Debra R. Ayres

Genetic diversity and structure of the narrow endemic Wyethia reticulata and its congener W. bolanderi (Asteraceae) using RAPD and allozyme techniques

Wyethia reticulata is an edaphic endemic in the Sierra Nevada foothills. Its sympatric congener, W. bolanderi, is also restricted to the foothills, but has a north-south range of 275 km, compared to 14 km for W. reticulata. The goals of this study were to determine clonal diversity, population size, genetic variation, and spatial and generic structure for each species from paired populations in El Dorado County, California, using allozyme and RAPD (random amplified polymorphic DNA) methodologies. Wyethia reticulata, spreading by rhizomes, had populations dominated by a few large individuals, while W. bolanderi, with a basal caudex, had populations of a few hundred evenly sized individuals. Genetic analyses indicated that W. reticulata, compared to its congener, had somewhat less genetic diversity (H(T): 0.28 vs. 0.38), had more of its genetic variation partitioned among populations (F(ST): 0.25 vs. 0.07), and showed a complete absence of inbreeding (F(IS): -0.03 vs. 0.22). Population membership in accord with populations defined by geographical location resulted only when all markers were included in the analysis. Ecological limits on recruitment of genets appears to result in small population size in W. reticulata. Limited gene flow, drift within small populations, and sexual reproductive dominance of large clones result in the genetic divergence of populations in this species, while genetic diversity is maintained by the longevity of clones and outbreeding.

Evolution of a new ecotype of Spartina alterniflora (Poaceae) in San Francisco Bay, California, USA

We report the discovery and spread of a dwarf ecotype of Spartina alterniflora in San Francisco Bay. Relative to typical S. alterniflora, this dwarf ecotype has one-fifth the tiller height (∼21 cm), tenfold the tiller density (∼4000 tillers/m(2)), and is restricted to growth in the upper intertidal zone. Chromosome counts of the dwarfs are identical to typical smooth cordgrass (2n = 62), and smooth cordgrass-specific random amplified DNA markers confirm the species identity of the dwarf. Field-collected clonal fragments of the dwarf grown for 2 yr under high-nutrient conditions maintained the dwarf syndrome, as did plants grown from the seed of a dwarf. The dwarf condition is not caused by endophytic fungi. The first dwarf smooth cordgrass patch was discovered in 1991, and by 1996 five separate dwarf patches had appeared within 200 m of the original. Since 1991, total area covered by the dwarf ecotype has increased sixfold to 140 m(2). The ecological range of the dwarf smooth cordgrass ecotype is similar to that of S. patens, a competitor on the Atlantic coast. We suggest that the absence of S. patens from most of San Francisco Bay has allowed the dwarf ecotype of smooth cordgrass to survive and spread.

Hybridization between invasive Spartina densiflora (Poaceae) and native S. foliosa in San Francisco Bay, California, USA.

Rapid evolution in contemporary time can result when related species, brought together through human-aided introduction, hybridize. The significant evolutionary consequences of post-introduction hybridization range from allopolyploid speciation to extinction of species through genetic amalgamation. Both processes are known to occur in the perennial cordgrass genus, Spartina. Here we report the existence of a third recent Spartina hybridization, discovered in 2002, between introduced S. densiflora and native S. foliosa in San Francisco Bay, California, USA. We used nuclear and chloroplast DNA analysis and nuclear DNA content with chromosome counts to examine plants of morphology intermediate between S. densiflora and S. foliosa in a restored marsh in Marin County, California. We found 32 F(1) diploid hybrids and two triploid plants, all having S. densiflora and S. foliosa as parents; there is also evidence of a genetic contribution of S. alterniflora in some hybrids. None of these hybrids set germinable seed. In 2007 we found a hybrid over 30 miles away in a marsh where both parental species occurred, suggesting hybridization may not be a localized phenomenon. The presence of diploid and triploid hybrids is important because they indicate that several avenues existed that may have given rise to a new allopolyploid species. However, such an event is now unlikely because all hybrids are targets of eradication efforts.

Extinction of a Common Native Species by Hybridization with an Invasive Congener1

Abstract In this article, we summarize previous and ongoing work examining the population and genetical consequences of the introduction of smooth cordgrass, a tidal marsh grass native to the Atlantic, into the range of California cordgrass in San Francisco Bay, CA, in the 1970s. The two species hybridized. A subset of hybrid genotypes outcompetes the native species, overgrows its niche space, produces much seed, and sires the majority of seed on native flowers. The result is the loss of existing native cordgrass plants and the usurpation of cordgrass regeneration sites by copious hybrid seed. These processes could lead to the extinction of the native species. Nomenclature: California cordgrass, Spartina foliosa Trin.; Smooth cordgrass, Spartina alterniflora Lois. Additional index words: Cordgrass, invasive Spartina, Spartina alterniflora, Spartina foliosa. Abbreviations: MLW, mean low water; MSL, mean sea level; RAPD, random amplified polymorphic DNA.

