Reproductive and recruitment dynamics of invasive hybrid cordgrasses (Spartina alterniflora x Spartina foliosa) in San Francisco Bay tidal flats.

摘要

Due to rapid invasion of open intertidal mudflats worldwide, maritime cordgrasses in the genus Spartina are of great conservation concern. Salt marshes and un-vegetated tidal flats characterize the San Francisco Bay estuary (SFB), California. Spartina alterniflora was introduced there from the U.S. east coast in the 1970s. Subsequent hybridization with California native S. foliosa has produced a genetically diverse swarm of hybrids, which, through deliberate plantation and the dispersal of floating seed on tidal waters, has spread rapidly in the Bay. In Chapter 1, I characterize 24 new disomic microsatellite loci, which I use, in addition to 11 previously described microsatellites, in Chapters 2 and 3. In Chapter 2, I investigate variation in Spartina, and Spartina hybrid breeding systems in field and greenhouse experiments, and via genetic methods. I show that hybrids in situ set 13% self-fertilized seed (range: 0% to 76%), while neither parent species did so in SFB. Some SFB hybrids colonizing tidal flats were more self-fertile than their progenitors in a nearby cordgrass meadow. I showed that inbreeding depression was absent in many hybrids in a controlled greenhouse environment. Self-fertilization ensures seed production at the leading edges of hybrid Spartina invasion by counteracting Allee effects. In Chapter 3, I examine the ecological consequences of self-fertile hybrids on tidal flat invasion by determining SFB hybrid genetic structure, potential source populations for regional tidal flat recruitment, and the local hybrid seedling recruitment dynamics at three SFB hybrid cordgrass populations. I combined microsatellite marker and GPS/GIS data via multivariate ordination, Bayesian clustering, parentage/sib-ship studies, and spatial genetic analysis. I show that the northern hybrid tidal flat population is genetically distinct from two distant, southern, sympatric populations. The majority of northern seedlings were produced by out-crossing meadow adults, while in the south, selfing tidal flat hybrids produced most seedlings; a single isolated individual contributed up to 55% of all seedlings. I conclude that, despite the dynamic nature of the tidal system, most seedling recruitment occurs from local seed sources. The production of large numbers of selfed, highly fit offspring will assure the continuing invasion of SFB by hybrid Spartina.