Evolution of increased vigour associated with allopolyploidization in the newly formed invasive species Salsola ryanii

A correlation between allopolyploidization and increased fitness is an explanation for the importance of allopolyploidy throughout evolution, specifically plant evolution. Although many authors have suggested correlation between allopolyploidy and increased fitness, common garden comparisons testing hypotheses about fitness shifts associated with allopolyploidy are lacking. In this study, we test the theory that allopolyploidy is associated with increased fitness in the newly formed allopolyploid weed Salsola ryanii. We conducted a common garden comparison over 2 years to determine how different fitness correlates (plant volume, plant mass and estimated seed number) of the newly formed allopolyploid species S. ryanii compares to its progenitors (S. tragus and S. australis) at a site within the range of all the newly formed allopolyploid and both of its progenitors. We document an increase in above-ground plant mass and above-ground volume in the newly formed allopolyploid compared to its progenitors. Plant mass and volume of the newly formed allopolyploid relative to its progenitors was dependent on year. The results of this study support the hypothesis that allopolyploidization is associated with increased growth, which is consistent with predictions that allopolyploid lineages experience vigour due to fixed heterozygosity.

Gametophyte morphology and reproductive biology in Elaphoglossum

Gametophyte morphology, behavior, and growth habit of Elaphoglossum alatum Gaud., Elaphoglossum callifolium (Bl.) Moore, Elaphoglossum crassifolium (Gaud.) Anderson & Crosby, Elaphoglossum marginatum (Wall ex Fee) Moore, and Elaphoglossum yoshinage (Yatabe) Makino were observed on artificial agar medium. The morphology of these gametophytes are very different from those of Bolbitis, a genus of Lomariopsidaecae thought to have a close relationship with Elaphoglossum. The growth habit of Elaphoglossum gametophytes, like many other epiphytic species, is perennial and clone forming. Isozyme analysis of naturally occurring sporophytes of E. alatum, E. crassifolium, and Elaphoglossum paleaceum (Hook. & Grev.) Sledge suggest that sporophytes of E. alatum and E. crassifolium are diploid and derived from highly outbreeding populations. Fixed heterozygosity in E. paleaceum suggests that sporophytes of this species are polyploid, but at least some outcrossing occurs. High genetic loads determined from single gametophyte cultures of E. callifolium and E. crassifolium indicate low probabilities of successful intragametophytic selfing. Several tests indicate, however, that these species neither produce nor respond to antheridiogens. Therefore, intergametophytic mating in these species is likely maintained by genetic load. This mating behavior is promoted by a perennial and clone-forming growth habit which may function to prolong the life-span and to enlarge the space occupied by individual gametophytes.Key words: Elaphoglossum, fern gametophyte, reproductive biology, breeding system.

Genetic resemblance of allotetraploid Aster ascendens to its diploid progenitors Aster falcatus and Aster occidentalis

Polyploidy is an important process in the evolution of the flowering plants. We used isozyme electrophoresis to examine genetic variation in populations of the western North American Aster ascendens Lindl., a dibasic allopolyploid species with n = 13 and 26, and its parent taxa, Aster falcatus Lindl. (x = 5) and Aster occidentalis Torr. & Gray (x = 8). Isozyme evidence confirms the derivation of A. ascendens from these two taxa. All populations and a high proportion of individual plants of A. ascendens possessed combinations of the alleles found in the parent species, and many individuals showed evidence of duplicated loci. However, fixed heterozygosity appeared to be less common than expected. Populations of A. ascendens resembled A. occidentalis much more closely than A. falcatus, and an UPGMA cluster analysis based on genetic identities showed that A. ascendens and A. occidentalis populations were closely intermingled, whereas A. falcatus populations were distinct. These results suggest that (i) A. ascendens has originated from multiple hybridization events involving different populations of A. occidentalis, and (ii) diploidization involving differential silencing of genes derived from A. falcatus may have occurred in A. ascendens.Key words: Aster, genetic variation, isozymes, allopolyploidy, diploidization, gene silencing.

