Diversidade genética, sistema de cruzamento e fluxo gênico da castanheira-do-brasil (Bertholletia excelsa Bonpl.) em floresta nativa do Mato Grosso

Abstract

Bertholletia excelsa Bonpl. is native to Amazon rainforest and is known worldwide for the commercialization of its almonds, which have a remarkable taste and high nutritive value. The species has its cut prohibited by law, which makes nut extractivism a sustainable income alternative for local communities. On the other hand, the inadequate occupation of man in forests can lead to problems such as forest fragmentation, which cause isolation of populations, impairing gene flow and consequently genetic diversity. A tool capable of assisting in the monitoring of the genetic diversity of these natural populations is the molecular markers. Considering the above, the objective of the research was to evaluate the genetic diversity, the gene flow and the crossing system of a native population of Brazil nut tree, located in the Amazon rainforest in Mato Grosso state. The study was conducted with a total of 518 individuals of Brazil nut tree, divided into three groups: adult (diameter at breast height - DBH≥50cm), juveniles (DBH<50cm) and progenies. The plant material (vascular exchange or leaf) of 218 genotypes was collected, being 96 adults and 122 juveniles. The group of progenies totaled 300 individuals from 98 fruits from 9 matrix trees located in the center of the population. The fruits contained a total of 1348 seeds, which were planted in a greenhouse to obtain the progenies. The analysis relied on the use of five microsatellite markers, previously developed for the species. With 309 individuals, 9 matrices and 300 progenies, the diversity among and within families was evaluated, the Analysis of Molecular Variance (AMOVA) in relation to each locus, the genetic distance between the genotypes based on the Non-Weighted Index and the grouping by Ward's method. Also, with the matrices and their progenies, the evaluation of the crossing system was performed, by the mixed models, correlated crosses and the fixation index of each tree. With 518 individuals, allele frequencies and genetic parameters (expected and observed heterozygosity, number of alleles and rate of inbreeding) were estimated. The gene flow was obtained by calculating the distances traveled by the pollinator and the disperser. The results showed a low dissimilarity between seed donor matrices, which caused low diversity among the progenies. It was possible to identify replicates among half-sib progenies. The observed heterozygosity was higher than expected for all groups analyzed (adults, juveniles, and progenies), as well as the rate of null inbreeding. The average number of alleles was 5.1 and both rare and private alleles were found in the population. The system of crossing by allogamy corroborates with what is expected for Brazil nut tree. The families of this population comprised different kinship levels as half-sibs and full-sibs. The geographic distances calculated through the analysis of the gene flow showed that the pollinator ran a greater distance compared to the disperser. It is concluded that in relation to seed donor matrices it is necessary to enlarge this group to obtain more diverse progenies. The identification of replicates among the progenies demonstrates the efficiency of the methodology in the indication of materials for seed collections. For this population evaluated, the follow-up of the next generations should be carried out to evaluate the genetic diversity and the gene flow between these individuals. The follow-up of the next generations of this population should be carried out to evaluate the genetic diversity and the gene flow between these individuals. The low genetic variation found can compromise natural populations against possible environmental changes. Although the Brazil nut tree is an allogamy species, the group of animals capable of pollination and dispersal of its flowers and seeds is restricted, so the conservation of the native forest to ensure the success of these events, allow the perpetuation of the species and maintain the levels of genetic diversity of the population is important.