Effect of Initial Planting Density on Growth Traits and Wood Properties of Triploid Chinese White Poplar (Populus tometosa) Plantation

Planting density primarily affects the yield and wood quality of plantations. There are multiple reports on the effects of planting density on growth traits and wood properties in young triploid Chinese white poplar (Populus tomentosa) plantations. Nevertheless, assessment of the effects of initial planting density is lacking for plantations older than ten years. Here, an 11-year-old plant density trial (2490, 1665, 1110, 832, 624, 499, and 416 trees/hm2) established with four hybrid clones (S86, B301, B331 and 1316) in northern China was used to determine the effect of initial planting density on growth traits (diameter at breast height (DBH), tree height (H), stem volume (SV) and stand wood volume (SWV)), basic wood density (BWD), and fiber properties (fiber length (FL), fiber width (FW), and the ratio of fiber length to width (FL/FW)). A total of 84 trees from four clones were sampled. In this study, the initial planting density had a highly significant effect on growth traits (p < 0.001) and had a moderate effect on FL. Overall, the reduction in initial planting density led to the increase in DBH, H, SV, and FL/FW. Triploid hybrid clones planted at 416 trees/hm2 had the largest DBH, H, SV, FL/FW and the smallest SWV and FW. Clonal effects were also significant (p < 0.05) for all studied traits except for FL. Clone S86 had a higher growth rate and the largest BWD and FW. Clones–initial planting densities interaction was insignificant for all growth traits and wood properties. A weak and positive estimated correlation between BWD and growth traits (H, SV, SWV) within each planting density was seen. Our results demonstrate that an appropriate reduction in initial density in triploid Chinese white poplar plantations with long rotation is a suitable strategy to promote tree growth and retain excellent wood processing characteristics.

Clonal gametogenesis is triggered by intrinsic stimuli in the hybrids germ cells but is dependent on sex differentiation

Interspecific hybridization may trigger the transition from sexual reproduction to asexuality, but mechanistic reasons for such a change in a hybrids reproduction are poorly understood. Gametogenesis of many asexual hybrids involves a stage of premeiotic endoreduplication (PMER), when gonial cells duplicate chromosomes and subsequent meiotic divisions involve bivalents between identical copies, leading to production of clonal gametes. Here, we investigated the triggers of PMER and whether its induction is linked to intrinsic stimuli within a hybrids gonial cells or whether it is regulated by the surrounding gonadal tissue. We investigated gametogenesis in the Cobitis taenia hybrid complex, which involves sexually reproducing species (Cobitis elongatoides and C. taenia) as well as their hybrids, where females reproduce clonally via PMER while males are sterile. We transplanted spermatogonial stem cells (SSCs) from C. elongatoides and triploid hybrid males into embryos of sexual species and of asexual hybrid females, respectively, and observed their development in an allospecific gonadal environment. Sexual SSCs underwent regular meiosis and produced normally reduced gametes when transplanted into clonal females. On the other hand, the hybrids SSCs lead to sterility when transplanted into sexual males, but maintained their ability to undergo asexual development (PMER) and production of clonal eggs, when transplanted into sexual females. This suggests that asexual gametogenesis is under complex control when somatic gonadal tissue indirectly affects the execution of asexual development by determining the sexual differentiation of stem cells and once such cells develop to female phenotypes, hybrid germ cells trigger the PMER from their intrinsic signals.

Genotype-environment interaction and stability of fiber properties and growth traits in triploid hybrid clones of Populus tomentosa

Background Clones provide a sensitive method for evaluating genotypic stability and detecting genotype-environment (G × E) interactions because of non-additive genetic effects among clones and there being no genetic effect among ramets of an ortet. With this study, we aimed to confirm and expand earlier findings, estimate stability parameters, and provide accurate estimates of clonal repeatabilities and genetic gains for a triploid breeding program of P. tomentosa Carr. Results Six 5-year-old clonal trials established in Northern China were used to determine the clonal variation, clone × site interactions, and the stability parameters of fiber properties of wood and growth traits. 360 trees from ten hybrid clones were collected from six sites. The clonal and site effects had a highly significant effect (P < 0.001) for all studied traits. While the clone × site interactions had a highly significant effect (P < 0.001) on fiber length (FL), coarseness (C), and tree growth (tree height [H], diameter at breast height [DBH] and stem volume [SV]), and a moderate effect (P < 0.05) on fiber width (FW) and fiber length/width (FL/W). For FL and SV, most of the triploid hybrid clones had higher reaction norms to the improvement in growth conditions and higher phenotypic plasticity. The estimated clonal repeatability of FW (0.93) was slightly higher than for FL (0.89), FL/W (0.83), C (0.91), DBH (0.76), H (0.85), and SV (0.80). Three clonal testing sites were sufficient to estimate quantitative parameters of fiber properties, however, more clonal testing sites would help improve the accuracy of quantitative parameters of the growth traits. Conclusions Our results highlight that accurate estimation of quantitative parameters for growth traits in triploid hybrid clones of P. tomentosa requires more clonal testing sites than the fiber properties.

