Weed evolution after crop gene introgression: greater survival and fecundity of hybrids in a new environment

Abstract

Crop-wild hybridization may produce offspring with lower fitness than their wild parents due to deleterious crop traits and outbreeding depression. Over time, however, selection for improved fitness could lead to greater invasiveness of hybrid taxa. To examine evolutionary change in crop-wild hybrids, we established four wild (Raphanus raphanistrum) and four hybrid radish populations (R. raphanistrum x Raphanus sativus) in Michigan (MI), USA. Hybrid and wild populations had similar growth rates over four generations, and pollen fertility of hybrids improved. We then measured hybrid and wild fitness components in two common garden sites within the geographical range of wild radish [MI and California (CA)]. Advanced generation hybrids had slightly lower lifetime fecundity than wild plants in MI but exhibited c. 270% greater lifetime fecundity and c. 22% greater survival than wild plants in CA. Our results support the hypothesis that crop-wild hybridization may create genotypes with the potential to displace parental taxa in new environments.

Competition alters life history and increases the relative fecundity of crop–wild radish hybrids (Raphanusspp.)

“…Typically F 1 hybrids produce approximately 50 -60% aborted pollen grains (Snow et al ., 2001). We found that F 1 hybrids can produce as many seeds as their wild relative (but see Snow et al ., 2001), and in some environments the fitness of advanced-generation hybrids may greatly exceed that of their weedy progenitor (Campbell et al ., 2006). Cultivated and wild radish species were introduced into California by the nineteenth century, and Panetsos & Baker (1967) suggested that 'introgression of raphanistrum characters appears to have been a major factor in converting the erstwhile crop plant, R. sativus , into a highly successful weed.'…”

“…1; part of a larger study described in Campbell et al ., 2006). The six populations were separated from each other and from local wild radish populations by at least 1 km, which is far enough to avoid unintended gene flow via pollinators (Ellstrand & Marshall, 1985 (Campbell et al ., 2006). Each spring through 2004, the plots were tilled, fertilized and handweeded for 2 wk to simulate agricultural management and to promote population persistence.…”

“…Because wild pollen fertility is uniformly high, a smaller sample size was justified (Campbell et al, 2006). After staining (Alexander, 1969), pollen fertility was assessed with a compound microscope as the proportion of aborted grains in samples of at least 100 grains per plant.…”

“…Meanwhile, little is known about the potential for complex, quantitative crop traits to enhance the fecundity of wild relatives. Although hybridization with some cultivated plants may reduce the fitness of weeds in natural environments (Stewart et al ., 2003), possible benefits of cropwild hybridization for weeds suggest this phenomenon should not be overlooked (Boudry et al ., 1993;Miura & Terauchi, 2005;Campbell et al ., 2006). Studies of advanced-generation hybrids find that some interspecific hybrid genotypes persist for multiple generations under certain environmental conditions and therefore long-term gene introgression is possible (Lexer et al ., 2003).…”

Hybrids between cultivated and wild carrots in natural populations in Denmark

“…In the case of hybridization between polyploid cultivars and diploid relatives, chromosomal imbalances may add to a low fitness of hybrids (Hauser et al, 1998a). However, hybrids between cultivated and wild plants may sometimes exhibit improved fitness relative to their parents (Ellstrand, 2003), due to inheritance of beneficial crop alleles (Campbell et al, 2006), heterosis (Hauser et al, 1998b), and novel combinations of genes and traits of the parents (Hauser et al, 1998a;Hooftman et al, 2005). Transgression of traits from the two parents is thought to be a major mechanism for creating evolutionary novelties in hybrids (Lexer et al, 2003).…”