The survey of 14 populations of T. chantrieri revealed a
high level of genetic variation at the species level, but there
was considerable variation (Table 1). This implied that a
large proportion of genetic variation was partitioned among
populations. In general, selfing species usually possess lower
genetic diversity within populations and higher genetic
differentiation among populations relative to outcrossing
species (Hamrick and Godt 1996). Therefore, the data on
the population genetic structure in T. chantrieri at large spatial
scales are consistent with the highly selfing mating system
documented at smaller spatial scales (Zhang et al.
2005). Among these 14 populations, the mating systems of
two populations (WEV and BB) have been quantified by
allozyme markers in our previous research. The WEV population
had the highest selfing rate (Sm = 0.941), and exhibited
the lowest genetic diversity (PPB = 15%, HE = 0.041).
The BB population also had a quite high selfing rate (Sm =
0.859) and contained very low genetic variation (PPB =
25%, HE = 0.079) (Zhang et al. 2005). Among the four
populations of T. chantrieri examined in the previous study,
estimates of the population level maternal selfing rate (Sm)
averaged 0.86 (range 0.76-0.94). This is a high figure, and
is similar to that of other obligately selfing species. Consistent
with this, the average genetic diversity in the south
Yunnan–Thailand region was very low (PPB = 23.125%,
HE = 0.075).
Comparing with genetic variation and structure based
on RAPD analyses of other wild plant populations (Nybom
and Bartish 2000; Nybom 2004), the amount and pattern of
genetic variation in T. chantrieri is more comparable to selfing
or mixed mating taxa than to outcrossing species. The
GST among populations was 0.5835, which was similar to
the average for selfing plant species (0.51) in Hamrick and
Godt’s (1990) analysis. A high level of population differentiation
may be explained by several factors, such as the species’
breeding system, genetic drift, demographic fluctuations,
or the genetic isolation of populations (Hogbin and
Peakall 1999). When populations are small and geographically
and genetically isolated from one another, genetic
drift influences the genetic structure and increases differentiation
among populations (Barrett and Kohn 1991; Ellstrand
and Elam 1993). Estimates of the effective gene flow
per generation (Nm) of T. chantrieri were lower (0.3568)
than one successful migrant per generation. This indicated
limited gene flow among populations, which may be insufficient
to counteract the effect of genetic drift. While inferences
of the migration rate from estimates of Nm are not
definitive for populations that do not exhibit metapopulation
dynamics or large demographic shifts (Whitlock and
McCauley 1999), the method is still a reasonable guide to
levels of gene flow among populations. The low estimates
of migration among T. chantrieri populations correspond