According to the results of the genetic similarity measures relating to the 13 molecular primers and estimated using the Jaccard index, it was possible to observe that ALHA05 access appears most frequently in relation to the formation of the most dissimilar pairs. The highest dissimilarity values were observed between ACO01 and MAR12 accessions (d = 100%), which were the most divergent pair, followed by ACO01 and ALHA04 (d = 80.70%). These results suggest that crosses between these pairs could produce progenies with greater variability, which could be a useful strategy for the breeding program.
The most similar pairs of accessions were Costa Rica and Flor Branca (d = 7.96%) and FP19 and CARP09 (d = 8.70%), showing that recombination between them could possibly result in progenies with a narrow genetic base. When taking into account the need to identify contrasting individuals as base material for exploitation within species breeding programs, much has been attributed to the recombination of similar accessions as a flawed strategy, but when attributes of these genotypes are observed, such as resistance to a certain stress and commercial characteristics of interest in another, recombination between the two can be advantageous, with the possibility of aggregating these characteristics in a single genotype.
This same scenario was observed between the Flor Branca and Junko accessions, which showed great similarity detected by the ISSR primers. Both are commercial clones, that is, plants with defined characteristics accepted by the market. Nonetheless, Flor Branca has stable flowering throughout the year and produces fruit with fragile flesh; on the other hand, Junko produces fruit with firm and durable flesh, but its production is greatly affected by temperature fluctuations throughout the year (Souza et al. 2013). Accordingly, it can be inferred that the result of crossing the two may be the most suitable resource for producing segregating populations in which genotypes with more stable production and more durable fruit can be selected. Moreover, the fact that both are commercial clones may result in less effort being made to discard individuals with undesirable characteristics (linkage drag).
Analyzing the dendrogram generated by the UPGMA hierarchical clustering method (Fig. 1), it is possible to perceive the existence of genetic divergence between the accessions. When a cut-off point of 50% genetic distance is established, 23 groups are formed, some of which are divided into subgroups. The first group consists of 10 accessions, namely: Costa Rica, Flor Branca, Junko, Clone71/2, LAG03, LAG04, LOND01, Clone47, Dominga and Eclipse. The accessions that make up Group 1 represent 10.42% of the total and comprise accessions that were collected in three regions (Petrolina-PE, Pacajus-CE and Londrina-PR). In Group 2, the accessions named Carolina, CARP07, CARP01, Coopama nº1 and CARP04 (5.21%) were allocated. Group 3 was formed by CARP06, CARP08, BRS Cabocla, CARP02, BRS Sertaneja, CARP09, FP19, Florida Sweet x BRS Cabocla, CARP03, LAG08, CARP05, Florida Sweet, LAG07, BRS Roxinha, BRS Rubra, ALHA06, Barbados, BRS Apodi, ALHA05, ALHA09 and BRS Jaburu (21.88). Group 4 was formed by Luisa, Manoela, LAG09, LAG05 and Lígia (5.21%). Group 6 was formed by the accessions named BV07 and CAMTA (2.08%). Group 7 comprises the accessions named LAG06, MAR10 and MAR08 (3.12%). Group 10 was formed by the accessions named BRS Frutacor, BV01, BRS Cereja, ALHA04 and LAG01 (6.25%). Group 13 formed by the accessions named ACO19, ACO33, ALHA03 and ACO35 (4.17%). Group 17 formed by the accessions named UEL01, Valéria, Samurai, Tropicana and UEL03 (5.21%). Group 19 formed by the accessions Olivier and REC01 (2.08%). Group 20 was made up of the accessions named Monami and Morena (2.08%). Group 21 was made up of the accessions named Natália, Nikki and Mineira (3.12%). In general, the accessions in each of these groups have individuals collected in at least two locations, and these results indicate that there may have been an exchange of genetic material between them at some point, given that they have alleles in common.
Still analyzing the dendrogram (Fig. 1), it was possible to observe individuals from the same location, which should have been grouped together with the others, composing separate groups, such as Group 8, formed by the accessions named MAR06, MAR07, MAR02, MAR03, MAR05 and MAR01 (7.28%). Group 11 was formed by the accessions named ACO31 and ACO34 (2.08%). Group 12 was formed by the accessions named ACO01 and ACO34 (2.08%). Group 14 was formed by the accessions named ACO08, ACO30, ACO09, ACO13, ACO10, ACO15, ACO17, ACO14, ACO05 and ACO07 (10.42%). Finally, Group 18 was made up of the Okinawa and Reci 02 accessions (2.08%). The possible explanation for this behavior can be attributed to the short time that these individuals spent in each location, which did not allow the exchange of alleles between all the individuals to take place, leading them to equilibrium. In addition, these results show that there is possible variation between individuals at each location, leading us to assume that selection within populations would also be an interesting strategy.
There were also groups formed by a single accessory, namely: 5, 9, 15, 16, 22, 23 and 24, which included the accessions named MAR11, MAR09, ACO18, ACO20, Neusa, Mulata and MAR12, respectively, corresponding to 1.04% of the total number of accessions in each group (Fig. 1). These accessions behaved exceptionally well, as individuals collected from the same location were generally grouped together. This behavior indicates that the isolated accessions have different alleles from the others. These alleles may be associated with traits of interest, reinforcing the need to continue collecting and studying the characterization of the AGB.
