Whole-genome assembly of Corylus avellana cv‘Tonda Gentile delle Langhe’ using linked-reads (10X Genomics)

The European hazelnut (Corylus avellana L.; 2n=2x=22) is a worldwide economically important tree nut that is cross-pollinated due to sporophytic incompatibility. Therefore, any individual plant is highly heterozygous. Cultivars are clonally propagated using mound layering, rooted suckers and micropropagation. In recent years, the interest in this crop has increased, due to a growing demand related to the recognized health benefits of nut consumption. C. avellana cv ‘Tonda Gentile delle Langhe’ (‘TGdL’) is well-known for its high kernel quality, and the premium price paid for this cultivar is an economic benefit for producers in northern Italy. Assembly of a high-quality genome is a difficult task in many plant species because of the high level of heterozygosity. We assembled a chromosome-level genome sequence of ′TGdL′ with a two-step approach. First, 10X Genomics Chromium Technology was used to create a high-quality sequence, which was then assembled into scaffolds with cv ′Tombul′ genome as the reference. Eleven pseudomolecules were obtained, corresponding to 11 chromosomes. A total of 11,046 scaffolds remained unplaced, representing 11% of the genome (46,504,161 bp). Gene prediction, performed with Maker-P software, identified 27,791 genes (AED ≤ 0.4 and 92% of BUSCO completeness), whose function was analysed with BlastP and InterProScan software. To characterise ‘TGdL’ specific genetic mechanisms, Orthofinder was used to detect orthologs between hazelnut and closely related species. The ‘TGdL’ genome sequence is expected to be a powerful tool to understand hazelnut genetics and allow detection of markers/genes for important traits to be used in targeted breeding programs.

Geographic Distribution of Incompatibility Alleles in Cultivars and Selections of European Hazelnut

The european hazelnut (Corylus avellana L.) is native to most of Europe and nearby areas in Asia Minor and the Caucasus Mountains. Cross-pollination is enforced by sporophytic incompatibility under the control of a single locus with multiple alleles (haplotypes). Fluorescence microscopy is routinely used to determine if a pollination is compatible or incompatible, and use of an array of known testers allows identification of the alleles of cultivars and selections. Both alleles are expressed in the stigmas, but often only one is expressed in the pollen because of dominance. Cultivars are highly heterozygous diploids (2n = 2x = 22) and clonally propagated. Most of the world's leading cultivars were selected from local wild populations near where they are now planted on a commercial scale. Genetic improvement efforts are recent and, although tremendous genetic variability is available, such efforts have had little impact outside of Oregon and France. Studies of genetic diversity using simple sequence repeat markers have placed most cultivars in one of the four main groups: Spanish-Italian, Central European, English, or Black Sea. This study presents 17 years of data on incompatibility in hazelnut, including the discovery of six new S-alleles and determination of the dominance relationships among 105 new pairs of alleles. The total number of alleles now stands at 33. The S-alleles of 284 cultivars, 13 interspecific hybrids, and 522 selections of diverse origin are presented. The S-alleles identified in hazelnut cultivars is information that should be useful to breeders in the planning of crosses, to curators of germplasm collections, and to growers and nurseries as they choose cultivars and pollenizers when designing orchards. Differences in S-allele frequency seen in the cultivars and selections are related to geographic origin. The most common alleles of cultivars in the major geographical groups are S2 in the Spanish-Italian group, S5 in the Central European group, S3 in the English group, and S4 in the Black Sea group. Most selections belonged to the Black Sea group, and S4 was by far the most common allele. Differences in allele frequency were also observed among seed lots within a country.

Genetic control of the development of the haploid generation in Oenothera

The haploid generation of higher plants has to be considered in its own individuality. Special experimental designs are needed to investigate the developmental processes of the male and female gametophytes between meiosis and fertilization. Experiments on <em>Oenothera</em> demonstrate the existence of genes, which action can be described as influencing the competition between meiospores or between gametophytes, or as interaction between different individuals, the gametophytic-gametophytic and gametophytic-sporophytic incompatibility. The development of the haploid generation is regulated by genes. Some of these genes are active only in this phase of the life cycle.

Development of female and male gametophytes in cereal species

The first steps in studies on the female and male gametophytes of wheat involved the light microscope analysis of semi-thin sections of embryo sacs containing egg-cells developing in planta. The information thus obtained on the development of the egg-cell from its initial formation to maturity contributed to the successful isolation of egg-cells. The morphological and ultrastructural details of egg-cells isolated 3, 6, 9, 12, 15 and 18 days after emasculation were examined to determine the function of the female gametophyte and its suitability for micromanipulation. A sufficient number of gametoplasts in the right stage of development are required for the successful microinjection and in vitro fusion of egg-cells and male gametes. No data are yet available on the fusiogenicity of wheat egg-cells in various stages of development. Various in vitro fertilisation and microinjection techniques could be of service in gamete fusion experiments aimed at the creation of interspecific and intergeneric hybrids which do not occur in nature due to sporophytic incompatibility. The results acquired in investigations on immature embryos can be used for the study of embryos developing from egg-cells fertilised in vitro.

Evaluation of two self-incompatibility alleles in three summer rape (Brassica napus L.) cultivars by UV fluorescence microscopy, seed set and outcrossing rates

S-alleles W1 and T2 and an incompletely dominant white petal character were introgressed into the self-compatible (SC) summer rape (Brassica napus L. ssp. oleifera {Metzg.}) cultivars Global, Topas and Westar. The derived self-incompatible (SI) lines were evaluated for strength of incompatibility by ultraviolet fluorescence of pollen tubes, and by seed set. Pollen tube and seed set analyses showed the W1 and T2 alleles were strongly, moderately and weakly expressed in Topas, Global and Westar, respectively. Seed set data showed a significant difference between SI lines, but not between S-alleles, or between homozygous or heterozygous lines from the same SI cultivar. SI cultivar yellow petal (wild type) lines were field pollinated with SC white petal lines. Seed collected from the SI cultivars were evaluated for proportion of outcrossed progeny by recording the frequency of yellow petal and cream petal plants, which were the result of self- and cross-pollination, respectively. The proportion of outcrossed progeny (i.e., outcrossing rates) ranged from 23% to 79%. Topas SI lines had significantly higher outcrossing rates than Global SI lines, which corresponded to SI line seed set data. Environment, S-allele selection and genotype significantly affected outcrossing rates. Key words: Brassica napus, sporophytic incompatibility, S-allele, outcrossing rate