The species Dimocarpus longan contains two subspecies i.e. subspecies longan and subspecies malesianus, each with several varieties. Within the subspecies longan, the most commonly cultivated taxon is Dimocarpus longan ssp. longan var. longan, which is the commercial longan (Figure 1). Three edible longan types can be distinguished within the variety longan in Thailand (Subhadrabandhu, 1990). The first type is a large forest tree with small fruits and a very thin aril, possibly of interest for breeding purposes. The second one is the native longan ('lamyai kradook' or small 'lamyai'), growing in the northern part of the country as an erect tree producing small fruits with large seeds and is recommended as a rootstock for commercial cultivars. The third type is the commercial longan ('lamyai kraloke') which produces large fruits and small seeds. Beside the variety longan, there exist at least three other varieties, viz. variety longepetiolulatus, variety obtusus and variety magnifolius. All three varieties have been found growing wild in China (Huang, 1999).
The other subspecies, that is the subspecies malesianus, contain mainly unexploited genetic materials which may have great potential to be developed into commercial fruits in their own right or as breeding materials for the commercial longan. These include variety malesianus and variety echinatus. While the commercial longan is adapted only to the subtropics and will not flower when grown in the true tropics, the subspecies malesianus are fully adapted to the unchanging heat, humidity, daylength and other conditions of the equatorial zone. For example, the subspecies malesianus var. malesianus are native to Southeast Asia with the greatest variation found in Borneo where it might be possible to distinguish between 30 to 40 local races (Leenhouts, 1971; van Welzen et al., 1988). The diversity of this subspecies in Sarawak in the Borneo island has been documented by Wong and Gan (1992) and Wong (2000). The fruits are globular to slightly oblong and smooth to warty. In Peninsular Malaysia, the most common form of this taxon is the one with globose smooth fruits which turn brown when ripe. This is the true 'mata kuching' or 'cat's eye' which has often been identified as Euphoria malaiensis. It has a very thin aril and is hardly worth eating. This form also exists in Borneo and Sumatra. The more superior forms are found in Sarawak, all with densely thick warty fruits and greater aril recovery percentage. These forms can be roughly grouped into three types (Figure 2) based on the fruit characteristics: the 'isau' with fruits which are globular and remain green when ripe; the 'sau' with fruits which are slightly oblong and also remain green when ripe; and the 'kakus' with globular fruits which turn brown when ripe. The leaves, flowers and tree forms also differ. The 'kakus' is more widespread in Sarawak, while the 'isau' and 'sau' are mainly confined to the river banks of the Rajang river and to the Bareo valley.
The variety echinatus differs from the variety malesianus in that the fruits have rather long spines resembling the 'rambutan' (Nephelium lappaceum). This variety is found in Sabah where the 'kakus' also exists (van Welzen et al., 1988).
Thus the true tropical longan offers the greatest opportunity for selecting superior material and thus offers an attractive possibility of longan becoming a new fruit crop for the humid lowlands throughout the tropics.
