Strong parallel evidence of selection during switchgrass sward establishment in hybrid and lowland ecotypes

Background:Switchgrass sward establishment results in up to 90%seedling mortality.The degree of selection during sward establishment has not been reported using modern genetic methods.Methods:Pooled leaf samples were sequenced from replicated swards of 46 half-sib families from two breeding groups(lowland and hybrid)before and through 3 years of stand establishment.Pooled allele frequencies were then assessed using fixation indices(Fst)and an independent data set was used to predict the polygenic impact of establishment selection on two traits(heading date and winter survivorship).Last,the DNA pools were assigned survival rankings to predict the sward survival genomically estimated breeding values within the training data set.Results:Strong and parallel selection occured in both breeding groups.Five genomic regions exceeded the significant threshold of 99.9%in>10 families,indicating consistent selection across families and breeding groups.Polygenic trait predictions determined that establishment selection was partially associated with winter survivorship but resulted in variable heading date alterations.The genomewide variation is consistent with selection for a small number of related parental lines.Conclusions:This study observed strong selection for a small number of hybrid and coastal ecotype individuals which are promising germplasm sources for improved sward survival.This confirms prior reports of sward selection during grassland establishment and highlights the strength of pooled DNA sequencing for survival traits.

Switchgrass sward establishment selection is consistent across multiple environments and fertilization levels

Background Strong selection can occur during switchgrass sward establishment.Differences in establishment selection due to environment or management could provide information on genotype-by-environment variation and could influence strategies for breeding perennial grasses.Methods Leaf samples were collected before sward establishment and from 3-year-old swards for two breeding groups(lowland and hybrid)at three locations.Within two locations,samples were collected from paired fertilized(112 kg N ha−1)and unfertilized plots.Allele frequencies from pooled DNA samples were studied through multivariate analysis of variance,genomewide trait predictions(heading date and winter survivorship),and genomically estimated breeding values(GEBVs)for individual sward survival within an independent data set.Results This study found only minor variations in selection due to location or management.Predicted heading dates of the hybrid population had significant changes due to fertilization and location.There were strong correlations among sward establishment survival GEBVs between growing environments(hybrid r=0.77;gulf r=0.97).Interestingly,this study found a small number of genotypes that were over-represented in established swards across all growing environments.