The enigmatic invasive Spartina densiflora: a history of hybridizations in a polyploidy context.

The aim of this study was to explore the origin of the invasive Spartina densiflora by analysing samples from the native region (South America) and from a recently colonized area (California). A combination of various molecular data (chloroplast and nuclear sequences, molecular fingerprint) and ploidy level estimations was used to answer the question whether the reticulate phylogenetic pattern previously detected in S. densiflora was restricted to California, or alternatively, whether a more ancient hybrid origin preceded formation of this species in its native area. We found that this species is heptaploid in both its native and introduced range. Identification of nuclear homeologous sequences indicate that this species has a reticulate origin in its native range, involving a lineage related to the hexaploid clade formed by S. alterniflora, S.foliosa, and S. maritima, and another lineage related to the sub-Antarctic endemic S. arundinacea that provided the chloroplast genome. The samples from California displayed similar multilocus patterns to the samples from Chile, supporting the hypothesis that this species originated on the southeast American coast (Argentina), from where it eventually spread to the west coast of South America (Chile) first and to the Northern Hemisphere (California) later.

Evolutionary dynamics of Waxy and the origin of hexaploid Spartina species (Poaceae).

We investigated the evolutionary dynamics of duplicated copies of the granule-bound starch synthase I gene (GBSSI or Waxy) within polyploid Spartina species. Molecular cloning, sequencing, and phylogenetic analyses revealed incongruences between the expected species phylogeny and the inferred gene trees. Some genes within species were more divergent than expected from ploidy level alone, suggesting the existence of paralogous sets of Waxy loci in Spartina. Phylogenetic analyses indicate that this paralogy originated from a duplication that occurred prior to the divergence of Spartina from other Chloridoideae. Gene tree topologies revealed three divergent homoeologous sequences in the hexaploid S. alterniflora that are consistent with the proposal of an allopolyploid origin of the hexaploid clade. Waxy sequences differ in insertion-deletion events in introns, which may be used to diagnose gene copies. Both paralogous and homoeologous coding regions appear to evolving under selective constraints.

Hybridization as a stimulus for the evolution of invasiveness in plants?

Invasive species are of great interest to evolutionary biologists and ecologists because they represent historical examples of dramatic evolutionary and ecological change. Likewise, they are increasingly important economically and environmentally as pests. Obtaining generalizations about the tiny fraction of immigrant taxa that become successful invaders has been frustrated by two enigmatic phenomena. Many of those species that become successful only do so (i) after an unusually long lag time after initial arrival, and/or (ii) after multiple introductions. We propose an evolutionary mechanism that may account for these observations. Hybridization between species or between disparate source populations may serve as a stimulus for the evolution of invasiveness. We present and review a remarkable number of cases in which hybridization preceded the emergence of successful invasive populations. Progeny with a history of hybridization may enjoy one or more potential genetic benefits relative to their progenitors. The observed lag times and multiple introductions that seem a prerequisite for certain species to evolve invasiveness may be a correlate of the time necessary for previously isolated populations to come into contact and for hybridization to occur. Our examples demonstrate that invasiveness can evolve. Our model does not represent the only evolutionary pathway to invasiveness, but is clearly an underappreciated mechanism worthy of more consideration in explaining the evolution of invasiveness in plants.

Hybridization and introgression in Carpobrotus spp. (Aizoaceae) in California. I. Morphological evidence.

Individuals of the invasive non-native plant species Carpobrotus edulis, its native or naturalized congener C. chilensis, and putative hybrids are found throughout coastal California. This study characterized morphological variation within the genus and assessed the extent of hybridization and direction of introgression between C. edulis and C. chilensis. Twelve randomly selected Carpobrotus individuals from each of 49 sample locations were classified as "C. edulis," "C. chilensis," or one of three intermediate morphotype classes, and compared for clonal, vegetative, and fruit characters. Univariate comparisons showed that C. edulis individuals had deeper mats with more cover within the mat boundary, longer, wider, more serrate leaves, and wider fruits with more carpels than intermediate and C. chilensis morphotype classes. Carpobrotus edulis and intermediates had more lateral branches than did C. chilensis, suggesting that hybrids tend to vary toward C. edulis. Discriminant function analysis (DFA) identified the following characters as discriminating significantly among Carpobrotus groups: leaf length, leaf width, leaf length/width, number of lateral branches, number of carpels, and fruit diameter. Using these characters, DFA correctly classified 98% of apparently pure C. edulis and C. chilensis, but only 76% of all Carpobrotus individuals when putative hybrids were included. Principal components analysis showed that C. edulis and C. chilensis cluster into distinct morphotypes, but that hybrids overlap with both groups, particularly with C. edulis. The genus Carpobrotus in California resembles a large hybrid swarm, with putative hybrids forming a large portion of the overall population and tending to vary toward C. edulis.

Hybridization and introgression in Carpobrotus spp. (Aizoaceae) in California. II. Allozyme evidence.

Morphological evidence has indicated that hybridization and introgression are occurring between Carpobrotus edulis L., a nonindigenous, invasive species in California (Bolus), and its putative native congener, C. chilensis. The identification of allozyme markers has enabled us to quantify hybridization and the extent of introgression between these species. Samples from 20 individuals of each of five morphological types (both parent species and three hybrid phenotypes) were collected from 39 populations along the coast of California from the Mexico to Oregon borders. Ten enzyme systems revealed a total of 17 resolvable loci, eight of which were polymorphic for the genus. Five of the polymorphic loci easily differentiate C. edulis and C. chilensis. Allele frequencies among the morphologically defined types are consistent with estimations of allele frequency changes due to first- and second-generation backcrossing. In comparison to long-lived, herbaceous perennials and widespread species, C. edulis and intermediate types have more variation in their populations (P = 41.73, Ap = 2.11, Ho = 0.246, and P = 38.13, Ap = 2.06, Ho = 0.216, respectively) than C. chilensis (P = 11.76, Ap = 2.00, Ho = 0.082). Indirect estimates of gene flow indicate four of the five morphological types are outcrossing. Gene flow between previously allopatric species may have broad implications if it results in an increase in fitness; further experimentation is needed to determine the ultimate ecological consequences of this phenomenon and its possible threat to limited, remaining natural habitat in California.