The work is carried out in the framework of the onion F1 breeding program. The practical purpose of the work is to carry out genetic selection of onion forms using molecular genetic markers of male sterility in the course of obtaining the maternal line for heterotic selection of F1 hybrids. The scientific component of these studies was the study of the population-genetic basis of various types of male sterility in onions of the Belarusian genmplasma. We used belarusian varieties Vetraz and Skarb litvinov and a collection of other varieties and hybrids. Markers of the mitochondrial genes orfA501, cob and the nuclear alleles Ms/ms, cosegregating with genes of fertility restorer/S-sterility maintainer genes, were studied. It has been shown that in the Vetraz variety are observed plants with the N- or T-cytotype in N-cytoplasm (TN-). The belarusian variety Scarb litvinov contains plants with the cytotypes N-, S, TN-, SN-, STN-. As a result, it was concluded that the male sterility of the Vetraz variety is T-type, and in the Scarp litvinov variety is complex, and is caused by the cytoplasmic alleles S- and T-. In both varieties: Vetraz and Scarb litvinov the sources of the ms locus causing the S-type of male sterility have been identified. Less than one-fifth of the number of world collection of varieties and hybrids was detected as Scytotype. This value is smaller than the value given in the scientific literature. However it may reflect the significant spread of the original S-cytoplasm of a single plant of the shortday Italian Red variety in the world onion germplasm.

. Heterotic selection is now widely used in crop production. The superiority of heterotic hybrids over varieties in many respects has long been known. However, the widespread use of heterosis in melon growing is hampered by the high cost of seed material, due to the lack of lines with male sterility. The aim of this work was to evaluate heterotic hybrids of watermelon obtained on the basis of a line with male sterility isolated at the Bykovskaya Experimental Station.Materials and methods. The studies were carried out in 2020 and 2021. at the Bykovskaya melon breeding experimental station - a branch of the FGBNU FNTSO. The object of research is heterosis hybrids of watermelon. The heterosis hybrid of watermelon Barkhan was used as a standard. Classical breeding methods were used in the work: incubation and hybridization.Research results. The assessment of watermelon hybrids according to the main economic characteristics revealed the best combinations in comparison with the standard. According to the content of dry matter, four hybrids of watermelon stood out, the excess over the standard ranged from 0.6 to 1.9%. The indicator of the average weight of the fetus in one LIR hybrid was at the level of the standard and amounted to 6.5 kg. The remaining hybrids exceeded the Barkhan standard by 1.2-4.0 kg. The average yield of the studied hybrids ranged from 19.0 to 22.5 t/ha. The calculation of such indicators as the degree of dominance, the effects of true and hypothetical heterosis made it possible to identify three combinations with high rates.Conclusion. Thus, it was found that the yield in heterotic hybrids of watermelon is inherited according to the principle of overdominance. Inheritance of traits dry matter content and average fruit weight ranges from minus overdominance to overdominance in various combinations. According to the complex of economically valuable traits, three combinations of table watermelon were distinguished: LKn, LMr and LFT.

Relevance. F1 hybrids breeding programs of onion are based on the use of nuclear cytoplasmic male sterility (NCMS). Breeding and seed production based on NCMS suggest a development of a female sterile maternal line, a maintainer line and a paternal component. The development of an isogenic pair of sterile line – maintainer line is one of the most labor-, time-consuming and intellectually-intensive stages of a breeding program, the implementation of which becomes more complicated with an increase in the number of traits / genes for which selection is carried out.Methods and results. This paper presents a genetic scheme for the development of a Downy mildewresistant (pathogen P. destructor) maintainer line of onion using marker-assisted selection (MAS). F2 populations were obtained from the cross of the female maintainer inbred line, Bn1- (13) (genotype – cytN msms pdpd) and male inbred line resistant to Downy mildew, No. 136 (genotype – cytT MsMs PdPd). DNA marker DMR1 were used for Downy mildew resistance gene Pd, jnurf13 – for maintainer gene ms and marker system 5`cob: orfA501 – for N, S and T cytoplasms. It was shown that the MAS application allow to develop a stable Downy mildew resistant maintainer line cytN msms PdPd in two generations.