Fenotipagem e genotipagem de populações de cebola com diferentes níveis de endogamia

Abstract

The production of onion hybrids depends on the development of genetically pure lineages. Traditionally, the pedigree method (applying successive self-fertilization in individuals) guarantees obtaining quickly the endogenous lines. However, the onion undergoes a strong process of inbreeding depression when self-fertilized for several generations, such as many other species of cross-pollination. Depression by inbreeding reduces plant vigor, bulb size and seed production, increasing the cost to produce the hybrid seed. Researchers have shown that cross-linking between a few plants promotes the reduction of depression by inbreeding in natural populations. Based on the same principle and taking in consideration the populations submitted to the selection of superior individuals, the objective of this paper is to elucidate if there is any effect of depression by inbreeding segregating lines of onion obtained by self-fertilization and by cross-linking of two and three plants. Therefore, we used 33 populations of onion and designed treatments in different ways of obtaining the populations (self-fertilization of one plant [A1], intercrossing of two [B2] and three plants [C3]), to study 11 different genetic backgrounds, grouped according to low or medium inbreeding. We evaluate throughout two consecutive years (2014 and 2015) the agronomic characteristics in the vegetative and reproductive phases of the crop, and conducted analysis with RAPDs and SSRs. The experiments were conducted at the experimental station of Bayer Vegetable Seeds in Uberlândia/MG. Plant vigor suffered the depressed by inbreeding in the early generations with greater heterozygosity. Similarly, the contrast between the self-fertilized compared to the two-or three-plant crossbred was more pronounced in the populations with lower endogamic level. The intercrossing of three plants reflected in larger production of seeds per plot in populations with low endogamic level. Changing the method of conducting the generations through the sib of plants provided greater divergence between the genotypes. Important characteristics such as bulb production per plot, seed survival, resistance to purple blotch (fungus Alternaria porri), and seed production by umbel, that can be used in the estimation of divergence in the selection of segregating onion lines. We observed a positive and significant genotypic correlation of 0.92 for the percentage of seed survival, and 0.82 for the resistance to purple blotch, with seed production per umbel. These results indicates that the weight of seeds per umbel can be increased through selection for resistance to purple blotch and percentage of seed survival. Furthermore, we concluded the depression by inbreeding could not be interpreted as due to homozygous effects for deleterious mutations, and that microsatellites markers may be useful for detecting variability within the Brazilian onion germplasm. These results suggest that sibs of two and three plants can be used to reduce the inbreeding effect in onion, and should start in low endogamous populations, for the best use of the method.