Sexual reproduction of cordgrass hybrids (Spartina foliosa x alterniflora) invading tidal marshes in San Francisco Bay

Hybrids between alien cordgrass, Spartina alterniflora, introduced in the 1970s, and native California cordgrass, S. foliosa are a menace to the ecology of tidal wetlands in San Francisco Bay, California, USA. Over several years we genetically analyzed cordgrass plants and seedlings throughout the estuary and found that hybrids are the principal cordgrass invaders and colonizers.  We hypothesized that this was due to higher seed set and siring ability by hybrids relative to the native species. Hybrid seed comprised from 91% to 98% of that set in the marsh study plants over the two years of the study. Total viable pollen production by hybrid plants was 400 times that of the native plants.  Seed and pollen production were highly skewed toward a few hybrid genotypes. In addition to seed produced by hybrid plants, hybrid seed was produced by S. foliosa due to pollination by hybrid plants. While the greatest advantage for hybrids was in pollen and seed production, hybrid seeds germinated, and seedlings survived and grew as well or better than the native species.  As native S. foliosa becomes increasingly rare, hybrid seed floating on the tides will predominate, overwhelming recruitment sites and resulting in further colonization by hybrids.   In an evolutionary context, hybrids with exceptional pollen and seed production will be initially favored by natural selection, leading to the evolution of even more fertile hybrid genotypes.