Targeted generation of polyploids in Hydrangea macrophylla through cross-based breeding

Background Up to now, diploid and triploid cultivars were reported for the ornamental crop Hydrangea macrophylla. Especially, the origin of triploids and their crossing behaviors are unknown, but the underlying mechanisms are highly relevant for breeding polyploids. Results By screening a cultivar collection, we identified diploid, triploid, tetraploid and even aneuploid H. macrophylla varieties. The pollen viability of triploids and tetraploids was comparable to that of diploids. Systematic crosses with these cultivars resulted in viable diploid, triploid, tetraploid and aneuploid offspring. Interestingly, crosses between diploids produced diploid and 0 or 1–94% triploid offspring, depending on the cultivars used as pollen parent. This finding suggests that specific diploids form unreduced pollen, either at low or high frequencies. In contrast, crosses of triploids with diploids or tetraploids produced many viable aneuploids, whose 2C DNA contents ranged between the parental 2C values. As expected, crosses between diploid and tetraploid individuals generated triploid offspring. Putative tetraploid plants were obtained at low frequencies in crosses between diploids and in interploid crosses of triploids with either diploid or tetraploid plants. The analysis of offspring populations indicated the production of 1n = 2x gametes for tetraploid plants, whereas triploids produced obviously reduced, aneuploid gametes with chromosome numbers ranging between haploid and diploid level. While euploid offspring grew normally, aneuploid plants showed mostly an abnormal development and a huge phenotypic variation within offspring populations, most likely due to the variation in chromosome numbers. Subsequent crosses with putative diploid, triploid and aneuploid offspring plants from interploid crosses resulted in viable offspring and germination rates ranging from 21 to 100%. Conclusions The existence of diploids that form unreduced pollen and of tetraploids allows the targeted breeding of polyploid H. macrophylla. Different ploidy levels can be addressed by combining the appropriate crossing partners. In contrast to artificial polyploidization, cross-based polyploidization is easy, cheap and results in genetically variable offspring that allows the direct selection of more robust and stress tolerant polyploid varieties. Furthermore, the generation of polyploid H. macrophylla plants will favor interspecific breeding programs within the genus Hydrangea.

Most of Tobacco Male Meiocytes Are Involved in Intercellular Nuclear Migration at the First Meiotic Prophase

Serial block-face scanning electron microscopy was used here to study tobacco male meiosis. Three-dimensional ultrastructural analyses revealed that intercellular nuclear migration (INM) occurs in 90–100% of tobacco meiocytes. At the very beginning of meiosis, every meiocyte connected with neighboring cells by more than 100 channels was capable of INM. At leptotene and zygotene, the nucleus in most tobacco meiocytes approached the cell wall and formed nuclear protuberances (NPs) that crossed the cell wall through the channels and got into the cytoplasm of a neighboring cell. The NPs did not separate from the migrating nuclei and never produced micronuclei. Approximately 70% of NPs reached nuclei of neighboring cells. The NPs and the nuclei they reached got very close, and the gap between their nuclear membranes became indistinguishable in some cases. At pachytene, NPs detached from the nuclei of neighboring cells and came back into their own cells. After that, the INM stopped. The reason for such behavior of nuclei is unclear. INM probably causes a short-lived fusion of two nuclei and thus has a potential to form aneuploid or unreduced pollen. We consider INM a normal part of tobacco meiosis.

Versatility of reproductive modes and ploidy level interactions in Sorbus s.l. (Malinae, Rosaceae)

This study focuses on the frequency of reproductive modes and the role of reproductive mode variation in diversification of the genus Sorbus s.l. and evolutionary success of individual taxa. Our sampling covers all four ploidies known in Sorbus and includes stabilized taxa and recently formed biotypes. We infer reproductive modes of 3660 seeds using the flow cytometric seed screen method; we also calculated ploidy of contributing pollen. Sorbus shows high variability in reproduction pathways (sexuality, apomixis, formation of BIII hybrids, haploid parthenogenesis). Diploids are sexual, whereas polyploids show predominantly pseudogamous apomixis. In half of the polyploids studied, several reproductive modes were present and even intra-individual variation was detected. Diploids, tetraploids and pentaploids are pollinated by conspecific pollen, whereas triploids are largely, but not fully, dependent on the pollen of other species. In pseudogamous seeds, fertilization of the central cell by both sperms is a dominant pathway of endosperm origin. Flow cytometry of pollen did not detect unreduced pollen at any ploidy. The principal mechanisms of diversification in Sorbus are residual sexuality in apomicts, production of triploid taxa from diploid–tetraploid crosses and production of BIII hybrids. Evolutionary success of Sorbus taxa is largely dependent on the breeding system and is determined by self-(in)compatibility and the interplay between stabilizing apomixis and variation-generating sexuality.