Strategies to Produce Grapefruit-Like Citrus Varieties With a Low Furanocoumarin Content and Distinctive Flavonoid Profiles

Pummelos and hybrids, such as grapefruits, have high furanocoumarin and low flavonoid contents. Furanocoumarins interact negatively with certain drugs, while flavonoids are antioxidant compounds with health benefits. To obtain new grapefruit-like varieties with low furanocoumarin and high flavonoid contents, diploid and triploid hybrid populations from crosses between diploid and tetraploid “Clemenules” clementine and diploid “Pink” pummelo were recovered and analyzed. With regard to furanocoumarins, triploids produce less bergapten, bergamottin and 6,7-DHB than diploids. Regarding flavonoids, triploids yielded more eriocitrin, narirutin, hesperidin and neohesperidin than diploids, whereas no differences were observed in neoeriocitrin and naringin. These results indicate that, the strategy to recover triploid hybrids by 4x × 2x crosses is more appropriate than the recovery of diploid hybrids by 2x × 2x crosses for obtaining grapefruit-like varieties of citrus with lower furanocoumarin and higher flavonoid contents.

Non-Transgenic CRISPR-Mediated Knockout of Entire Ergot Alkaloid Gene Clusters in Slow-Growing Asexual Polyploid Fungi

The Epichloë species of fungi include seed-borne symbionts (endophytes) of cool-season grasses that enhance plant fitness, although some also produce alkaloids that are toxic to livestock. Selected or mutated toxin-free endophytes can be introduced into forage cultivars for improved livestock performance. Long-read genome sequencing revealed clusters of ergot alkaloid biosynthesis (EAS) genes in Epichloë coenophiala strain e19 from tall fescue (Lolium arundinaceum) and Epichloë hybrida Lp1 from perennial ryegrass (Lolium perenne). The two homeologous clusters in E. coenophiala—a triploid hybrid species—were 196 kb (EAS1) and 75 kb (EAS2), and the E. hybrida EAS cluster was 83 kb. As a CRISPR-based approach to target these clusters, the fungi were transformed with ribonucleoprotein (RNP) complexes of modified Cas9 nuclease (Cas9-2NLS) and pairs of single guide RNAs (sgRNAs), plus a transiently selected plasmid. In E. coenophiala, the procedure generated deletions of EAS1 and EAS2 separately, as well as both clusters simultaneously. The technique also gave deletions of the EAS cluster in E. hybrida and of individual alkaloid biosynthesis genes (dmaW and lolC) that had previously proved difficult to delete in E. coenophiala. Thus, this facile CRISPR RNP approach readily generates non-transgenic endophytes without toxin genes for use in research and forage cultivar improvement.

Genotype-Environment Interaction and Stability of Fiber Properties and Growth Traits in Triploid Hybrid Clones of Populus Tomentosa

Background: Clones provide a sensitive method for evaluating genotypic stability and detecting genotype-environment (G × E) interactions because of non-additive genetic effects among clones and no genetic effect among ramets of an ortet. The experiments aimed at confirming and expanding the earlier findings, estimating stability parameters, and providing accurate estimates of clonal repeatabilities and genetic gains for an triploid breeding programme of Populus tomentosa Carr. Results: Six 5-year-old clonal trials established in Northern China were used to determine the clonal variation, clone × site interactions and the stability parameters of fiber properties of wood and growth traits. Three hundred sixty trees from ten hybrid clones were collected in the six sites. The clonal and site effects had a highly significant effect (P < 0.001) for all studied traits. The clone × site interactions had a highly significant effect (P < 0.001) on fiber length (FL), coarseness (C), and tree growth (tree height [H], diameter at breast height [DBH] and stem volume [SV]), and a moderate effect (P < 0.05) on fiber width (FW) and fiber length/width (FL/W). For FL and SV, most of the triploid hybrid clones had higher reaction norms to the improvement in growth conditions and higher phenotypic plasticity. The estimated clonal repeatability of FW (0.93) was slightly higher than for FL (0.89), FL/W (0.83), C (0.91), DBH (0.76), H (0.85), and SV (0.80). Three clonal testing sites were sufficient to estimate quantitative parameters of fiber properties. However, more than three clonal testing sites will help improve the accuracy of quantitative parameters of growth traits.Conclusions: Our results unraveled that accurate estimation of quantitative parameters for growth traits in triploid hybrid clones of Populus tomentosa required more clonal testing sites than fiber properties.

Patterns of Genetic Diversification in the Invasive Hybrid Plant Pathogen Phytophthora × alni and Its Parental Species P. uniformis

In pathogenic fungi and oomycetes, interspecific hybridization may lead to the formation of new species having a greater impact on natural ecosystems than the parental species. From the early 1990s, a severe alder (Alnus spp.) decline due to an unknown Phytophthora species was observed in several European countries. Genetic analyses revealed that the disease was caused by the triploid hybrid P. × alni, which originated in Europe from the hybridization of P. uniformis and P. × multiformis. Here, we investigated the population structure of P. × alni (158 isolates) and P. uniformis (85 isolates) in several European countries using microsatellite markers. Our analyses confirmed the genetic structure previously observed in other European populations, with P. uniformis populations consisting of at most two multilocus genotypes (MLGs) and P. × alni populations dominated by MLG Pxa-1. The genetic structure of P. × alni populations in the Czech Republic, Hungary and Sweden seemed to reflect the physical isolation of river systems. Most rare P. × alni MLGs showed a loss of heterozygosity (LOH) at one or a few microsatellite loci compared with other MLGs. This LOH may allow a stabilization within the P. × alni genome or a rapid adaptation to stress situations. Alternatively, alleles may be lost because of random genetic drift in small, isolated populations, with no effect on fitness of P. × alni. Additional studies would be necessary to confirm these patterns of population diversification and to better understand the factors driving it.