Spartina alterniflora x foliosa, the hybrid of native Spartina foliosa and introduced Spartina alterniflora, is a highly invasive weed in San Francisco Bay salt marshes. Concerned that it would overgrow open mudflats and outcompete native marsh vegetation, government agencies have funded eradication efforts since the year 2000. The control program herbicides hybrids that are visually distinct from native Spartina, using morphological characteristics such as taller tillers, wider culms, and pink stems to identify hybrids. Management has drastically reduced the area occupied by morphologically evident hybrids. However, it is possible that exotic alleles persist in morphologically cryptic hybrids that resemble native plants. These cryptic hybrids could pose problems for the ecology of San Francisco Bay. To investigate the existence of cryptic hybrids, I genotyped 92 plants in four marshes using ten microsatellite and seven RAPD markers. Hybrids and natives can be distinguished by the presence/absence of private S. alterniflora alleles. Samples were also measured for six morphological features and tracked for application of herbicide by the control program. I classified the hybrids into visually evident and cryptic hybrids, where cryptic hybrids had morphological measurements within one standard deviation of the mean found for native plants.
Results showed that 25% of tillers tested were hybrids. 78% of hybrids were categorized as cryptic, and among these, an average of 29% of their alleles derived from exotic S. alterniflora, compared to 55% in evident hybrids. MANOVA showed a significant relationship between the percent of S. alterniflora alleles and morphology: plants with a high percent of exotic markers deviate further from the native phenotype. Of the 24 hybrids identified, only two were herbicided: these were the two individuals with the highest percent S. alterniflora and the most distinct morphologies. My work suggests that highly introgressed, morphologically cryptic hybrids may act as a reservoir of hybrid alleles that are able to escape detection by the control program. Resampling in 2010 will show if artificial selection for a cryptic morphology is causing an increase in cryptic hybrids relative to evident hybrids. A common garden experiment is underway to test for the ecological differences between native Spartina, cryptic hybrids, and evident hybrids; early results indicate that cryptic hybrids as well as evident hybrids can survive across a broader intertidal range than native Spartina, which could disrupt the functioning of valuable marsh habitat. The ongoing evolution of introduced Spartina demonstrates the challenges of managing hybrid invasions in general.