The dendrogram generated by the UPGMA method is in line with the dissimilarity values generated by the Jaccard index, since the accessions that make up Group 1 are those with the greatest similarity (Costa Rica and Flor Branca; d = 7.96%), with the greatest distance being between accessions from different groups (ACO01 and MAR12; d = 100%). This behavior was expected due to the fact that the cophenetic correlation coefficient of the dendrogram showed a value considered satisfactory (r = 88%), which translates into greater accuracy of the dendrogram in the graphical representation of the data matrix.
In addition to the dendrogram, the Analysis of Molecular Variance (AMOVA) was used to quantify the genetic diversity between and within groups. The results showed that 19% of the contrast between individuals was observed between groups and 81% within groups (Table 2), meaning that selection is more effective within them. These results are in line with those observed in the dendrogram, which suggested greater variation within the regions in which the materials were collected. This result may be associated with the existence of a greater genetic difference, taking into account the mode of reproduction, between individuals that show allogamous reproduction and, especially, if propagated via seed, as is the case with some of the accessions that make up the AGB.
Table 2
Analysis of Molecular Variance (AMOVA) using ISSR primers, considering groups of acerola accessions by collection.
Source of variation | ¹DF | SS | VC | % Variation |
Between groups | 8 | 338.138 | 2.956 | 19 |
Within groups | 86 | 1079.756 | 12.555 | 81 |
TOTAL | 94 | 1417.895 | 15.511 | - |
¹ Degree of Freedom (DF), Sum of Squares (SS), Variance Component (VC)
When observing the parameters obtained using ISSR molecular primers (Table 3), the Total Number of Amplified Bands (NTB) in each population ranged from 101 to 846 in Groups 9 and 1, respectively. Lima et al. (2015), when evaluating the amplification of 20 ISSR primers in 56 acerola accessions, obtained a total of 186 amplified fragments, a much lower number compared to the present study, which shows the existence of polymorphism within the groups. This information is important from the point of view of breeding, as this variability is the raw material for obtaining new cultivars and may contain alleles favorable to tolerance or resistance to biotic and abiotic stresses that are potentially harmful to the commercial cultivation of acerola trees.
Another approach that can be taken in relation to the polymorphism of individuals is that it be attributed to the mode of reproduction, that is, predominantly allogamous. In this case, the exchange of pollen between plants results in great variation in the offspring.
For the number of different alleles (Na) (Table 3), Groups 1 and 8 showed the highest values, with 1.15 and 1.19, respectively. This behavior was confirmed by the dendrogram (Fig. 1), where some individuals were isolated in the clusters. For the effective number of alleles (Ne), all the groups had values above unity, with Groups 4 and 8 standing out, with 1.22 and 1.28, respectively.
For the Shannon-Wiener Index (I), the highest values were expressed in Groups 1, 4 and 8, the first two with 0.21 and the last with 0.28 (Table 3). According to Bolstein et al. (1980), this parameter ranges from 0 to 1 and indicates that the closer it is to 1, the greater the genotypic diversity within the group. Using this definition as a basis, it is possible to observe the presence of low diversity between the groups, which agrees with the results of the dendrogram and AMOVA (Table 2), where greater variation was also observed within the groups.
The expected heterozygosity (He) was low for all populations, with values ranging from 0.05 to 0.18 (Table 3), which can be explained by the nature of the primer being dominant and not being able to distinguish heterozygous from homozygous dominant individuals, which results in a smaller number of alleles compared to the actual amount, revealing low values for this parameter.
The percentage of polymorphic loci (%P) is a parameter used as an aid in the effective detection of polymorphism. In the groups in this study, the average value was 36.09, with a range from 13.63 to 57.40 (Table 3). According to Felix et al. (2010), molecular primers with %P values above 0.5 are considered to be highly informative, those with values between 0.25 and 0.5 are classified as moderately informative, while ladder primers with values below 0.25 are poorly informative. Given this classification, in this study, the primers were effective in terms of capturing high polymorphism in Groups 1 (57.40) and 8 (58.85), moderate polymorphism in Groups 3 (44.97), 4 (47.34), 6 (27.81) and 7 (38.87), and low polymorphism in the others.
Table 3
Genetic diversity parameters obtained using dominant ISSR primers in acerola tree populations.
Groups | ¹N | NTB | Na | Ne | I | He | %P |
1 | 26.00 | 846 | 1.15 | 1.21 | 0.21 | 0.13 | 57.40 |
2 | 5.00 | 119 | 0.39 | 1.09 | 0.08 | 0.05 | 15.98 |
3 | 14.00 | 391 | 0.94 | 1.16 | 0.16 | 0.10 | 44.97 |
4 | 7.00 | 236 | 0.97 | 1.22 | 0.21 | 0.17 | 47.34 |
5 | 9.00 | 226 | 0.49 | 1.08 | 0.08 | 0.05 | 20.71 |
6 | 8.00 | 238 | 0.61 | 1.14 | 0.13 | 0.07 | 27.81 |
7 | 11.00 | 454 | 0.85 | 1.20 | 0.18 | 0.12 | 38.46 |
8 | 13.00 | 522 | 1.19 | 1.28 | 0.27 | 0.18 | 58.58 |
9 | 2.00 | 101 | 0.50 | 1.10 | 0.08 | 0.06 | 13.61 |
¹N = Number of Individuals Per Population, NTB = Total Number of Amplified Bands, Na = Number of Different Alleles, Ne = Effective Number of Alleles; I = Shannon-Wiener Index, He = Expected Heterozygosity, %P = Percentage of Polymorphic Loci.