Citrus and The Forbidden Fruit of Barbados

Grapefruit

Everyone thinks they can identify an orange or a lemon, however, the shrubby tropical and sub-tropical trees that we call Citrus have been cultivated for so long that after thousands of years of hybridisation and selection their division into meaningful species is not as easy as you may think. This kind of phenomenon is not unusual in crop plants, but with Citrus mankind appears to have made this kind of change more difficult to achieve by selecting for seedless varieties and asexuality.

Citrus are not unique in the plant world in that frequently when they produce what are apparently perfectly normal seeds, they are not the product of sexual reproduction but are in fact genetically identical to the mother plant, they are clones.  Indeed one of the reasons why the pioneering genetic studies of Gregor Mendel were overlooked for so many years, was because of his failure to repeat his observations of the laws of inheritance, by unlucky chance his second species was one that produced its seeds asexually. Citrus do more than just produce seeds asexually, they also produce seeds with multiple embryos. Thus it is possible to grow several plants from a single orange pip. In fact within an orange pip it is often possible to find both asexual copies of the mother tree and sexual embryos; the true offspring of the same tree.

Within Citrus fruits there is a trend that the more intensively a species has been cultivated the more likely it is to produce seeds via asexual means. Thus, oranges, grapefruit and mandarins are highly asexual, lemons and limes are partly so and citrons and pummelos reproduce via sexually produced seeds.  This relationship may well have a causal basis, in that, part of the ‘domestication’ process of any crop involves the selection for superior types. Once this has been achieved, then growers are interested in selecting types which breed true, and what breed more true to type than an asexual species. But there is a problem here. All this occurred years before Mendel, when mankind had no understanding of the mechanics of genetics. Selection for asexuality is all well and good, but it does rather limit the scope for future of hybridisation and plant breeding.

Even with such a high degree of asexuality the Citrus family has still managed to produce an embarrassing mix of individuals of uncertain parentage. Take the grapefruit for example, until Columbus’ second voyage to the new world the Citrus clan were entirely unknown in the Americas, however, the grapefruit appears to have spontaneously arisen on the island of Barbados, being first reported in 1750. But from where did it appear? Opinions have been divided. It may have resulted from a chance mutation from the pummelo (also called the shaddock after Captain Shaddock who introduced this large fruited Citrus to Barbados, while the French know it as pamplemousse which sounds more like something from childrens’ fairy tale).  Alternatively the grapefruit may have arisen as the love child of an illicit liaison between an orange and a pummelo, during those hot Caribbean nights! Perhaps this is why it was originally called the ‘forbidden fruit of Barbados’. Whatever its origins the grapefruit has been unable to shake off both these habits. It has subsequently given rise to pink fruited grapefruit apparently by random mutations. Branches of yellow-fruited grapefruit trees have been known to miraculously start to produce fruit with ruby coloured flesh. Furthermore, the grapefruit and the mandarin orange have sired a love child of all of their own, the unromantically named ugli fruit.

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