Impact of cytomixis on the microspogenesis and formation of unreduced pollen grains in monocots

Aim. Despite significant progress in the investigation of cytomixis its functional role and effect on the course of meiosis as well as mechanisms of 2n pollen grains formation is still not completely clear. We have studied the destination of cytomictic chromatin as well the mechanisms of unreduced microspores formation in species of monocots with spontaneous cytomixis. Methods. Light and fluorescent microscopy. Results. The cytomictic chromatin forms additional meiotic chromosomes in the recipient microsporocytes. Many of these meiotic chromosomes undergo rearrangement and fragmentation but retain their bivalent organization. Conclusions. Cytogenetic anomalies of microsporogenesis caused by activation of cytomixis in prophase may reflect meiosis stabilization mechanisms by assimilation and adaptation or diminution and reutilization of the introduced DNA. The main mechanism of polyploidization of microsporocytes and pollen grains in studied monocots is the restitution of the first meiotic division via the formation of a meta-anaphase 1 block.Keywords: cytomixis, microsporogenesis, additional (recipient) chromosomes, meta-anaphase 1 block, Lilium croceum Chaix., Allium cepa L., Allium fistulosum L.

Ploidy and Genome Size in Lilac Species, Cultivars, and Interploid Hybrids

Genome size variation can be used to investigate biodiversity, genome evolution, and taxonomic relationships among related taxa. Plant breeders use genome size variation to identify parents useful for breeding sterile or improved ornamentals. Lilacs (Syringa) are deciduous trees and shrubs valued for their fragrant spring and summer flowers. The genus is divided into six series: Syringa (Vulgares), Pinnatifoliae, Ligustrae, Ligustrina, Pubescentes, and Villosae. Reports conflict on genome evolution, base chromosome number, and polyploidy in lilac. The purpose of this study was to investigate genome size and ploidy variation across a diverse collection. Flow cytometry was used to estimate monoploid (1Cx) and holoploid (2C) genome sizes in series, species, cultivars, and seedlings from parents with three ploidy combinations: 2x x 2x, 2x x 3x, and 3x x 2x. Pollen diameter was measured to investigate the frequency of unreduced gametes in diploid and triploid Syringa vulgaris cultivars. Three triploids of S. vulgaris were observed: ‘Aucubaefolia’, ‘Agincourt Beauty’, and ‘President Grévy’. Across taxa, significant variations in 1Cx genome size were discovered. The smallest and largest values were found in the interspecific hybrids S. ×laciniata (1.32 ± 0.04 pg) and S. ×hyacinthiflora ‘Old Glory’ (1.78 ± 0.05), both of which are in series Syringa. Series Syringa (1.68 ± 0.02 pg) had a significantly larger 1Cx genome size than the other series. No significant differences were found within series Pubescentes (1.47 ± 0.01 pg), Villosae (1.55 ± 0.02 pg), Ligustrina (1.49 ± 0.05 pg), and Pinnatifoliae (1.52 ± 0.02 pg). For S. vulgaris crosses, no significant variation in 2C genome size was discovered in 2x x 2x crosses. Interploid crosses between ‘Blue Skies’ (2x) and ‘President Grévy’ (3x) produced an aneuploid population with variable 2C genome sizes ranging from 3.41 ± 0.03 to 4.35 ± 0.03 pg. Only one viable seedling was recovered from a cross combination between ‘President Grévy’ (3x) and ‘Sensation’ (2x). This seedling had a larger 2C genome size (5.65 ± 0.02 pg) than either parent and the largest 2C genome size currently reported in lilac. ‘Sensation’ produced 8.5% unreduced pollen, which we inferred was responsible for the increased genome size. No unreduced pollen was discovered in the other diploids examined. Increased ploidy may provide a mechanism for recovering progeny from incompatible taxa in lilac breeding.