Maternal Effects on Multiple Generations of Helianthus Annuus Crop-wild Hybrid Seed

Book excerpt: Abstract: Although gene flow between crop and wild relatives is not a new area of study, much can still be learned through the investigation of hybrids between divergent taxon. Most crops exchange alleles with wild and weedy relatives, especially when grown in crop centers of origin. Such gene flow can result in evolutionary consequences for wild populations ranging from genetic erosion and demographic swamping to the evolution of enhanced weediness. All of these scenarios are concerning, since wild populations constitute a breeding pool in which valuable allelic diversity is stewarded and weeds already cause problems in agriculture. Helianthus annuus was domesticated in North America where its cultivated and wild forms have been known to hybridize when grown in proximity. However, cultivated sunflower and its relative, common sunflower, differ in morphology, ecological habit, and life history. For example, dormancy differs between crop and wild sunflower types since it was selected against during domestication, but has remained necessary for maintaining fitness for wild plants. Sunflower has physiological dormancy, but the strength and duration of dormancy differs between wild and crop types. Achenes resulting from hybridization between crop and wild sunflower have been shown to have reduced dormancy compared to wild achenes. Although the introgression of crop alleles into wild populations cannot occur unless the F1 generation survives to reproduce, the dormancy of later generations is important for determining if this trait could provide significant resistance to introgression. The objectives of this work were twofold: to identify how maternal parent and differing crop allele percentages each affect seed germination, dormancy, and mortality. Here, fifteen hybrid crosstypes were created on three different maternal plant types - wild F1 crop-wild hybrid, and crop. Since the pericarp and seed coat of sunflower achenes are maternal tissue, contrasting the three maternal types allowed for investigation of the impact of maternal effects in crop-wild sunflower hybrids on achene germination, dormancy, and survival. The genetic composition of these crosstypes varied from 0% to 100% crop alleles, with six intervening levels. All fifteen crosstypes were surveyed in a seed burial study, while two crosstypes were included a companion pericarp manipulation experiment. In the field experiment, three removal dates were chosen to assess hybrid seed germination, dormancy, and mortality. The contribution of crop alleles, maternal parent, and their interaction affected achene germination, dormancy, and survival. While crosstypes with higher crop allele percentages had increased early spring mortality compared to wild achenes, all crosstypes had high overwinter survival, indicating that reduced dormancy is not a complete barrier to crop allele introgression. However, hybrid achenes produced on wild maternal parents had high levels of dormancy that was comparable to wild dormancy, while crop-produced and F1-produced achenes had little to no dormancy by the last removal date, indicating that only hybrids produced on wild maternal plants will persist in the seed bank. Hybrid achenes remaining in the seed bank can be recruited in future years to backcross with wild plants or cross with remaining hybrids and may increase the risk of genetic erosion and demographic swamping.