Why does an orange have rind? A lot of fruit do not have peel. It is not clear, what exactly is it for: even succulent berries and fruit like plums and peaches do quite well with the thinnest of skins. A fruit with leathery peel containing volatile oils and fluid-filled carpels is a feature of hesperidia alone. Pomelo has the skin (flavedo) and pith (albedo) that take nearly as much room as the flesh. Buddha hand citron is almost all rind, juiceless and sometimes seedless. The circumstances for evolution of this kind of fruit must be very unusual. If fruit is to be consumed by an animal, the rind is not helping it to be consumed, at least in the obvious way. Hard shells on nuts make perfect sense, as these protect the seed from a possible damage, but citrus fruit seems to be putting a barrier that makes little sense. The astrigency, the odd shape, occasional seedlessness only add to the confusion.
Citrus fruits might have originated in Australia, New Caledonia and New Guinea around 30 Mya. North-eastern India and Burma - or China - have been suggested, with a demarkation line ("Tanaka line") from Hanoi to Ledo: to the south of this line the citrus evolved into an acidic fruit, to the north it evolved into a sweat one. Citron is considered as the oldest extant citrus.
(Trends in Genetics 17(2001)536; Citrus genetics, Breeding & Biotechnology, I Khan, p. 20)...Recent molecular studies indicate that the closest relatives to the citron - long-considered to have originated in India and one of the parents of the lemon - are species from New Ireland (off eastern New Guinea) and others from New Caledonia. Conventional wisdom holds that citrus evolved in Southeast Asia but this is based on ignorance about what constitutes a true species, the relationships between plants in Asia and Australasia, and how the two landmasses were related. There are about 25 true species of citrus and some 50% of these are from New Guinea, New Caledonia and Australia, while most commercial species and cultivars originated in Asia.
The genetics of citrus is bewildering. For example, lemons are hypothesized to be (i) a hybrid of citron and lime, (ii) a trihybrid of citron, pomelo, and finger lime (iii) a hybrid of sour orange and citron, and (iv) a fruit of polyphyletic origin. The most basal of citrus fruits tend to have thicker rinds, and it seems that such rinds are ancestral. An unknown proto-citrus fruit, perhaps on a tropical island, has developed thick, waxy peel around the flesh, for reasons unknown.
Perhaps the animal for which it was designed is long gone. Or maybe (given its island origin) it was something else and truly uncommon: citrus are good at floating and the "fingers" might've worked as a lure for fruit-eating fish (that do not care about the acidity). There are examples of fig tree seed dispersal by tropical fish living in the streams in the rain forest,http://www.springerlink.com/content/yxu965vujn0bfhcm
Pirhaha fish, actually, subsist on seeds and fruits falling into water and flooded forests.http://www.springerlink.com/content/yxu965vujn0bfhcm
There are also fruit-eating reptilians (eg, Brazilian caimans) that would not mind acid. Bright color, strong smell via hydrophobic scent molecules make sense from the standpoint of luring an aquatic animal.
Maybe the proto-citrus targeted some extinct fish on the islands? Why do we peel oranges?
Buddha hand citron, one of the basal citrus fruits