Genetic evidence for hybridization between the native Spartina maritima and the introduced Spartina alterniflora (Poaceae) in South-West France: Spartina × neyrautii re-examined

 Spartina alterniflora, a perennial grass native to the North American Atlantic coast, was introduced during the 19th century in western Europe (Southern England and western France) where it hybridized with the native Spartina maritima. In England, the sterile hybrid S. × townsendii gave rise by chromosome doubling to the highly fertile allopolyploid Spartina anglica, which has now invaded many salt marshes and estuaries in western Europe, and has been introduced in several continents. In South-West France, another sterile hybrid was discovered in 1892 in the Bidassoa Estuary, and named Spartina × neyrautii. According to their morphology, some authors suggested that S. × neyrautii and S. × townsendii result from reciprocal crosses. During the 20th century, the hybridization site was severely disturbed, and surviving of S. × neyrautii was questioned. In this paper, various Spartina populations are investigated in the Basque region (France and Spain), and compared to the hybrid taxa formed in England (S. × townsendii and S. anglica). The samples were analyzed using molecular fingerprinting (RAPD and ISSR) and Chloroplast DNA sequence (trnL-trnT spacer, trnL intron and trnL-trnF spacer). In the Bidassoa estuary, a hybrid isolated clone has been found, that displays additive species-specific nuclear markers of S. maritima and S. alterniflora, and that is subsequently considered as a surviving clone of S. × neyrautii. The molecular analyses indicate that S. × neyrautii and S. × townsendii share the same maternal (S. alterniflora), and paternal (S. maritima) parental species, but also that the two independent hybridization events have involved different parental (nuclear) genotypes in England and in South-West France. Received July 12, 2002; accepted October 4, 2002 Published online: March 20, 2003     

A comparison of the seed dormancy characteristics of Spartina patens and Spartina alterniflora (Poaceae)

Seed dormancy characteristics of Spartina alterniflora were delineated previously by Plyler and Carrick (American Journal of Botany, vol. 80, pp. 752–756, 1993). This study was undertaken in order to determine whether or not the dormancy characteristics of S. patens are similar. As in the previous study, the site of a dormancy mechanism was determined by assessing the germinability of surgically altered dormant seeds. Likewise, the effects of three growth-regulating substances (abscisic acid, fusicoccin, and gibberellic acid), prechilling, and light were studied by assessing the germinability of appropriately treated dormant seeds. Surgical injury to the endosperm, and to a lesser extent the scutellum, produced significant germination in S. patens, whereas only injury to the scutellum produced germination in S. alterniflora. Exogenously applied abscisic acid was highly effective in maintaining dormancy in injured seeds of both species. Gibberellic acid and fusicoccin were ineffective in promoting germination in S. patens, but fusicoccin was highly effective in breaking dormancy in 5. alterniflora. Prechilling was effective in breaking dormancy in S. patens but not in S. alterniflora. In S. patens, treatments that broke dormancy were only successful when seeds were exposed to light during the germination period. It is concluded that the dissimilarities in the dormancy mechanisms may be manifestations of the different environmental adaptations these closely related species display.     

Effect of fluctuating rhizosphere redox potential on carbon assimilation ofSpartina alterniflora

Summary Spartina alterniflora Lois. plants from a Louisiana salt marsh were subjected to fluctuating levels of soil redox potential under controlled environmental conditions. The experiment was designed to examine the changes in carbon assimilation rates in response to the change in rhizosphere sediment redox condition representing a broad range of reduction normally associated with oxygen deficient environments. Variation in sediment redox potential is frequently encountered by this species in its natural environment in Louisiana's Gulf Coast marshes as a result of tidal patterns. Results indicated some adverse effects of extreme anoxic conditions on carbon assimilation ofS. alterniflora, a possible reflection of this species limited ability for maintaining root oxygenation under rapid, intense reduction in soil redox potential. It was also demonstrated that gas exchange limitations may be temporary and apparently may follow by some recovery. Carbon assimilation rates declined 15 to 21% when soil redox level decreased rapidly to below-200 mV which was followed by substantial recovery. A system for accurate control and measurement of rhizosphere redox potential and simultaneous measurement of plant photosynthetic activity is described.     

