does not fit populations in Taiwan, but suitably describes the relationship between populations of T. aralioides in Japan and Taiwan.
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