Tumbleweed (Salsola, section Kali) species and speciation in California

Tumbleweeds (Salsola species, section Kali) are road side and rangeland pest plants throughout the 48 contiguous states in the US. Three described tumbleweed species and two undescribed Salsola taxa occur in California. The known species are Russian thistle, Salsola tragus, introduced from Eurasia in the 1800s, Russian barbwire thistle, S. paulsenii, which grows in the desert regions of California, and is also native to Eurasia, and the recently identified S. kali subspecies austroafricana, possibly native to South Africa. Our goals were to investigate karyology, genome size, and molecular genetic affinities of the described species and the other taxa within their ranges in California using recently developed microsatellite loci, dominant nuclear DNA markers (RAPD and ISSR), and DNA sequence data. Chromosome counts and genome size assessments made with flow cytometry were compared. These analyses indicated that one undescribed taxon is a new allopolyploid hybrid between S. tragus and S. kali subspecies austroafricana, and the other undescribed taxon appears to be a complex hybrid involving all three described species. The invasion potentials for the hybrid taxa are unknown. Tumbleweeds are the focus of biological controls efforts but the identification of suitable agents for the hybrid taxa may be problematic because of the large amount of genetic variability encompassed within this evolving Salsola complex.

What can mathematical modeling tell us about hybrid invasions

Compared to the vast theoretical literature on the dynamics of single species invasions, relatively few models have dealt with the emergence of invasive hybrids. Here, we review the variety of modeling approaches that have been used to study the dynamics of hybridization, outlining the underlying assumptions and highlighting their advantages and disadvantages. We summarize the predicted outcomes for the persistence for the native species and the resulting genetic composition of the native–exotic–hybrid complex under a variety of model scenarios. We highlight promising future directions for theoretical investigation and its integration with experimental studies.

Responses to salinity of Spartina hybrids formed in San Francisco Bay, California ( S. alterniflora × foliosa and S. densiflora × foliosa )

San Francisco Bay (SFB), which supports large populations of the California native cordgrass Spartina foliosa, has been the recipient of introductions of S. alterniflora and S. densiflora. Hybrids have arisen between the native and these exotic species. Sterile F1 S. densifloraxa0×xa0foliosa hybrids have formed numerous times in a number of marshes, while introgressing S. alternifloraxa0×xa0foliosa hybrids are fully fertile and invaded widely in SFB, especially onto naturally-open low tidal flats by inundation-tolerant hybrids. Sarcocornia pacifica, pickleweed, dominates the mid-to-upper marsh zones where the hypersaline conditions that occur during the summer drought, characteristic of this climate, exclude S. foliosa. Here we report on two glasshouse experiments investigating the salinity tolerance of hybrid Spartina. Some hybrids of both origins grew well and flowered at high salinity levels while the parental species grew little and did not flower. Our results imply that mid-zone marshes are also vulnerable to invasion by salinity-tolerant Spartina hybrids. Herbicide control implemented over the last 10xa0years targeting both the exotic species and their hybrids have reduced their extent. However, efforts in monitoring and management of exotic Spartina and its hybrids must continue as vast areas of tidal marsh restoration are underway and planned in SFB; colonization by Spartina hybrids tolerant to inundation and/or salinity will greatly alter restoration trajectories. These concerns are all the more vital given projections of climate change and its effects on salinity and sea level rise in SFB salt marshes.

Introduction to “Ecological and Evolutionary Consequences after Invaders Hybridize”

The motivation for this special issue was a symposium organized by Ayres and Schierenbeck at the 2006 American Botanical Society annual meeting that took its title from Ellstrand and Schierenbeck’s highly cited 2000 paper in Proceedings of the National Academy of Science, ‘‘Hybridization as a Stimulus for the Evolution of Invasiveness’’. Ellstrand was the keynote speaker. As explored previously by Ellstrand and Schierenbeck (2000), the mixing of divergent genotypes can result in the creation of populations of hybrids that are more invasive than the parental species. The symposium sparked such interest in the participants and audience that Ellstrand suggested we collaborate on a special issue. Here we revisit the earlier examples (Schierenbeck and Ellstrand 2009), and in a series of papers, present case studies examining the ecological and evolutionary consequences of hybridization in plants which range from hybridization between cultivars in ornamental pears (Culley and Hardiman 2008), to introgressive hybridization between species (Gaskin and Kazmer 2008; Sloop et al. 2009), to the formation of diploid (Abbott et al. 2008) and allopolyploid hybrid species (Ainouche et al. 2008; Ayres et al. 2008). We also consider the interplay between sexual and clonal reproduction in invaders (Bailey et al. 2008) whether hybrid plants are by their nature more invasive than non-hybridized taxa in a meta-analysis (Whitney et al. 2008), and outline approaches to modeling hybrid population dynamics (Hall and Ayres 2008). The case studies should serve as cautionary tales against the casual introduction of nonnative taxa into areas occupied by conspecifics or congeners while this special issue illustrates the potential for hybrid studies to increase our understanding of evolution.