Chromosomal status, gene exchange and evolution inDactylis: 3. The rôle of the inter-ploid hybrids

Complete Chloroplast Genomes of 14 Subspecies of D. glomerata: Phylogenetic and Comparative Genomic Analyses

Orchardgrass (Dactylis glomerata L.) is a species in the Gramineae family that is highly important economically and valued for its role in ecology. However, the phylogeny and taxonomy of D. glomerata are still controversial based on current morphological and molecular evidence. The study of chloroplast (cp) genomes has developed into a powerful tool to develop molecular markers for related species and reveal the relationships between plant evolution and phylogenetics. In this study, we conducted comparative genomic analyses and phylogenetic inferences on 14 cp genomes of D. glomerata originating from the Mediterranean and Eurasia. The genome size ranged from 134,375 bp to 134,993 bp and exhibited synteny of gene organization and order. A total of 129–131 genes were identified, including 85–87 protein coding genes, 38 tRNA genes and 8 rRNA genes. The cp sequences were highly conserved, and key sequence variations were detected at the junctions of inverted repeats (IRs)/small single–copy (SSC) regions. Moreover, nine highly variable regions were identified among the subspecies based on a sequence divergence analysis. A total of 285 RNA editing sites were detected that were relevant to 52 genes, where rpoB exhibited the most abundant RNA editing sites. The phylogenetic analysis revealed that all Dactylis subspecies clustered into a monophyletic group and most branches provided a high support bootstrap. The main divergence time of D. glomerata was dated to the Miocene era, and this could have been due to changes in the climate. These findings will provide useful insights for further studies on phylogeny, the identification of subspecies and the development of hypotheses for the evolutionary history of the genus Dactylis and of the Gramineae family.

Exploiting Unreduced Gametes for Improving Ornamental Plants

The formation of gametes with somatic chromosome number or unreduced gametes (2n gametes) is an important process involved in the origin of polyploid plants in nature. Unreduced gametes are the result of meiotic mutations occurring during micro- and mega-sporogenesis. 2n gametes have been identified or artificially induced in a large number of plant species. Breeding of plants through 2n gametes can be advantageous because it combines genetic effects of polyploidy with meiotic recombination and sexual hybridization to produce tremendous genetic variation and heterosis. 2n gametes also occur in ornamental plants, but the potential of using 2n gametes in ornamental plant breeding has not been extensively exploited. Ornamental plants are primarily produced for their esthetic appearance and novelty, not for food and yield, and they can be readily propagated through vegetative means. Triploids, tetraploids, and plants with even higher ploidy levels produced through 2n gametes can be propagated through tissue culture to fix their phenotypes, thus leading to the development of new cultivars. In this review article, we intend to discuss the mechanisms underlying the formation of 2n gametes, techniques for 2n gamete identification, methods for enhancing 2n gamete formation, and the current status in the use of 2n gametes for development of novel ornamental plants. We believe that polyploidy breeding through 2n gametes represents a viable way of developing new cultivars, new species, and even new genera of ornamental plants.

Phylogenetic and Diversity Analysis of Dactylis glomerata Subspecies Using SSR and IT-ISJ Markers

The genus Dactylis, an important forage crop, has a wide geographical distribution in temperate regions. While this genus is thought to include a single species, Dactylis glomerata, this species encompasses many subspecies whose relationships have not been fully characterized. In this study, the genetic diversity and phylogenetic relationships of nine representative Dactylis subspecies were examined using SSR and IT-ISJ markers. In total, 21 pairs of SSR primers and 15 pairs of IT-ISJ primers were used to amplify 295 polymorphic bands with polymorphic rates of 100%. The average polymorphic information contents (PICs) of SSR and IT-ISJ markers were 0.909 and 0.780, respectively. The combined data of the two markers indicated a high level of genetic diversity among the nine D. glomerata subspecies, with a Nei’s gene diversity index value of 0.283 and Shannon’s diversity of 0.448. Preliminarily phylogenetic analysis results revealed that the 20 accessions could be divided into three groups (A, B, C). Furthermore, they could be divided into five clusters, which is similar to the structure analysis with K = 5. Phylogenetic placement in these three groups may be related to the distribution ranges and the climate types of the subspecies in each group. Group A contained eight accessions of four subspecies, originating from the west Mediterranean, while Group B contained seven accessions of three subspecies, originating from the east Mediterranean.

Fitness differences among diploids, tetraploids, and their triploid progeny in Chamerion angustifolium: mechanisms of inviability and implications for polyploid evolution

Theoretical models indicate that the evolution of tetraploids in diploid populations will depend on both the relative fitness of the tetraploid and that of the diploid-tetraploid hybrids. Hybrids are believed to have lower fitness due to imbalances in either the ploidy (endosperm imbalance) or the ratio of maternal to paternal genomes in their endosperm (genomic imprinting). In this study we created diploids, tetraploids, and hybrid triploids of Chamerion angustifolium from crosses between field-collected diploid and tetraploid plants and evaluated them at six life stages in a greenhouse comparison. Diploid offspring (from 2x x 2x crosses) had significantly higher seed production and lower biomass than tetraploid offspring (from 4x x 4x crosses). Relative to the diploid, the cumulative fitness of tetraploids was 0.67. In general, triploids (from 2x x 4x, 4x x 2x crosses) had significantly lower seed production, lower pollen viability, and higher biomass than diploid individuals. Triploid offspring derived from diploid maternal parents had lower germination rates, but higher pollen production than those with tetraploid mothers. Relative to diploids, the cumulative fitness of 2x x 4x triploids and 4x x 2x triploids was 0.12 and 0.06, respectively, providing some support for effect of differing maternal:paternal ratios and endosperm development as a mechanism of hybrid inviability. Collectively, the data show that tetraploids exhibit an inherent fitness disadvantage, although the partial viability and fertility of triploids may help to reduce the barrier to tetraploid establishment in sympatric populations.

