Rate of Shattercane x Sorghum Hybridization In Situ

Cultivated sorghum [Sorghum bicolor (L.) Moench subsp. bicolor] can interbreed with a feral weedy relative shattercane [S. bicolor nothosubsp. drummondii (Steud.) de Wet ex Davidse]. Traits introduced from cultivated sorghum could contribute to the invasiveness of a shattercane population. An experiment was conducted to determine the potential for pollen-mediated gene flow from grain sorghum to shattercane. Shattercane with juicy midrib (dd) was planted in soybean [Glycine max (L.) Merr.] fields during 2 yr in concentric arcs at varying distances from a 0.39 ha sorghum pollen source with dry midrib (DD). The arcs were placed so that prevailing winds would carry sorghum pollen to the shattercane. Seven hundred twenty seeds from each of over 300 shattercane panicles in anthesis during sorghum pollen shed each year were collected. Progeny were evaluated by phenotype to determine rate of hybridization. Hybridization averaged 3.6% within the source in 2008 and 16.0% in 2009 and declined as distance increased. Hybridization as high as 2.6% for an individual panicle was measured at the farthest distance evaluated (200 m). Wind direction and speed were also measured and their product affected hybridization rate for all pollination periods. Results indicate that genes from cultivated sorghum will likely be introduced into shattercane populations at distances of at least 200 m and that rate of hybridization is dependent on weather factors such as wind. Source size is also important in determining hybridization rate but was not studied here.