One hundred years of Spartina altemiflora (Poaceae) in Willapa Bay, Washington: random amplified polymorphic DNA analysis of an invasive population

Spartina altemiflora (smooth cordgrass) has invaded Willapa Bay, Washington, covering bare mud flats and seagrass beds. At current expansion rates it threatens to occupy most of the intertidal habitat in the bay within 40 years. Although a major mitigation effort is underway, nothing is known about the genetic structure of the Willapa Bay S. altemiflora population or the underlying causes for the dramatic increase in seed set that has occurred in the last 15 years. Random amplified polymorphic DNA (RAPD) analysis was assessed as a method of DNA fingerprinting to analyse the genetic history and current structure of the Willapa population. The results suggest that all S. alterniflora clones in Willapa Bay are descended from a single genet. Given the limited genetic variability expected in a clonally founded population, we discuss the utility of RAPDs as genetic markers to explore the large morphological and reproductive differences reported among Willapa clones. Cluster analysis of RAPD fragments was used to show relatedness among S. alteniflora genets and may be helpful in locating the most prolific clones for mitigation efforts. Overall, RAPDs show great potential in genetic analysis of S. altemiflora, particularly in invasive populations where isozyme variability is low or nonexistent.     

Plant Traces Resembling Skolithos

The ichnogenus Skolithos Haldeman 1840 is a simple tubular trace fossil that was initially described as a “fucoid” or seaweed impression, i.e., a fossilized marine plant. Today the tracemaker is commonly considered to have been a vermiform marine invertebrate, although this interpretation is not acceptable to all. It is the name giver to the archetypal, Seilacherian Skolithos ichnofacies, which is a widely acknowledged indicator of relatively high energy, shallow water, nearshore to marginal marine environments. Here we record and describe analogous cylindrical structures, unquestionably reflecting plant tracemakers. These have been recognized in Quaternary consolidated “coffee” sandrock and Recent coastal dune, marsh or swamp, and salt meadow settings from northern New Zealand and Sapelo Island, USA.

We conclude that Skolithos is not an unequivocal indicator of shallow marine settings. Our observations confirm the opinions of a number of previous workers who have expressed the need for caution when using this ichnotaxon as a shallow-water, higher energy palaeoenvironmental indicator, especially when supporting evidence is otherwise lacking. In particular we note that in high intertidal and non-aquatic settings, Skolithos may be a plant trace fossil rather than serve as evidence for invertebrate activities. At the present time there is pressing need for a thorough systematic revision of the Skolithos ichnotaxon.     

The use of molecular assays to identify plant pathogenic organisms vectored by biological control agents

The planthopper Prokelisia marginata VanDuzee (Homoptera: Delphacidae) has beenconsidered for the biological control of theweed Spartina alterniflora Loisel(Poaceae) in Willapa Bay, Washington, U.S.A. Prokelisia marginata is a stenophagousphloem-feeding insect with the potential totransmit bacterial plant diseases that could bemoved by less-specific vectors to other plantspecies. Initial assays with PCR primers thatare putatively specific for phytoplasmas gavepositive results in Spartina. However,subsequent analyses did not indicate thetransmission of the pathogen by theplanthopper. We sequenced the 16S ribosomalRNA (rRNA) gene of the bacterial species thatgave positive results in PCR. Comparisons withsequences available in GenBank suggested thatthe positive results using the putativelyspecific PCR primers were due to the presenceof such bacteria as Pseudomonas, Holomonas, Vibrio, and Acinetobacter. We did not find phytoplasmasin either Spartina or the planthopperP. marginata.