Recurrent genome recovery
After the first BC generation the average of recurrent genome recovery of all BC1 populations was 78.29%. In group 1 the average of recurrent genome recovery was on 78.49%, in group 2 77.36% and in group 3 79.03% (Figure 1). The difference in the recurrent genome recovery observed between groups 2 and 3 was significant.
By the backcross theory, in the BC1 generation the proportion of the recurrent recovered genome is expected to be 75% (Borem and Miranda 2005) for all populations. The proportion expected by the backcrossing theory is based on infinite size populations, and deviations from that expected can be observed according to the number of samples evaluated. In the BC1 generation, as there was no selection, these differences from the expected may have been observed due to the sample size. As real populations are always limited in size regarding to theoretical populations or ideal populations, these deviations could be expected.
The difference in the proportion of the recurrent genome recovery in groups 2 and 3 was significant, but the difference of this proportion between groups 1 and 3 was not significant, indicating that there is no relationship between the initial genetic similarity between parents and the proportion of the recurrent genome recovery in the BC1 generation. This is also demonstrated by the lack of significant correlation between the average of recurrent genome recovery of BC1 plants with the genetic distances among parents in each population (Table 2). The deviation of the recurrent genome proportion observed in group 2 regarding group 3 can be associated with the lowest number of plants evaluated in group 2 (Table 1).
The five plants of each BC1 population with the highest recurrent parent genome recovery was selected for the next backcrossing generation. Considering only the selected plants, the average of recurrent genome recovery mean among all populations was 86.73%, and in group 1 the average was 87.29%, in group 2 was 85.38% and, in group 3, the average was 87.51% (Figure 1). The difference between the recurrent genome recovery from the selected plants and the average of recurrent genome recovery observed in whole population is the effect of using molecular markers in MABC to accelerate genome recovering in BC program.
The average of recurrent genome recovery in the selected plants from group 1 and group 3 differed significantly from the average of recurrent genome recovery of the plants selected from group 2. This difference is not related to the genetic distances among the parents, since the correlation between the genetic distance and the recurrent genome recovery was not significant (Table 2). The total number of plants evaluated by markers on group 2 was 253, which is 5.6% lower than group 1 and 8.6% lower than group 3. As the number of selected plants was fixed in each population, there were different selection intensities. The lowest proportion of recurrent genome in group 2 is related to the lowest selection intensity and the lowest average of the recurrent genome on this group (Figure 1).
In the BC2 generation the average of recurrent genome recovery among all populations was 92.57%. In group 1 the average was 93.57%, in group 2 was 91.86%, and in group 3 was 92.27% (Figure 1). The differences of the average of recurrent genome recovery among the three groups in the BC2 generation were not significant. The correlation between the average of recurrent genome recovery in the BC2 generation and the genetic distances among parents were also not significant (Table 5).
The average of recurrent genome recovery in the BC2 generation (92.57%) was only slightly lower than it would be expected for the BC3 generation (93.75%). This was possible because the plants used for the BC that gave origin to the BC2 (BC1 selected plants) already had recovery genome proportion equivalent to the expected for the BC2 generation (Table 3).
The average of recurrent genome recovery in the five selected plants of all populations was 96.18% in BC2. In group 1 the average of recovery genome in the selected plants was 96.56%, in group 2 was 96.00% and in group 3 was 95.99% (Figure 1). The differences of the average of recurrent genome recovery among the three population groups were not significant. The correlation of the recurrent genome recovery with the genetic distances among the parents was also not significant (Table 2).
In the BC3 generation the average of recurrent genome recovery in plants of all populations was of 96.97%. In group 1 it was 96.90%, in group 2 was 96.89% and, in group 3 was 97.14% (Figure 1). The differences among these averages were not significant and the correlation between the recurrent genome recovery and the genetic distances among the parents was also not significant (Table 2).
Taking only the five plants with the highest recurrent genome recovery of each population, the average of proportion of the recurrent genome recovery was 98.92% in BC3. In group 1 the average of recurrent genome recovery was 98.69%, in group 2 was 98.77%, and in group 3 was 99.29%. These averages was not statistically different and were not correlated with the genetic similarities among the parents.
