A review of dipterocarps - Center for International Forestry Research

Conservation of Genetic Resources in the dipterocarpaceae 46
Table 1. Infraspecific variation in chromosome number
(from Ashton 1982).
Species Chromosome
Number
Dipterocarpus alatus 20, 22
D. tuberculatus 20
D. tuberculatus var. turbinatus 30
Hopea beccariana 20, 21, 22
H. odorata 14, 20, 21, 22
H. subalata 20, 21, 22
Aneuploid series are common in Anisoptera and
Dipterocarpus. Both genera have species with 2n=20 or
2n=22. In some taxa, both variants occur within the same
species (Table 1).
Thus, both polyploidy and aneuploidy indicate the
importance of chromosomal variation in diversification
at the species level. However, the largest genus, Shorea,
shows remarkable uniformity in chromosome number;
31 out of the 34 species, for which chromosome numbers
are known, have the same diploid number, viz. 2n=14.
Intraspecific variation in chromosome number has
been reported in several species, particularly in
Dipterocarpus and Hopea (Table 1). Of the 68 species
for which chromosome numbers are available, 6 species
have been recorded to show intraspecific variation. Such
variation has not been reported for species of Shorea, the
largest genus of the family even though data are available
for 34 species.
Inter and intraspecific variation in chromosome
numbers is difficult to interpret for two reasons. First,
more than one chromosome numbers for the same taxon
have been reported by different rather than the same
author. Second, much of the reported variation due to
reports of a single author, Tixier (1960) and most of
Tixier’s counts have not been confirmed by others.
It should, also be kept in mind that information on
chromosome number for large tropical trees is usually
obtained from very small sample sizes. Often only one
or two individuals in a population are examined and rarely
is there data from more than one population. Thus, it is
impossible from available data to determine the magnitude
of intraspecific variation in chromosome number.
Furthermore, even in these cases, where such variation
has been reported, one cannot estimate the extent of
variation and therefore its significance. For example, for
species of Dipterocarpus as well as Hopea listed in Table
1, variation is in the form of either aneuploid or
polyploid chromosomal series, but whether this variation
is in the form of occasional aneuploid or polyploid
populations is not known (Ashton 1982).
Breeding Systems
Breeding systems are one of the primary determinants of
the pattern of genetic diversity in natural populations of
plants (Hamrick 1982, Hamrick and Godt 1989).
Outcrossing combined with extensive movement of pollen
and seed can lead to a high degree of genetic variation
within populations but reduce differentiation among
populations. Selfing and limited mobility of pollen and
seed can have the opposite effect of reducing variation
within, but promoting differentiation among populations.
Dipterocarpaceae have bisexual flowers which are
pollinated by a variety of animal vectors (see below).
Controlled pollinations have revealed the presence of selfincompatibility
systems in a large number of species. At
least 14 out of 17 species appear to be self-incompatible
(Table 2.) The self-incompatibility system in several
species is apparently weak, as is the case in many other
tropical species. In most of the species subjected to
controlled pollination so far, a certain proportion of selfpollinated
flowers set fruits. Dayanandan et al. (1990)
and Momose et al. (1994) suggest that fruit set in self
and cross-pollinated flowers is initially high but during
development, fruits from self-pollinated flowers suffer
from higher abortion rates than fruits from crosspollinated
flowers. T. Inoue (personal communication)
has implicated the existence of a post-zygotic
incompatibility system in Dryobalanops lanceolata. Such
systems have also been reported for other tropical forest
trees (Bawa 1979, Seavey and Bawa 1983).
On the basis of controlled pollinations, most
dipterocarps appear to be strongly cross-pollinated.
Outcrossing is the usual mode of reproduction in tropical
forest trees (Ashton 1969, Bawa 1974, 1979, 1990, and
references therein.) However, in dipterocarps, studies of
breeding systems conducted so far are based on very small
sample sizes in very few species. The data of Dayanandan
et al. (1990) are from 2-3 trees, mostly two of each
species; of Chan (1981) from 1-2 trees, and of Momose
et al. (1994) from only one tree. Considering the
variability among trees and that the distinction between
self-compatibility and self-incompatibility in the family
appears to be quantitative, large sample sizes will be
required to precisely define the self-incompatibility
systems.