Does size matter? Morphological and genetic similarities between cashew (Anacardium occidentale) and cajuí (A. humile)

Cashew trees are recognized for their economic and ecological importance in the tropics, but their species delimitations and phylogenetic relationships remain unclear. Here, we analyzed the genetic diversity of their germplasms from the Cerrado and restinga biomes in Northeastern Brazil. Twenty-four accessions of A . occidentale L. and two of A . humile A.St.-Hil. were evaluated using morphological and genetic markers. The morphological variability extended beyond species boundaries, challenging their differentiation. Furthermore, molecular analyses of selected nuclear ( ITS ) and plastid ( matK , trnLF , ycf1 and rps16 ) DNA regions revealed no clear genetic separation between A . occidentale and A . humile , resulting in an unresolved clade. We also performed cytogenetic analyses and genome size measurements on twelve accessions of A . occidentale along with two of A . humile looking for possible undetected genomic differentiation. The results indicated a strong chromosomal stability, with 2n  = 40, CMA ⁺ /DAPI ⁻ bands in the terminal region of the short arm of three chromosome pairs, one pair of 5S rDNA, and three pairs of 35S rDNA sites co-localized with CMA ⁺ /DAPI ⁻ bands. Genome sizes were similar among the accessions, with an average value of 0.44 pg/1C (435 Mbp), suggesting no significant intra- or interspecific variation and no evidence of polyploidy. The lack of differentiation among the accessions may be attributed to incomplete lineage sorting, hybridization with introgression, or the possibility that they represent a single species, with domesticated accessions exhibiting larger fruits and pseudofruits.