AN intimate glimpse into the mating habits of oak trees has revealed many long-distance liaisons and possibly a taste for the unusual. Faraway fathers sire a surprisingly large share of bur oak acorns, and the mothers may prefer it that way, says Beverly Dow, a graduate student at the University of Illinois at Chicago.
Dow used genetic analysis to trace the parentage of 282 individual acorns collected from three mother trees in a single stand of bur oak (Quercus macrocarpa) on an abandoned farm in northern Illinois. For each acorn, Dow and her supervisor Mary Ashley examined four short stretches of DNA in which the same two DNA bases are repeated over and over again in a molecular stammer. Extra bases are easily added or lost when the DNA is copied, so the number of repeats in each of these stretches, known as microsatellites, varies widely from one individual to another. By matching microsatellite lengths found in acorns against those of all the adult trees in the stand, Dow was able to identify the father of each acorn with a high degree of certainty – if the father was present in the stand.
For 174 of the 282 acorns she studied (62 per cent), Dow found that none of the 62 mature oaks in the 6-hectare stand could have been the father. Pollen for those fermions must therefore have come from trees outside the stand, the nearest of which was 100 metres farther off. “To my knowledge, this is the first time there’s been an exact paternity analysis [for trees] that revealed exactly which offspring are coming from outside,” says Dow.
Oak pollen is carried by the wind, so pollen grains should be most abundant near the parent tree and get scarcer with distance as the breezes scatter them. To garner such a high proportion of distant fathers, therefore, Dow speculates that the oaks actively favour seeds fertilised by pollen from outside the stand – perhaps by selectively aborting embryos fertilised by nearby trees which may compete less well for nutrients during seed development.
Until recently, most biologists thought that long-distance pollination occurred only rarely, with fewer than one plant in a hundred having parents separated by more than 100 metres, says Norman Ellstrand, a plant population geneticist at the University of California at Riverside. But over the past decade, a handful of paternity analyses by Ellstrand and others have suggested that at least some plants regularly interbreed across such distances.