Nature weekend 29-31 January 2016

It’s already halfway through summer in the Drakensberg. One indication is that the Red-chested Cuckoo is still calling, but the Black Cuckoo is not. The Half-collared Kingfisher has returned after at least six months away, a pair was poised in their favourite Buddleja overhanging the water. Another bird we have not seen for some time is the African Firefinch – a pair was foraging among proteas above Cowslip Dam.

It’s not easy to find a new tree species along the established trails at The Cavern, but what’s this scrappy character close to the 70s block? It’s Pavetta cooperi, well known on the edges of Berg forests. The leaves have black spots, suggesting some affliction, but are actually diagnostic of Pavetta. This is a group of 20-odd shrubs and small trees noted for the profusion of white flowering displays giving them their popular name of brides’ bushes.

However, the black spots are the most interesting feature. Each is a colony of bacteria of a species unique to its partner Pavetta, and found nowhere else in nature. Every leaf has them, and indeed every host cell contains at least one bacterium. This means that it will be present in flowers and fruits to be transmitted to the next generation. How did the association begin? Long ago the bacterium must have been an invading pathogen, which the ancestral Pavetta resisted successfully. From here it’s a short step to some form of symbiosis where the bacterium gives up its former nefarious lifestyle. It becomes a harmless passenger, and its waste products might even benefit its new protector.

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