Rafflesias in the tropical rain forests of Southeast Asia produce some of the biggest flowers in the world—up to one meter in diameter. Adding to their notoriety is their carrion-like smell to attract insects for pollination service. They’re not microbes by anyone's definition of the term. The reason they make an appearance in these pages is that they have stolen a larger proportion of their genome via horizontal gene transfer (HGT) than any other organism we know of. Between 24% and 41% of their mitochondrial DNA is of foreign origin, and even in their nucleus a whopping 2.1% of the genes were imported.
Fully opened flower of Rafflesia arnoldii from the Palupu
Reserve near Bukittinggi, Indonesia. Photo by Troy Davis.
Plant genomes seem to be especially rife in genes acquired by HGT, and this is particularly true for the parasitic kinds, of which the Rafflesias are impressive examples. These freeloaders have no leaves or stems, no plastids anywhere—thus no photosynthesis. They live off host vines whose nutrients they harvest via haustoria, absorptive organs that penetrate into the host’s tissues. These un-plantlike plants exist concealed within their host vines, only occasionally offering a showy reproductive structure to the outer world—reminiscent of the mushrooms formed by underground fungal mycelia. Now we have learned that they are not only trophic parasites, but also genomic thieves.