Fitness of Crop-Wild Hybrid Sunflower under Competitive Conditions: Implications for Crop-to-Wild Introgression

Abstract

Understanding the likelihood and extent of introgression of novel alleles in hybrid zones requires comparison of lifetime fitness of parents and hybrid progeny. However, fitness differences among cross types can vary depending on biotic conditions, thereby influencing introgression patterns. Based on past work, we predicted that increased competition would enhance introgression between cultivated and wild sunflower (Helianthus annuus) by reducing fitness advantages of wild plants. To test this prediction, we established a factorial field experiment in Kansas, USA where we monitored the fitness of four cross types (Wild, F1, F2, and BCw hybrids) under different levels of interspecific and intraspecific competition. Intraspecific manipulations consisted both of density of competitors and of frequency of crop-wild hybrids. We recorded emergence of overwintered seeds, survival to reproduction, and numbers of seeds produced per reproductive plant. We also calculated two compound fitness measures: seeds produced per emerged seedling and seeds produced per planted seed. Cross type and intraspecific competition affected emergence and survival to reproduction, respectively. Further, cross type interacted with competitive treatments to influence all other fitness traits. More intense competition treatments, especially related to density of intraspecific competitors, repeatedly reduced the fitness advantage of wild plants when considering seeds produced per reproductive plant and per emerged seedling, and F2 plants often became indistinguishable from the wilds. Wild fitness remained superior when seedling emergence was also considered as part of fitness, but the fitness of F2 hybrids relative to wild plants more than quadrupled with the addition of interspecific competitors and high densities of intraspecific competitors. Meanwhile, contrary to prediction, lower hybrid frequency reduced wild fitness advantage. These results emphasize the importance of taking a full life cycle perspective. Additionally, due to effects of exogenous selection, a given hybrid generation may be especially well-suited to hastening introgression under particular environmental conditions.

Seed fates in crop–wild hybrid sunflower: crop allele and maternal effects

“…, Mercer et al. ). Thus, following a hybridization event where wild, F 1 , and crop plants each produce crop–wild hybrid seeds, the substantial number of surviving seedlings has the potential to cross and backcross with each other.…”

“…The three removal dates were as follows: late fall (removed on December 10, 2010), for monitoring maladaptive fall germination; early spring (removed on March 8, 2011), for investigating early germination; and spring (removed on April 8, 2011), for capturing peak emergence. Removal dates were selected based on a previous seed burial study at this location conducted from 2009 to 2010 Mercer et al 2014). Each block was then covered by hardware cloth to prevent seed predation.…”

Life history traits and phenotypic selection among sunflower crop–wild hybrids and their wild counterpart: implications for crop allele introgression

“…However, seed production can be lower among crop–wild hybrid sunflower cross types than wild plants, although differences diminish under more competitive conditions (Mercer et al. ). Thus, predicting the occurrence of demographic swamping is not straight forward and conclusions will likely hinge on both the fitness measure used and the hybrid zone environment.…”

Influence of intergenotypic competition on multigenerational persistence of abiotic stress resistance transgenes in populations of Arabidopsis thaliana

“…For example, although Brassica rapa and B. napus both suffered when in competition with Lolium perenne , plant density, regardless of species, had a greater impact on performance, even in the presence of insect herbivore pressure (Damgaard & Kjaer, ). Similarly, conspecific plant density had a greater influence on fitness parameters of sunflower and crop‐wild hybrids than did the presence of interspecific competition from other weed species (Mercer et al., ).…”

“…Results from the growth chamber‐grown plants indicate potential for complex fitness effects that could ultimately influence transgene establishment (Chan et al., , ). Several studies have indicated the importance of evaluating fitness based on performance throughout the life cycle, in a range of environments, and under competitive conditions (Agren et al., ; Bhatnagar‐Mathur et al., ; Hartman et al., ; Mercer et al., ; Mittler & Blumwald, ). Therefore, in these experiments, we sought to examine fitness impacts of overexpression of the three mechanistically different abiotic stress resistance transgenes ( CBF3, M6PR, SOS1 ) in A. thaliana as observed throughout the life cycle over six generations in variable field environments, and to monitor transgene persistence when in competition with wild‐type (WT) plants as would occur during introgression into natural populations.…”