If the hybrid species theory has any validity, one thought is that Mangosteen may have descended from Garcinia globulosa (which appears very similar to G. atroviridis). Asam kandis is common and sometimes cultivated in Malaysia and Indonesia. It bears grapefruit-size fruits whose sweet pulp is eaten raw.

The wild mangosteen species Garcinia forbesii, native to Kalimantan, seems to be very closely related to Mangosteen. The rinds are used by locals in cooking to impart sourness to food dishes. The inside of the fruit is not bad, but does not taste as good as mangosteen, slightly bitter and herbal, and more sour.

Garcinia nigrolineata: Kandis jantan (big kandis) has orange fruits like G. globulosa , but larger and sweeter. The edible fruit is gathered from the wild for its delicious fruits and the species is sometimes used as a rootstock for mangosteen.

Garcinia hombroniana: Manggis hutan (jungle mangosteen), from Indonesia and Malayasia, is a species bearing fruit with a flavour like a slightly sour peach when it is eaten raw. The exterior of the fruit is a bright red-orange, while the inside is translucently opaque, appearing similar to Mangosteen.

This paper says that Mangosteen was thought to have arisen from two wild species, G. celebica (apparently alternatively known as G. hombroniana ) and G. malaccensis, but that researchers had originally mistook another species (G. penangiana ) for G. malaccensis (not at all uncommon to confuse closely related obscure species in the Garcinia genus) and that now that true G. malaccensis has been identified it has been found to be nearly genetically identical to Mangosteen. Mangosteen highly resembles G. malaccensis and shares a similar taste of ripe fruits.

This paper speculates that Mangosteen may have come from G. celibica rather than G. hombroniana. G. celibica and G. hombroniana appear extremely similar. It appears they may have been mistaken for the same species, or possibly mistaken for different species.

I do have to agree that the fruits of Garcinia Malaccensis look the closest to Mangosteen out of any of the wild species.

To make matters more interesting, there was a report in 1987
(Idris and Rukayah) of the existence of a male mangosteen tree, about 70 years
old, growing in the Negeri Sembilan region of peninsular Malaysia. The
characteristics were similar to those of the female tree but the male flowers were
smaller than female flowers, and the stamens in the male flower were numerous
and arranged in a mass, the filaments of the male flowers being much shorter
than those of the female flowers. Later studies by Richards in 1990 suggested
that this plant could have been a hybrid between mangosteen and either G.
malaccensis or G. hombroniana. There can be found mangosteen trees growing wild in the forests of Kemaman, in peninsular Malaysia, though it is unclear whether they were endogenous to this region or escaped into the wild from human cultivation.

Seedless Mangosteens

Often some of the fleshy white segments inside the mangosteen rind do not contain seed. These segments tend to be much smaller, but also make for a more enjoyable eating experience. In rare cases a mangosteen fruit may not contain any seed. The best Mangosteen fruits are those with the highest number of stigma lobes, which indicate the highest number of fleshy segments and the fewest seeds. The number of stigma lobes and the number of fleshy segments always match.

I think the rare mangosteen variety "masta" / "mesta" only has 2 to 3 hard seeds, so the fruit is relatively seedless.

(This is still G. mangostana, same species as regular Purple Mangosteen)

Here's a picture, as you can see the shape of the mesta variety is a little bit oblong, compared to the regular variety, with a more pointed tip. Fruits are slightly smaller and the exterior rind a little thicker.

The variety mesta is cultivated in Pahang and Sabah, Malaysia. The fruit in the latter picture was taken from a tree growing in Temerloh.