In the three groups of populations, it is possible to observe a great variability in the recurrent genome recovery among plants in the BC1 generation, and gradual decrease of variability as they advance the BC generations (Figure 1). This variability among plants within the BC populations is the basis for the markers assisted selection in the BC programs to accelerate the recurrent genome recovery (Guimarães et al. 2016, Wang et al. 2007, Peng et al. 2013).
Genetic similarity with recurrent genome
The recurrent genome recovery implies the amount of the genome that was divergent between the donor and recurrent parents and that became similar to the recurrent genitor genome in each BC generation (Peng et. al. 2013), expressed in percentage. In this case, only the divergent part of the genome is considered in the similarity evaluation, which means to say that the original similarity between the donor and recurrent parents was zero. For this reason, it is not expected that the genetic distance among the parents has influence in the recurrent genome recovery, as was observed in this study.
However, it is to be expected that if the recurrent and donor parents are genetically more similar, the total donor genome recovery, and not percentage, could be faster. Nonetheless, to observe this, it is necessary to evaluate the genetic similarity between the BC plants and the recurrent parent considering the whole genome, and not just the proportion of the genome that is different between recurrent and donor parents. For this reason, it was also estimated in each backcrossing generation the genetic similarity of the plants with the recurrent parent considering the whole genome, including the monomorphic markers.
After the first backcross generation, the average of genetic similarity between the plants of all BC1 populations with the recurrent parents was 92.15%. In BC1, the average of genetic similarity from group 1 with the recurrent parents was 94.01%, from group 2 was 92.34% and from group 3 was 90.10% (Figure 2). The differences among the average of genetic similarity from each group were statistically significant, demonstrating the genetic similarity effect between the parents and the genetic similarity of plants in BC1 with the recurrent parent (Figure 2). The correlation between the average of genetic similarity of each BC1 population and the genetic similarity between the donor and recurrent parents was 0.97 (Table 2).
Considering only the five selected plants in all populations, the average of genetic similarity with the recurrent parents was 95.11% in BC1 (Figure 2). In group 1 the average of similarity was 96.42%, in group 2 was of 94.98% and in group 3 was of 93.92%. Considering only the selected plants, the average of similarity of group 1 populations with the recurrent parents was statistically superior to the average of similarity in groups 2 and 3. The correlation between average of similarities of the selected plants in each population with the genetic similarities among the parents of each backcrossing population was 0.917.
In the BC2 generation the average of genetic similarity of all populations with the recurrent parents was 97.15%. In group 1 the average of similarity was 98.22%, in group 2 was 97.08%, and in group 3 was 96.14% (Figure 2). In BC2 generation, the average of similarity was statistically different only between group 1 and group 3 (Figure 2). After two BC generation the similarity between the BC plants and the recurrent parents became closer between the groups. BC populations from group 3 were equal to group 2 populations, and BC populations from group 2 were equal group 1. In BC2 generation the genetic similarity between the BC plants and recurrent parents were also correlated positively (r=0.94) with the genetic similarity among the parents (Table 2).
Among the five plants with higher similarity the recurrent parent plants in each BC2 population, the average of similarity was 98.40. In group 1 the average of similarity was 98.95%, in group 2 was 98.48% and, in group 3, was 97.77% (Figure 2). Among the selected plants there was also significant difference between the similarity of BC plants with the recurrent parents between groups 1 and 3. The correlation between the similarity of these selected plants with the recurrent parents and the genetic similarities among the parents of each population was not significant (Table 2).
In the BC3 generation the average of genetic similarity from all populations with the recurrent parents were 98.61%. In group 1 average of similarity was 98.84%, in group 2 was 98.60%, and in group 3 was 98.39% (Figure 2). The differences among this average were not significant, and the correlation between BC the similarities of plants and the recurrent parents with the genetic similarity among the population parents was also not significant (Table 3).
Considering the five selected plants in each population, the average of similarity with the recurrent parents was 99.44%. In group 1 this average was 99.40%, in group 2 was 99.37%, and in group 3 the average of similarity with the recurrent parents were 99.55%. These average of similarity with the recurrent parents did not differ statistically and were not correlated with the genetic similarity among the parents.