Genetic diversity in Mediterranean diploid and tetraploid Bromus L. (section Bromus Sm.) populations

The levels of genetic diversity assessed from allozyme data were investigated in 25 populations of Mediterranean Bromus intermedius, B. squarrosus, B. lanceolatus, and B. hordeaceus from Algeria. The geographically restricted diploids B. intermedius and B. squarrosus displayed less genetic diversity (the mean population gene diversity of Nei (Hu) ranged from 0.03 to 0.12) than the widespread tetraploid colonizers B. lanceolatus and B. hordeaceus (Hu = 0.07–0.27). Deviations from Hardy–Weinberg expectations in diploid populations of B. intermedius and B. squarrosus were observed owing to heterozygote excess at several loci and suggested that these self-fertilizing species may have substantial amounts of allogamy. Tetraploid populations of B. lanceolatus and B. hordeaceus were largely homozygous at homologous loci and frequently exhibited intergenomic fixed heterozygosity in accordance with their alloploid origin. Genetic variation at the infraspecific level was mostly distributed within populations in the four species, B. hordeaceus showing the lowest level of interpopulation differentiation (Gst = 0.06) and the highest level of gene flow (Nm = 3.75). Consistent gene flows are in agreement with the strongest intercontinental invasive behaviour of B. hordeaceus. Less differentiation was reported in the literature among later introduced B. hordeaceus populations from England and Australia, indicating reduced differentiation under the process of colonization. Moderate divergence occured among the four taxa, with interspecific genetic identities ranging from 0.87 to 0.93. In spite of substantial genetic similarity, species were clearly differentiated, with each tetraploid being more closely related to a diploid: B. hordeaceus to B. squarrosus and B. lanceolatus to B. intermedius.Key words: Bromus, enzyme electrophoresis, population genetics, colonizing abilities, allopolyploidy.

Genetic structure of Trypanosoma cruzi populations from Argentina estimated from enzyme polymorphism

SUMMARYIsolates of Trypanosoma cruzi from human patients, domestic and sylvatic animals and vector insects were obtained in different areas of Argentina. Electrophoretic patterns of enzymes from extracts of 95 isolates were analysed. On the basis of zymograms providing information on 10 loci, 12 zymodemes are described according to their genotypes. Data presented show fixed heterozygosity, absence of segregation of genotypes, significant departures from Hardy–Weinberg equilibrium, and over-represented genotypes. This evidence supports the hypothesis that sexual reproduction is very restricted or absent in this parasite. The proportion of polymorphic loci is 80%. The expected mean heterozygosity per locus (He) is 0·43, while the observed value (Ho) is 0·24. Differences between these values may be explained by accepting a basically clonal structure for T. cruzi. The data matrix of 12 zymodemes using 28 characters was analysed using a Wagner parsimony algorithm. Two equally most parsimonious unrooted trees were generated; both have 39 steps. The results show clusters clearly separated according to the geographical origin of the stocks. There are some indications of some correlations between genetic composition of the parasite and the clinical picture of the infection in human patients.

Haploids of the wild tetraploid potato Solanum acaule ssp. acaule: generation, meiotic behavior, and electrophoretic pattern for the aspartate aminotransferase system

Solanum acaule Bitt. (acl) is a wild tetraploid potato, with bivalent pairing in meiosis. This species has been regarded as a segmental allotetraploid by cytological genome analysis, and one of its subspecies, acaule, as a fixed heterozygote for one of the two loci that codify the dimeric enzyme aspartate aminotransferase (AAT). Since haploid plants of acl could constitute unique tools to prove these previous views, controlled crosses between acl and a haploid inducer were carried out to try to obtain gynogenetic haploids. Two haploid plants (2n = 2x = 24) were identified among the progenies derived from 100 pollinations. Meiotic and electrophoretic analyses were performed in both of these plants. The mean frequency of univalents and bivalents per cell were 10.64 and 6.72, respectively. At the tetrad stage, monads (1.1%), dyads (15.0%), and triads (36.0%) were observed in addition to tetrads. Male fertility, however, was very low. It is conjectured that chromosome distribution in anaphase I was irregular and that dyads originated by a second division restitution mechanism. For the AAT system, two zones of activity, with one gene each, were detected in haploids and control tetraploids. In the slow zone, the same three-banded phenotype was observed in all individuals, although the bands stained with less intensity in the haploids. These observations confirmed that 4x acl is a segmental allotetraploid and that alleles in homoeologous loci, conforming to a fixed heterozygous genotype, do not segregate in meiosis.Key words: haploid, Solanum acaule, fixed heterozygosity, segmental allotetraploid.

