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.

Apomictic development during different flower development stages in Crataegus tanacetifolia (Lam.) Pers., endemic to Turkey

In this study, aposporic apomictic development and its relation to the different flower development stages were investigated by light and fluorescence microscopy in Crataegus tanacetifolia (Lam.) Pers. (Rosaceae). At pre-anthesis stage, aposporic initial cell differentiated at the somatic nucellus tissue shortly after the megaspore mother cell formation. The volume of aposporic initial cell increased during the generation of dyad and megaspore tetrad respectively by regular meiosis. At this stage, linear megaspore tetrad and vacuolated aposporic initial cell were located side by side into the same ovule. At anthesis stage, before pollination, four nucleated aposporic embryo sac was formed while sexual development came to end by atrophy of megaspores completely. At this stage, atrophied megaspores and two nucleated aposporic embryo sac were located side by side into the same ovule. At post-anthesis stage, pollination still had not begun and ovule contained only eight nucleated aposporic embryo sac. Mature aposporic embryo sac was composed of two synergid cells and one egg cell on the micropylar side, three antipodal cells on the chalazal side and a central cell with two polar nuclei in the middle of the sac. The absence of filiform apparatus in the synergid cells was quite remarkable. No callose accumulation around the aposporic initial cell was observed in any development stage. Pollination started shortly after the proembryo formation. Embryo and endosperm developed without fertilization due to the problems encountered in reaching pollen tubes to the ovary.

Creation of fertility-restored materials for Ogura CMS in Brassica oleracea by introducing Rfo gene from Brassica napus via an allotriploid strategy

Key message Ogura CMS fertility-restored materials, with 18 chromosomes, normal seed setting, stable fertility and closer genetic background to the parent Chinese kale, were successfully developed in B. oleracea via a triploid strategy for the first time. Abstract Ogura cytoplasmic male sterility (CMS) is the most widely used sterile type in seed production for commercial hybrids of Brassica oleracea vegetables. However, the natural Ogura CMS restorer line has not been found in B. oleracea crops. In this study, the triploid strategy was used with the aim to create euploid B. oleracea progenies with the Rfo gene. The allotriploid AAC hybrid YL2 was used as a male parent to backcross with Ogura CMS Chinese kale. After successive backcrosses, the BC2Rfo-positive individual 16CMSF2-11 and its BC3 progenies, with 18 chromosomes, were developed, which were morphologically identical to the parent Chinese kale. Compared with F1 and BC1 plants, it showed stable fertility performance, and regular meiosis behavior and could produce seeds normally under natural pollination. The genomic composition analysis of Rfo-positive progenies by using molecular markers showed that more than 87% of the C-genome components of BC3Rfo-progenies recovered to the parent Chinese kale, while most or all of the An-genome segments were lost in 16CMSF2-11 and its progenies. The results suggested that the genetic background of Rfo-positive individuals was closer to that of the parent Chinese kale along with backcrossing. Hereof, the Ogura CMS fertility-restored materials of Chinese kale were successfully created via triploid strategy for the first time, providing a bridge for utilizing the Ogura CMS B. oleracea germplasm in the future. Moreover, our study indicates that the triploid strategy is effective for transferring genes from B. napus into B. oleracea.

Genetic and cytogenetic structure of wild lemon grass (Elionurus muticus ) populations

Elionurus muticus is a native aromatic grass from the Pampa biome that produces an essential oil that is rich in citral. Despite the importance of citral, few studies have examined this species. The aims of this work were to evaluate the genetic structure and to characterize cytogenetically natural populations collected from Brazil. Genetic characterization was performed using AFLP markers, and cytogenetics assessed the chromosome number, karyotype and meiosis. The studied populations had genetic variability, especially within populations, indicating the possibility of selecting plants with relevant characters. High variability also suggests the preferential occurrence of outcrossing in natural populations. Regular meiosis was observed in the cytogenetic analysis with chromosome number 2n=20. The karyotype of the species is presented for the first time, with the karyotype formula 3sm + 4a + 1saSAT.

Microsporogenesis of Betula oycoviensis Bess. and of its progeny

The crossing of two specimens of <i>Betula oycoviensis</i> Bess. described as a separate taxon of the subsection Albae, unexpectedly yielded a progeny among which three types were distinguished: <i>oycoviensis, verrucosa</i>, and <i>nova</i>, in the approximate ratio 1:2:1. The first two types were trees, whereas the third one was a small shrub. Moreover, the three types of birches differed from one another in a number of morphological and biological characters. Microsporogenesis was in vestigated in the parental specimens of B. oycoviensis and in several specimens of the progeny representing three types of birches. It was only one birch of the <i>oycoviensis</i> type, a mixoploid specimen, that exhibited disturbances in microsporogenesis. In the other examined birches of the <i>oycoviensis, verrucasa</i>, and <i>nova</i> types meiosis 'proceeded normally and resulted in normally developed tetrads an-d pollen grains. Therefore, B. oycoviensis belongs to hybrids of regular meiosis, its hybrid character being revealed only at crossing.

