Essays on non-virtual reality #172

In the first essay of this three-part series on American chestnuts, I discussed the tree's near-extinction together with a bit of the ecological, economic, and cultural reasons why that mattered. Once estimated to number one of every four trees in the eastern deciduous forest, the species was brought low by the chestnut blight fungus, which came into the States on shipments of Japanese chestnuts around the turn of the last century. For today, let's consider the four main methods that have been widely discussed for bringing the species back: to let nature take its course, making the fungus less virulent, traditional genetic engineering and state-of-the-art genetic engineering. In 1982, the eminent ecologist Bernd Heinrich planted four American chestnut seedlings obtained from a breeder in Michigan on his 600 acre property in western Maine. He did so without much hope, knowing full-well that blight invariably finds and kills the trees. However, to his surprise and delight, by 2013 the trees were 35-feet tall and producing seeds with no sign of blight. In a 2020 YouTube video, Heinrich pointed out some of the over 1,000 seedlings his trees have produced, and he (and others) argue that given a chance, natural variability in genetic resistance to the fungus might possibly lead to recovery of American chestnuts. Several concerns, here. Heinrich's woods near Weld Maine is outside the chestnut's traditional range and, last I could discover, he had not known if the trees he planted might have been American chestnut-Asian chestnut hybrids or if blight was in fact present in the area. Blight had also decimated the European chestnut. But in the 1950's surviving trees were discovered in Italy that turned out to be protected by a virus that attacked the blight fungus. CHV1 (Cryphonectria hypovirus 1) causes the fungus to become "hypovirulent," reducing its virulence without killing it. While CHV1 has helped European chestnuts recover some of their former abundance, the introduction of hypovirulent strains of the fungus (containing the virus) into the U.S. hasn't met with much success. Although blight jumps quickly from tree to tree, the virus has trouble spreading from fungus to fungus on our side of the Atlantic. Which brings us to "traditional genetic engineering," the method we humans have long used for restructuring the genetics of plants and animals for our own benefit. Also known as artificial selection, this is where we only allow organisms possessing traits we like to reproduce. It gets fancier when we crossbreed different species to get hybrids with desirable traits. The lemon, for example, is a hybrid between a bitter orange and citron, and apples are a hybrid of at least four species of wild apples from Central Asia and Europe. Since Asian chestnuts are resistant to the blight fungus, many American chestnut-Asian chestnut hybrids have been developed with some, like the Duncan hybrid, widely available for purchase as ornamentals. But there are problems with using such hybrids in forest restoration. For one thing, Asian chestnut species are shorter, tend to have a forking trunk, and their nuts are less suited to American herbivores. So after crossing, say, a Chinese and American Chestnut to get a hybrid (with half its genes Chinese and half American), that hybrid — if it lives long enough to produce flowers — is typically "backcrossed" to an American chestnut. That process is commonly repeated three times to get a hybrid with 15/16 of its genes American, and 1/16 Chinese. The downside is that it's largely a crap-shoot which Asian genes end up in a given hybrid. Therefore, the next step is to plant a lot of hybrids and see which ones show the most resistance while still maintaining the form and ecology typical of an American chestnut. However, Dr. William Powell, Director of the American Chestnut Research and Restoration Program at SUNY-ESF in Syracuse, has observed that after 35 years of such work, the best results provide only intermediate resistance to the fungus, with the highest resistance found in hybrids possessing only 70% American chestnut genes — not good enough for successful reforestation. In 2020, Dr. Powell and his collaborators submitted a 300 page proposal to the USDA to deregulate a genetically engineered American chestnut variety known as Darling 58 for unrestricted planting. The transgenic chestnut has a single gene from a wheat plant added to its genome (without removing or altering any of its own genes). This gene (nicknamed OxO) codes for the production of oxalate detoxifying enzyme, and it protects a transgenic tree by breaking down the oxalic acid the blight fungus produces as a means of killing the tree's tissues. The plan is to cross Darling 58 trees with a lot of different American chestnuts to get the OxO gene into a genetically diverse group of trees. In the final column on this topic, I'd like to explore some of the hot-button ethical and political questions surrounding who makes decisions on if, when and where large-scale chestnut restoration projects might be initiated, a topic which seems likely to come to a head this year. Dr. Baker is professor emeritus of biology at Heidelberg University and a local guest columnist.