Molecular diversity assessment of a pantropical cash crop

Tese de mestrado, Biologia Molecular e Genética, 2022, Universidade de Lisboa, Faculdade de CiênciasThe cashew tree (Anacardium occidentale L.) is a tropical plant from Anacardiaceae family, that has acquired a high economic importance as a cash crop in several tropical countries. It is assumed that in the middle of the sixteenth century the Portuguese brought the cashew tree from Brazil (center of origin) to India, and later expanded to Southeast Asia, and introduced in Africa at the same time in Mozambique. The socio-economic importance of the cashew tree in tropical regions is recognized, and in recent decades, the demand for cashew has increased at the global market. Despite the recognized value that this specie represents worldwide, few studies have addressed molecular markers to assess the distribution and structuring of cashew genetic diversity. In this work, microsatellites were used to study the intraspecific diversity of cashew across different tropical regions, from Brazil (South America) to Guinea-Bissau and Mozambique (Africa) and East Timor, Indonesia (Asia), using CPLP countries as a case study, which also cover cashew pantropical distribution. Despite its huge commercial value, few studies have been pushed forward towards the molecular diversity assessment of cashew genetic resources, which highlights the importance of the study conducted. As a result of extensive fieldwork in Guinea-Bissau and East Timor, complemented with samples collected in Brazil, Cabo Verde, Mozambique, and São Tomé and Principe, a set of cashew specific microsatellites or simple sequence repeats (SSRs) were selected for the characterization of the intra-specific diversity and population structuring of cashew. Overall, about 343 individuals (i.e. 309 of A. occidentale; 16 of A. giganteum and 18 A. humile) corresponding to 20 different populations were screened and genotyped with 16 SSRs. mAoR12 and mAoR33 loci were discarded due to presence of null alleles in almost all populations. The remaining 14 markers were found to be polymorphic and further genetic diversity and population structuring analysis were conducted. The Analysis of Molecular Variance (AMOVA) was performed to quantify the genetic variability between and within populations, revealing that most of the genetic diversity lies within individuals, reflecting the heterozygous nature of cashew. In terms of diversity by population, the populations with the greatest diversity were found in Guinea-Bissau and Timor-Leste. The population structure can be observed using the DCINA distance matrices in which the presence of 4 main clusters was observed. Individual-based clustering methods using a Bayesian approach (STRUCTURE) and a multivariate analysis by DAPC allowed to assess the population structuring, thus highlighting that genetic diversity scattering does follows a geographical trend under a continental distribution. Overall, our data reports the first comprehensive study on cashew intraspecific diversity using a continental approach thus highlighting the need to perform conservation programs focused on a country standpoint. This is especially important considering that the result of a limited gene flow across countries were observed, which may be due to local selection of accessions within each country. This work demonstrates a national and continental signature, highlighting that cashew introduction was not a single event in history, and that there were several points of entry into Africa. The findings obtained in this work could be a baseline for the assessment of the diversity of cashew genetic resources at a pantropical scale, as one of the most economically important cash crops in tropical regions, still largely understudied