Cytological and reproductive aspects in the Caespitosa group of Paspalum

Somatic chromosome numbers are reported for thirty four germplasm accessions of Paspalum, Caespitosa group, representing five different species. All five species have shown x=10 as the basic chromosome number. The diploid 2n=20 chromosome number was confirmed for P. chacoense and P. indecorum, as well as sexuality for the latter. This is the first report of the chromosome number and cytological behavior for P. ligulare (2n=20 and 40), P. pleostachyum (2n=20, 30 and 40) and P. redondense (2n=20 and 40). The present results document regular meiosis in diploid accessions, with primarily bivalent pairing at diakinesis and metaphase I. Polyploids of these species had irregular meiosis, with univalent, trivalent, and quadrivalent chromosome associations. Diploid accessions of P. pleostachyum and P. ligulare have shown a single meiotic embryo-sac, indicating sexual reproduction, while the triploid and one of the tetraploid accessions of P. pleostachyum have shown aposporic embryo-sacs of nucelar origin, and a few meiotic sacs, suggesting facultative apomixis. Identification of the new diploid accessions may prove useful for phylogenetic studies of Paspalum, as well as for breeding programs focusing on the forage potential of species of the Caespitosa group.

Karyotypic diversification and its contribution to the taxonomy of Eleocharis (Cyperaceae) from Brazil

A karyotype analysis of 147 populations of 25 Brazilian species of Eleocharis (Cyperaceae) was carried out, including representatives of the three subgenera that occur in the country: Limnochloa, Scirpidium and Eleocharis. The analyses showed chromosomes without centromeres, but with terminal nucleolar constrictions (satellites) in some chromosomes. The chromosome numbers varied from 2n = 6 in E. subarticulata and E. maculosa to 2n = 60 in E. laeviglumis, but the chromosome basic number x = 5 was confirmed. Species of the subgenera Eleocharis and Scirpidium possess fewer and larger chromosomes, while those in the subgenus Limnochloa have small and more numerous chromosomes. These features indicate that the karyotypes of the subgenera Eleocharis and Scirpidium are more closely related, in agreement with morphological and phylogenetical data. The representatives of the section Eleocharis exhibited the largest differences in chromosome number and size, probably due to chromosome fission and fusion. Polyploidy was the most common event in this group. Nevertheless, most of the studied species exhibited regular meiosis with only bivalent formation, even the polyploids, such as in E. geniculata and E. sellowiana. The cytogenetic information obtained showed quite variable karyotypes with chromosomes gradually decreasing in size, and predominance of polyploidy. These results are useful in the differentiation of the subgenera.

Reconstruction of allopolyploid Brassicas through non-homologous recombination: introgression of resistance to pod shatter in Brassica napus

SummaryPod shattering of rapeseed (Brassica napus) causes serious yield loss. Genetic resistance to shattering has been introgressed into B. napus from B. juncea. This followed from allosyndetic pairing between chromosomes of B and C genomes in the interspecific F1 hybrid, B. juncea × B. napus (2n = 37, AABC). The reconstituted B. napus plant showed regular meiosis with 19 bivalents and had pollen and seed fertility of 84 and 23% respectively. An approach is suggested for achieving introgression from monogenomic diploids to digenomic allopolyploids that exploits non-homologous recombination.

Cytogenetics and reproductive behavior of induced and natural tetraploid guayule (Parthenium argentatum Gray)

Tetraploid guayule (Parthenium argentatum Gray) plants with 72 chromosomes were obtained by colchicine application to the shoot apices of the diploid (2n = 36) seedlings. Interspecific hybridization of guayule as female with Parthenium rollinsianum Rzedowski (2n = 36) was used to compare the mode of reproduction of the induced tetraploids with that of natural tetraploids. Plants produced from crosses of induced tetraploid guayule and P. rollinsianum were all hybrids. Hybrid plants showed 2n = 54 chromosomes, indicating that the artificially produced tetraploids reproduce sexually. Interspecific crosses between natural tetraploids and P. rollinsianum produced predominantly maternal type progeny. This suggests that the apomictic mode of reproduction in guayule is a consequence of mutations which accumulated in the natural polyploids. Diploid guayule plants showed regular meiosis with an average of 14.86 rod and 3.14 ring bivalents. Induced tetraploids produced 0.25 I, 29.21 II, 0.13 III, and 3.24 IV. In comparison, natural tetraploids showed 1.20 I, 30.85 II, 0.06 III, and 2.23 IV. The frequency of multivalents, which was limited in the induced tetraploids, showed further reduction in natural polyploid guayule, which may indicate diploidization in these plants. Percent pollen stainability was 98.5 ± 1.03, 83.7 ± 12.24, and 89.5 ± 3.41 for diploids, induced tetraploids, and natural tetraploids, respectively. Both induced and natural tetraploids had larger capitula, achenes, and pollen and thicker leaves than the diploids.Key words: guayule, natural rubber, reproductive behavior, meiosis, polyploid induction.

Influence of parental strains on the germination ofPhycomyces blakesleeanuszygospores

SummaryPhycomyces blakesleeanuswild-type NRRL1555( − ), the standard strain, when crossed with UBC21( + ), another wild type, gives zygospores that germinate in 50–60 days. By backcrossing to UBC21 and selecting for shorter dormancy we have isolated a ( − ) strain, A803, and a ( + ) strain A804, which when crossed give zygospores that germinate in 32 days, the shortest dormancy period found inPhycomyces.The same result was obtained when A803 was crossed with UBC21. Zygospore dormancy decreased as the parental strains became more isogenic with UBC21, but the number of zygospores giving germsporangia with viable germspores also decreased to zero in the third backcross. The existence of germspore-killer alleles in the strain UBC21 is postulated. The strains of shortest dormancy can be used as helper strains (Orejaset al.1985) in sexual crosses. Tetrad analysis of the cross NRRL1554 × S102, a two-factor cross, showed 90% of reciprocal ditypes plus tetratypes in the progeny, indicating that the ( + ) wild-type strain NRRL1554, when crossed with the standard strain, gives regular meiosis and, contrary to current beliefs, may be used inPhycomycesgenetic analysis.