Competition between diploid and derivative autotetraploid Dactylis glomerata L. from Galicia. Implications for the establishment of novel polyploid populations

The competitive ability of a tetraploid, and its morphologically indistinguishable progenitor diploid Dactylis glomerata L., from Galicia (Spain), was tested in a two-year study, using isolated plants and the same individuals grown at two densities (16 and 36 plants per pot, respectively). Inverse linear regression equations were used to quantify the response of the different ploidy levels (hereafter cytotypes) to density. For the diploids, inter-cytotype competition coefficients were significantly higher than intra-cytotype coefficients. The substitution rate increased from 1. in the first year to 3.9 in the second year, when most of the diploids failed to flower and some died. The competitive superiority of the tetraploids was associated with higher mean tiller weight, particularly in the mixed cultures. The tetraploids also had heavier seeds and faster leaf production in early spring, and they flowered earlier than the diploids. Several of these differences between cytotypes appear to be due to differences in ploidy level and may play a major role in the establishment of the tetraploids in Galician pastures, where competition is high.

Flowering characteristics and fertility of interploidy progeny from normal and 2n gametes in Dactylis glomerata L

We performed an extensive experiment to compare male fertility, flowering period, seed set and germination rate between triploid and tetraploid cross-progeny of Dactylis glomerata L. originating from reciprocal diploid (2x) × tetraploid (4x) crosses. Four hundred and forty-five triploid and tetraploid individuals, constituting the cross progeny of 2x-4x reciprocal crosses between diploids from nine distinct subspecies, and tetraploids from five subspecies, were tested for male fertility and phenology under homogeneous conditions. A significant negative correlation was observed between the parental genetic distance and the frequency of sterile panicles (ST) in 3x progeny, suggesting that selection may occur against fertility in triploid progeny from interploid crosses involving closely related subspecies. Genetic distance between the parental subspecies was not significantly correlated with the frequency of fertile panicles (FT), non-spreading panicles (NT, non-dehiscent anthers with pollen inside), and ST panicles in the 4x progeny, nor with the frequency of FT and NT panicles in the 3x progeny. Among the 4x progeny individuals that produced panicles, more than 60% were fully fertile, but 24% possessed only sterile panicles. In the triploids, 63% of the reproductive plants were male-sterile, but 31% had variable proportions of fertile panicles and 6% possessed only fertile panicles. Male fertility in the progeny was significantly affected by individual ploidy level, parental diploid subspecies identity, and by the interaction between these 2 factors in the case of the fertile progeny plants. The frequency of vegetative plants was higher in 4x progeny than in 3x progeny, more particularly when the seed parent was tetraploid. However, when the seed parent was diploid, the frequencies of vegetative plants were similar in 3x and 4x progeny. Pollen viability was significantly higher in anthers from fertile panicles than in anthers from non-spreading panicles; viability of pollen from non-spreading panicles was, in turn, significantly higher than in sterile panicles. Also, seed set was higher in panicles with fertile anthers than in the two other types. However, seeds from plants with fertile, non-spreading and sterile anthers showed no differences in germination rate. Therefore, in Dactylis glomerata, tetraploid cross progeny arising from 2n gametes possess higher fertility, on average, than triploid cross progeny involving normal gametes. In addition to lower male fertility (non-spreading or atrophied anthers), the triploids also showed significantly lower pollen fertility in anthers and lower seed set in open pollinating conditions, probably because disturbance in meiosis, due to odd chromosome associations, also occurs in eggs. Moreover, triploids were observed to reproduce preferentially with individuals possessing the same ploidy levels as their seed parent. These results suggest that the production of 4x individuals, derived from 2n gametes in interploid crosses, is likely to be the main effective means of direct gene transfer in contact areas of populations or subspecies possessing distinct ploidy levels.

Relationship between species in the genus Rosa, section Pimpinellifoliae

The morphology of twelve species of Rosa is described and similarities between these species are assessed. Possible origins of the tetraploid species from diploid species are indicated on grounds of comparative morphology. The wild origins of living and herbarium specimens are given in order to supplement published data on geographical distribution. Meiosis in pollen mother cells, viability of pollen grains at anthesis and ability to set seed was studied in several F₁ hybrids: no indication of complete or even partial sterility was found. Reproductive isolation is therefore unlikely to be maintained by reduced fertility of interspecific hybrids. Three species are reduced to synonymy with three other species, being retained as subspecific taxa. Two species are transferred from section Pimpinellifoliae to section Cinnamomeae.