Estratégias de condução de populações segregantes utilizando o método BULK/F2 ou S0

Abstract

In the breeding programs of bean and other autogamous species cultivated in Brazil, the Bulk method within F2 or S0 progenies (Bulk/F2) is widely used. Although the method has been proposed more than sixty years ago, very little has been discussed about its theoretical basis, especially regarding what occurs in frequency terms of the different genotypic combinations, genetic variance and the effect of natural selection within progenies along with generations of inbreeding. To answer many questions still pending with this method, the present work was carried out with the following objectives: to apply the Mendelian principles, using the bulk/F2 method, considering a large number of segregating genes, for some generations, to obtain information that can help in the autogamous plant selection programs; to verify if there is variability among S0 plants, and within the progenies of these plants with when evaluating the resistance to P. griseola and C. lindemuthianum, to infer if the S0 plants were or not heterozygous for the possible resistance to the two pathogens; and to verify if the resistant plants frequency varies as a function of the moment of bulk opening; was to show that the variation released within the progenies conducted by the bulk / F2 or S0 method is the same as that occurring among progenies, and to verify whether natural selection acts at grain yield as endogamy goes in the dry bean crop. To achieve these objectives, three research activities were carried out: a) Use of the simulation to understand what occurs between and within the progenies in terms of the frequency of loci in heterozygosis or homozygosis and genetic variance. A bi-parental crossing was made, obtaining 2000 individuals in the F2 generation. From these individuals, 150 F2:3 progenies with 100 individuals were obtained. A thousand simulations were performed in the F2, F2:3, F2:5 and F2:8 generations considering two types of allele interaction: additive and complete dominance. From these simulations, the percentage of loci in homozygous and heterozygosis, as well as the amplitude of these values, considering 10, 50, 100 and 200 loci for the F2 population and 10, 50 and 100 loci for the segregating progenies were obtained. b) Evaluate the existence of variation between and within SR progenies with respect to resistance to Pseudocercospora griseola, which causes angular leaf spot (ALS) and Colletotrichum lindemuthianum responsible for anthracnose (AT). Progenies from the fourteenth cycle (C.XIV) of RS were used. From each of the ten S0:2 progenies identified as most productive in the S0:1 generation, ten plants were harvested individually, progenies S2:3, and stored in a cold chamber. The remaining plants continued to be advanced to S0:5, when the same procedure was applied, and S5:6 progenies were obtained. The 100 S2:3 progenies and the 100 S5:6 progenies were multiplied by successive generations and S2:6 and S5:8 progenies were obtained. In separate experiments for each pathogen, the offspring of these progenies were evaluated. The seedlings were inoculated with the isolate of the race 63-63 of P.griseola and of the race 65 of C. lindemuthianum. c) Estimation of genetic and phenotypic parameters among and within progenies with different levels of inbreeding and genetic structure of populations for grain yield in common bean. For this, two distinct populations were used, the first one from the fourteenth cycle of the UFLA (C.XIV) recurrent selection program, the second obtained from the crossbreeding between the Pérola and Madrepérola lines. The procedures described for experiment (b) were also applied to the biparental population. With reference to the three activities, it was concluded that: As expected, computational simulation demonstrated with "n" genes what happens when a gene is considered. Each F2 plant has, on average, the same frequency of loci in homozygosis and heterozygosis that is expected between plants of the F2 population. The amplitude of variation between the expected and the observed reduces with the number of genes increase. Thus, the variability released among the offspring of an F2 plant is similar to that between plants of the F2 population. With a large number of loci it is impossible to have all the expected genotypes in the offspring. Thus, it is evident that the F2 population don’t need to be very large, since over 20 loci segregating, for example, the number of individuals that must be handled is out of the possibilities of any breeding program. It is understood that when using the bulk within F2 method, at the end of the process, that is, when the frequency of loci in heterozygosity tends to zero, there is a mixture of lines with the same variability of the existing progenies has been explored with the selection. Thus, it is up to the breeder to decide between selecting pure lines within the progenies, or selecting only progenies by choosing a mixture of pure lines. It was found that the UFLA’s RS progenies, C.XIV, have variability with regarding the resistance to the two evaluated pathogens, P. griseola and C. lindemuthianum. This variability is detected both among plants within populations and am ong descendants of some S0 plants, indicating that they were probably heterozygous. The frequency of progenies / plants resistant to both pathogens in the population is high, even though the selection was not performed specifically for these pathogens in previous cycles. The opening moment of the bulk does not significantly affect the occurrence severity of the two pathogens. It was found that a low experimental precision associated with a poor sampling does not make it possible to obtain estimates of the genetic variability among and within the progenies as it is terorically expected in the bulk / F2 or S0 method. However, the fact that the segregating population has allelic frequency equal to or different from 0.5 does not affect what is expected with variation among and within progenies using the bulk / F2 or S0 method. In principle, even with only two or three generations of difference, there is evidence that natural selection acted to maintain the plants/progenies with higher grain yield.