The Effect of Altered Soil Moisture on Hybridization Rate in a Crop-Wild System (Raphanus spp.)

Abstract

Since plant mating choices are flexible and responsive to the environment, rates of spontaneous hybridization may vary across ecological clines. Developing a robust and predictive framework for rates of plant gene flow requires assessing the role of environmental sensitivity on plant reproductive traits, relative abundance, and pollen vectors. Therefore, across a soil moisture gradient, we quantified pollinator movement, life-history trait variation, and unidirectional hybridization rates from crop (Raphanus sativus) to wild (Raphanus raphanistrum) radish populations. Both radish species were grown together in relatively dry (no rain), relatively wet (double rain), or control soil moisture conditions in Ohio, USA. We measured wild and crop radish life-history, phenology and pollinator visitation patterns. To quantify hybridization rates from crop-to-wild species, we used a simply inherited morphological marker to detect F1 hybrid progeny. Although crop-to-wild hybridization did not respond to watering treatments, the abundance of hybrid offspring was higher in fruits produced late in the period of phenological overlap, when both species had roughly equal numbers of open flowers. Therefore, the timing of fruit production and its relationship to flowering overlap may be more important to hybrid zone formation in Raphanus spp. than soil moisture or pollen vector movements.

Hybridization Slows Rate of Evolution in Crop-Wild Compared to Wild Populations of Weedy Raphanus Across a Moisture Gradient

“…In the F 0 generation, gene-flow naturally occurred within mixed plots of wild and cultivatedcrop plants and gave rise to the first generation (i.e., F 1 ) of wild and crop-wild hybrid (R. sativus × R. raphanistrum) seeds (Teitel et al, 2016a). As previously described (e.g., Campbell et al, 2016a;Teitel et al, 2016a), we manipulated soil moisture using rainout shelters and imposed one of four watering treatments within these plots/shelters to impose a natural selection experiment on replicated wild and crop-wild hybrid radish populations:…”

“…The crop radish (Raphanus sativus) cultivar used was Red Silk (Harris-Moran Seed Company, Modesto, CA, USA). As in (Campbell et al, 2016a), in 2010, both cultivated and wild plants (nine seedlings per genotype) were planted in 36 plots as part of a randomized block design at the Waterman Farm at Ohio State University in Columbus, Ohio USA, within a larger experiment (Sneck, 2012;Campbell et al, 2016a; Figure 1). Ancestral seedlings were planted in one of four watering treatments with one plot per treatment, per block, for a total of 10 blocks (thus we originally planted 40 populations; however four populations did not produce any F 1 seeds).…”

Growth and fecundity of colonizing hybrid Raphanus populations are environmentally dependent

“…Differences in pollen and seed production are known to drive differences in short‐term gene flow among populations (Loveless and Hamrick, 1984; Campbell, 1985; Brunet and Sweet, 2006). Thus, environmental conditions across landscapes may influence weed invasion dynamics (Arnold and Hodges, 1995; Arnold and Martin, 2010; Hovick et al, 2012; Campbell et al, 2016b).…”

“…For gene flow to promote the invasion success of a hybrid population, hybrid offspring must survive and reproduce at rates that are at least as successful as, or more successful than, rates of non‐hybrid offspring (Wolf and Wade, 2009; Campbell et al, 2016a). Environmental differences notoriously drive survival and reproduction, in hybrid and non‐hybrid plant populations alike (Arnold and Martin, 2010; Campbell and Wendlandt, 2013; Campbell et al, 2016b). For example, introgression of the herbivore resistance gene Bt from crop into wild populations of both Helianthus and Brassica increases the relative fecundity of crop‐wild hybrids only when herbivory is present (Stewart et al, 1997; Snow et al, 2003; Chapman and Burke, 2006).…”

“…Similarly, selection on crop and wild traits varied significantly among crop‐wild hybrid Helianthus populations grown in Indiana and Nebraska (Baack et al, 2008). Although many studies have manipulated environmental conditions to measure early‐generation crop‐wild hybrid fitness (summarized in Campbell and Snow, 2007), perhaps the most extensive examination of this general phenomenon is reflected in our experiments of crop‐wild hybrid radish performance and fecundity that now span more than a decade (Campbell et al, 2006, 2009a,b, 2014, 2015, 2016a,b; Campbell and Snow, 2007; Pirimova et al, 2015; Teitel et al, 2016a,b). Typically, we find that hybrid populations germinate earlier, flower later, and grow to a larger size than wild plants; however, we did not have a priori expectations of how soil moisture may affect these patterns.…”