Population genetic structure of Titanotrichum oldhamii (Gesneriaceae), a subtropical bulbiliferous plant with mixed sexual and asexual reproduction

Abstract

Background and aims: Titanotrichum oldhamii is a monotypic genus distributed in Taiwan, adjacent regions of China and the Ryukyu Isands of Japan. Its conservation status is vulnerable as most populations are small and widely scattered. Titanotrichum has a mixed system of reproduction with vegetative bulbils and seeds. The aim of this study was to understand the population genetic structure of Titanotrichum in relation to its specific reproductive behaviour and to determine possible implications for conservation strategies.

Methods: After an extensive inventory of most wild populations of Titanotrichum in East Asia, samples from 25 populations within its major distribution were carried out utilizing RAPD and inter-SSR molecular fingerprinting analysis.

Key results: The findings support the conclusion that many populations reproduce predominantly asexually but that some genetic variation still exists within populations. However, significant amounts of variation exist between populations, perhaps reflecting population differentiation by drift. This partitioning of genetic diversity indicates that the level of inter-population gene exchange is extremely low. These findings are consistent with field observations of very limited seed production. The Chinese populations are similar to those of Northern Taiwan, while the Ryukyu populations fall within the range of variation of the north-central Taiwan populations. The Taiwanese populations are relatively variable and differentiation between north, east and south Taiwan is evident.

Conclusions: The distribution of Titanotrichum seems to be consistent with a former land connection between China, Taiwan and the Ryukyu Islands at a glacial maximum during the Quaternary, followed by progressive fragmentation of the populations. North-central Taiwan is the centre of genetic diversity, possibly due to the proximity of the former land bridge between the regions, together with the variety of suitable habitats in north Taiwan. The significance of these findings for conservation is discussed.

Fig. 1. Map of East Asia showing the distribution of Titanotrichum oldhamii (circles). Twenty‐five populations analysed are plotted. Arrows indicate the hypothesized directions of migration at the glacial maximum. Dashed lines indicate the hypothetical coast line during the Pleistocene glacial maximum (after Ota, 1998). This simplified relationship among populations of Titanotrichum oldhamii in different geographic regions is proposed according to the clustering results given in this paper.

Fig. 2. Three‐dimensional representation of a principal coordinate analysis of the genetic relationships among 283 individuals (207 genotypes) of Titanotrichum oldhamii, inferred from a distance matrix using the Jaccard index. Arrows indicate individuals in two Japanese populations. Scattered dots without grouping identify individuals in ‘north‐central’ Taiwan.

Fig. 3. UPGMA phylogram constructed using pair‐wise Jaccard distances based on combined RAPD and inter‐SSR data among 283 individuals of 25 Titanotrichum oldhamii populations from Taiwan, China and Japan.