Data from: Fitness of crop-wild hybrid sunflowers under competitive conditions: implications for crop-to-wild introgression

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.

解析杂交带中新型等位基因渐渗（introgression）的可能性与程度，需比较亲本与杂交后代的终生适合度（fitness）。然而，不同杂交类型间的适合度差异会随生物环境条件发生变化，进而影响渐渗模式。基于既往研究，我们提出假说：竞争加剧可通过削弱野生植株的适合度优势，提升栽培向日葵与野生向日葵（Helianthus annuus）之间的等位基因渐渗水平。为验证该假说，我们在美国堪萨斯州开展了析因田间试验，监测了四种杂交类型（野生型、F1代、F2代及回交野生型（BCw）杂交种）在不同种间竞争（interspecific competition）与种内竞争（intraspecific competition）强度下的适合度。本试验的种内竞争调控变量涵盖竞争者密度与栽培-野生杂交种的出现频率两个维度。我们记录了越冬种子的出苗率、植株存活至开花结实的比例，以及每株结实植株所产种子的数量。此外，我们还计算了两项复合适合度指标：每株出苗幼苗所产种子数，以及每粒播种种子所产种子数。杂交类型与种内竞争分别对出苗率和结实存活率产生显著影响。进一步分析显示，杂交类型与竞争处理间存在交互作用，共同影响其余所有适合度性状（fitness traits）。更强的竞争处理，尤其是种内竞争者密度升高的处理，在考量每株结实植株产种量与每株出苗幼苗产种量时，持续削弱了野生植株的适合度优势，且F2代植株的适合度通常与野生植株无显著差异。若将幼苗出苗率纳入适合度考量范畴，野生植株的适合度仍保持优势；但当引入种间竞争者并提高种内竞争者密度时，F2代杂交种相对于野生植株的适合度提升了四倍以上。与此同时，与假说相悖的是，较低的杂交种出现频率反而削弱了野生植株的适合度优势。本研究结果凸显了从完整生活史视角开展研究的重要性。此外，由于外源选择（exogenous selection）的作用，特定的杂交后代类群在特定环境条件下，可有效加速等位基因渐渗过程。