A review of dipterocarps - Center for International Forestry Research

Conservation of Genetic Resources in the dipterocarpaceae 50
Genetic Diversity Within and Among Populations
The existence of self-incompatibility and high
outcrossing rates suggest that populations of
dipterocarps should harbour high levels of genetic
variation. Indeed, recent studies, based on analysis of
variation at isozyme loci, have revealed considerable
genetic variation in natural populations. A high level of
enzymatic polymorphism in natural populations of
Shorea leprosula was first detected by Gan et al. (1977).
More recently, genetic diversity within and among
populations of several species of Hopea (Wickneswari
et al. 1994), Shorea (Harada et al. 1994),
Stemonoporus (Murawski and Bawa 1994, Dayanandan
and Bawa, unpublished data) has been quantified. Genetic
diversity in many Malaysian species of Hopea and
Shorea were studied using Random Amplified
Polymorphic DNAs (RAPD). Considerable variation was
found both within and among populations. The level of
diversity in species of Hopea (Wickneswari et al. 1996)
was less than in species of Shorea (Harada et al. 1994).
In these studies, methods to characterise genetic
diversity depended upon several assumptions about the
segregation and homology of bands. Moreover, results
from most RAPD surveys cannot be compared with those
obtained from isozyme surveys because dominance at
RAPD ‘loci’ makes it impossible to distinguish
heterozygotes from homozygotes. Thus, genetic diversity
cannot be characterised in conventional terms. Bawa and
his associates have used isozymes to estimate genetic
diversity in species of Stemonoporus and Shorea. In
Stemonoporus oblongifolius, the percent of
polymorphic loci range from 89% to 100%, the average
number of alleles per polymorphic locus is 3.1 and mean
genetic diversity for the species is 0.297. The number
of loci sampled was 9 and was the same sampled for other
dipterocarps and tropical trees. The estimates of genetic
diversity are among the highest reported for plant species
(Murawski and Bawa 1994). The values for the above
parameters are lower for Shorea trapezifolia, but remain
toward the higher end of the value reported for tropical
trees. Similarly, a high level of genetic variation has been
observed in Shorea megistophylla (Murawski et al.
1994b) and several other species of Stemonoporus
(Murawski and Bawa, unpublished). Wickneswari et al.
(1994) also report high levels of variation in Hopea
odorata on the basis of isozyme studies.
Inter-population differentiation on the basis of
isozyme surveys has been studied in only three species:
Stemonoporus oblongifolius (Murawski and Bawa
1994), Shorea trapezifolia (Dayanandan and Bawa, in
preparation) and Hopea odorata (Wickneswari et al.
1994). In all cases, there is a high level of variation among
populations. In Stemonoporus oblongifolius, the mean Gst
value, which is a measure of population differentiation,
is 0.16. In other words, 16% of total genetic diversity is
due to differences among populations. Interestingly, the
distance among sampled populations ranged from 1.3 to
9.7 km. Thus, populations seem to differ over a relatively
small spatial scale. In Shorea trapezifolia too the Gst value
was high (0.11); in this case the most distant were
separated by 43.5 km. The mean genetic distance between
populations in Hopea odorata was 0.10 (Wickneswari et
al. 1994).
The high level of genetic differentiation could be due
to either restricted gene flow or local selection. Direct
observations of gene flow in dipterocarps are lacking.
Seed dispersal in Stemonoporus oblongifolius seems to
be passive; the one seeded, heavy, resinous fruit drop
under the maternal tree and the seed germinates without
being removed by any disperser (Murawski and Bawa
1994). In Shorea trapezifolia, the seeds are dispersed by
gyration, assisted by wind with most seeds falling within
the vicinity of the parent. Gyration of fruits, referred to
earlier, may have evolved as an adaptation to restrict
dispersal to the sites in which the parents are found. Thus,
gene dispersal via seeds in both species does not generally
occur over large distances.
The degree of gene dispersal via pollen would depend
upon the pollinators. Medium sized to large bees should
be able to bring about long-distance dispersal more
frequently than small bees or thrips. Both Stemonoporus
oblongifolius and Shorea trapezifolia are pollinated by
medium-sized bees (Apis spp.).
Gene dispersal has been indirectly measured in
Shorea trapezifolia (more than one migrant per
generation). Nm estimates the degree of migration
between populations, and a value of Nm>1 is enough to
prevent population differentiation due to drift for neutral
loci (Wright 1931, Maruyama 1970, Slatkin and
Maruyama 1975). In S. trapezifolia, the value of Nm is
1.62. This high value indicates that differentiation in S.
trapezifolia is not due to restricted gene flow.
Ashton (1982, 1988) has shown that congeneric
species in the family often occupy different edaphic zones.
Moreover, within the same habitat related species may
be differentiated along environmental gradients that