Genetic diversity of farmer-preferred cassava landraces in Tanzania based on morphological descriptors and single nucleotide polymorphisms

Cassava germplasm collection is important for the preservation of genetic variability, allowing the development of improved cultivars with desirable traits such as drought and disease tolerance, better starch quality and yield. Therefore, the assessment of diversity in cassava germplasm maintained by farmers is important for maintaining biodiversity and crop improvement. Herein, we report genetic diversity relationships of 52 farmer-preferred cassava landraces from the eastern zone of Tanzania based on morphological descriptors and single nucleotide polymorphisms (SNPs). Cluster analysis was performed for both morphological traits (genetic distance 1.18–0.15) and SNPs (genetic distance 0.078–0.002). The analysis revealed that there were a total of 17,393 variant positions, and that several of the SNPs were distributed across all the chromosomes. The abundance of SNP varied remarkably among the 18 cassava chromosomes, with chromosome 2 having the highest number of SNPs (1335) and chromosome 18 having the lowest number of SNPs (734). The power of SNPs in distinguishing morphologically similar landraces was shown. Both analyses did not group landraces according to geographical locations, suggesting that farmers were moving cassava germplasm to different areas. Their diversity was mainly due to adaptation and preferential selection by farmers. This further implied that within a geographical location, the cultivars were more diverse and there was no misnaming of cassava cultivars by farmers. The collection revealed a wide range of genetic diversity, and represented a valuable resource for trait improvement, allowing the capture of farmer-preferred traits in future cassava breeding programmes.