Polyploid evolution in arctic-alpine Draba (Brassicaceae)

The mainly arctic-alpine genus Draba is well known for its complex morphological and chromosomal vanation. This paper reviews a larger study of Nordic Draba, aimed to provide insights into evolutionary processes that confound taxonomic relationships in the genus. The populations analyzed were referred to 16 currently recognized species of the sections Draba (petals white), Chrysodraba (petals yellow), andDrabella (petals yellow), and investigated using enzyme electrophoresis, restriction site analysis of cpDNA and rDNA, and analysis of chromosome numbers, artificial and natural hybrids, reproductive biology, habitat differentiation, and morphology. Section Draba comprises three diploids and seven polyploids (4x-10x) based on x = 8, sect. Chrysodraba comprises five polyploids (4x-16x) based on x = 8, and sect. Drabella comprises D. crassifolia, which probably is octoploid based on x = 5. Artificial F1 hybrids were obtained in 19 interspecific combinations. Later-generation hybrids were obtained in seven of these combinations. The genetic data suggest that 1) all polyploids are genetic allopolyploids, i.e., they show disomic inheritance and are highly fixed-heterozygous; 2) several of the polyploids have originated recurrently, some of them even polyphyletically; 3) some of the alloploid populations may have originated from cross-incompatible, sibling species that all belong to a single diploid taxonomic species; 4) interspecific gene flow across chromosome number barriers is possible and probably occurs in natural situations; 5) each of three of the polyploids represents an independent alloploid lineage, whereas sect. Draba and two species of sect. Chrysodraba form an intricate phylogenetic network; 6) some of the polyploids have originated locally, others have migrated repeatedly into the Nordic area; and 7) the phenotypic expression of genes encoding taxonomically important morphological characters does not follow consistent patterns in hybrids; this result may explain the discrepancies between genetic and taxonomic relationships in this highly reticulate genus. All species are sexual autogams, but there was large variation in autogamous seed set and traits promoting cross-pollination. The diploids are genetically depauperate, extremely inbreeding stress-tolerators occupying restricted ecological niches, whereas most of the polyploids have high levels of genetic variation, occupy a wide range of niches, and are either stress-tolerant competitors with a mixed mating system or primarily inbreeding ruderals. Mixed mating appears advantageous in the polyploids occurring in competitive habitats, although their fixed heterozygosity buffers the effect of selfing with respect to loss of variability. The ecological amplitude, heterozygosity, and biochemical diversity in the species were positively correlated and increased significantly with ploidal level. Two hypotheses are advanced to explain these correlations: 1) the general-purpose genotype hypothesis, which suggests that a high level of fixed heterozygosity in an allopolyploid genotype per se allows for exploitation of several different niches; and 2) the special-purpose genotype hypothesis, which suggests that repeated alloploidizations involving genetically divergent progenitors result in different fixed-heterozygous genotypes, each of which may exploit a particular niche. The evidence for multiple polyploid origins, differentiation into sibling diploids, and interploidal gene flow · in Draba add to a growing data base suggesting that polyploid complexes represent considerably more dynamic genetic systems than previously envisioned. In Draba, the principal evolutionary importance of these processes is probably that they serve as escapes from genetic and ecological depauperation caused by uniparental inbreeding at the diploid level. These processes inevitably result, however, in incongruities between taxonomic and evolutionary entities in the genus, supporting the use of a wide species concept.

Isozyme evidence for independently derived, duplicate G3PDH loci among squamate reptiles

We report evidence for several independent gene duplications for the locus encoding the enzyme glycerol-3-phosphate dehydrogenase (G3PDH) in squamate reptiles. Evidence for the duplication comes from population genetic studies demonstrating "fixed" heterozygosity in all members of some lizard species, the documentation of independent allelic heterozygosity at each of the two G3PDH loci in these same species, and tissue-specific gene expression surveys in a taxonomically diverse array of groups. The duplicated condition is present at both low and high taxonomic levels (selected populations of the phrynosomatid lizard Sceloporus grammicus, and almost all snakes, respectively), and appears to represent the derived condition in most of these groups. One notable exception is the colubrid snake genus Masticophis, which appears to be characterized by an apomorphic secondary silencing event. Evolutionary implications of the duplication and silencing events within squamates are discussed, and we suggest that the overall phylogenetic utility of this marker is low in this radiation as a result of extensive homoplasy.

Cytogenetics of Australian scorpions. I. Interchange polymorphism in the family Buthidae

Male scorpions from Australian species of the family Buthidae exhibit a unique combination of cytogenetic features including achiasmate meiosis, holocentric chromosomes, and extensive interchange heterozygosity. Chromosome number is highly conserved, with all species having a basic diploid number of 2n = 14. There is evidence that inbreeding has contributed to the establishment of populations with interchange heterozygotes, some exhibiting rings of up to 12 chromosomes. Although most populations contain both structural homozygotes and interchange heterozygotes, one population may exhibit fixed heterozygosity. It is argued that the interchange heterozygosity observed in buthids is of adaptive significance.Key words: interchange, chromosome polymorphism, achiasmate meiosis, holocentric chromosomes.