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