A review of dipterocarps - Center for International Forestry Research

Conservation of Genetic Resources in the dipterocarpaceae 49
wing like structures in the fruits that allow the single
seeded fruits to gyrate toward the ground. Many species
growing in swamps or river banks have fruits with short
sepals and may be dispersed by water (Ashton 1982). In
some dipterocarps, such as species of Stemonoporus,
fruits are without wing-like sepals. When mature, they
simply fall on the ground and are apparently not removed
by any disperser (Murawski and Bawa 1994) although
rodents are known to hoard the seeds and, perhaps, aid in
dispersal (P. Ashton, personal communication). Seeds
disseminated by wind and water can potentially disperse
over long distances. In Shorea albida, dissemination by
wind up to 2 km has been documented (Ashton 1982)
and although dispersal by water has not been observed in
any species seeds may move over long distances in water
channels.
Apomixis
Apomixis has been reported in several taxa of the family.
There is embryological evidence for the existence of
multiple embryos originating from a single ovule in
Shorea ovalis ssp. sericea and S. agamii ssp. agamii
(Kaur et al. 1978). Multiple seedlings from a single fruit,
indicative of polyembryony, have been reported in
Anisoptera curtisii, Dipterocarpus baudii, D. cornutus,
D. costulatus, Dryobalanops aromatica, Hopea
odorata, H. subalata, Parashorea densiflora, Shorea
argentifolia, S. gratissima, S. macrophylla, S.
parvifolia, S. pauciflora, S. smithiana, Vatica pallida
and V. pauciflora (Kaur et al. 1978 and references
therein) and in Shorea trapezifolia (S. Dayanandan,
personal commnication). The percentage of multiple
seedlings is low in all these species except for S.
macroptera, S. resinosa, H. odorata, and H. subalata
in which 30-70%, 98%, 90% and 21% seeds respectively
have multiple seedlings.
Interestingly, a recent study by Wickneswari and
Norwati (1994) indicates that multiple seedlings from
the same seed in Hopea odorata have different
genotypes raising the possibility that multiple seedlings
may not necessarily involve apomixis. Furthermore,
using genetic markers, a high outcrossing rate has been
estimated for the species (Table 3). Isozyme surveys also
reveal high amounts of genetic diversity within
populations (Wickneswari et al. 1994).
Apomixis is associated with triploidy in Shorea
resinosa and Hopea subalata (also possibly in H.
latifolia) but other species displaying polyembryony are
mostly diploid. The ovary in Dipterocarpaceae is usually
three locular with two ovules in each loculus. Normally,
only one ovule develops into a seed thus, multiple
seedlings can result from occasional development of
seeds from more than one ovule and the presence of such
seedlings need not always imply apomixis.
Apparently, in some species apomixis is widespread
while in others it occurs occasionally. Obligate apomixis
for either individual trees or populations (and species)
remains to be demonstrated but is a possibility in taxa
with a triploid chromosome number. Certainly among
tropical woody families apomixis at a scale comparable
to Dipterocarpaceae has not been reported. Moreover,
considering that most species in the family have a low
diploid chromosome number, the common occurrence
of apomixis is puzzling because apomixis is usually
associated with polyploidy and hybridisation.
Apomixis could have played an important role in
evolution of the family. New genetic combinations
arising through mutations or hybridisation that may be
partially or completely sterile can be perpetuated by
apomixis. Vegetative multiplication can also maintain
heterozygosity for a long time. In addition, apomixis and
self-pollination may allow new genetic variants to spread
at new sites. Subsequent restoration of sexual
reproduction and outcrossing, combined with mutation,
can introduce genetic variation in the new isolates.
Hybridisation
Hybridisation has played an important role in the
evolution and diversification of angiosperms (Stebbins
1950). Hybrids in tropical trees are assumed to be rare
(Ashton 1969). In dipterocarps, however, hybrids have
been frequently reported. Ashton (1982) suggests that
many triploid taxa in the family could be of infraspecific
hybrid origin. His list includes the following: Hopea
subalata, H. odorata, Shorea ovalis ssp. sericea,
Neobalanocarpus heimii, Shorea leprosula, S. curtisii,
and hybrids between Vatica rassak and V. umbonata, and
Anisoptera costata and A. curtisii. Many examples of
putative hybrids between species of Dipterocarpus have
also been reported (Symington 1943). Apomixis, already
noted in several taxa of the family, could certainly allow
the hybrids to persist until sexual fertility is restored.
Although several of the interspecific hybrids are
polyploids, polyploidy in the family has so far been
recorded in relatively few taxa. On the other hand, the
base number x=11 observed in several genera of the
family itself could be of ancient alloploid derivation.