The rapid evolution of self-fertility in Spartina hybrids (Spartina alterniflora × foliosa) invading San Francisco Bay, CA

Abstract

Plant hybridization can lead to the evolution of invasiveness. We wished to determine whether hybrids between the largely self-sterile Atlantic Spartina alterniflora and California native S. foliosa had evolved self-fertility during their ca 30 year existence in San Francisco Bay, CA. In pollination experiments we found that neither of the parental species was self-fertile, nor were early generation hybrids. A large fraction of later generation hybrids were profusely self-fertile. Inbreeding depression was high in the parental species and early generation hybrids, but was much reduced in later generation hybrids—some even showed outbreeding depression. We found that populations of later generation hybrids and their seedling progeny were almost two-fold more homozygous than early generation hybrids, consistent with the evidence of increased selfing shown by our parentage analyses based upon 17 microsatellite markers. We posit that evolved self-fertility has contributed substantially to the rapid spread of hybrid Spartina in San Francisco Bay.