Evolution and Phylogenetic Diversity of Yam Species (Dioscorea spp.): Implication for Conservation and Agricultural Practices

Complete Chloroplast Genome of Four Thai Native Dioscorea Species: Structural, Comparative and Phylogenetic Analyses

The chloroplast genomes of Dioscorea brevipetiolata, D. depauperata, D. glabra, and D. pyrifolia are 153,370–153,503 bp in size. A total of 113 genes were predicted, including 79 protein-coding sequences (CDS), 30 tRNA, and four rRNA genes. The overall GC content for all four species was 37%. Only mono-, di-, and trinucleotides were present in the genome. Genes adjacent to the junction borders were similar in all species analyzed. Eight distinct indel variations were detected in the chloroplast genome alignment of 24 Dioscorea species. At a cut-off point of Pi = 0.03, a sliding window analysis based on 25 chloroplast genome sequences of Dioscorea species revealed three highly variable regions, which included three CDS (trnC, ycf1, and rpl32), as well as an intergenic spacer region, ndhF-rpl32. A phylogenetic tree based on the complete chloroplast genome sequence displayed an almost fully resolved relationship in Dioscorea. However, D. brevipetiolata, D. depauperata, and D. glabra were clustered together with D. alata, while D. pyrifolia was closely related to D. aspersa. As Dioscorea is a diverse genus, genome data generated in this study may contribute to a better understanding of the genetic identity of these species, which would be useful for future taxonomic work of Dioscorea.

Sex Determination in Dioscorea dumetorum: Evidence of Heteromorphic Sex Chromosomes and Sex-Linked NORs

Yams (Dioscorea spp.) are a pantropical genus located worldwide that constitute an important source of nutrients and pharmaceutical substances. Some Dioscorea crop species are widely grown in West Africa. One species that is mainly cultivated in Cameroon is Dioscorea dumetorum. This is a dioecious root crop whose sex-determining system was unknown until now. To address the possible presence of sex chromosomes in D. dumetorum, we performed a karyotype characterization of male and female individuals using classical and molecular cytogenetic approaches. It was determined that 2n = 40 was the most common number of chromosomes in all of the investigated samples. One chromosome pair was longer than the others in the chromosome set and was a heteromorph in male and homomorph in female individuals. This pair corresponded to sex chromosomes, and we also confirmed this with molecular cytogenetic experiments. The results of chromomycin banding revealed the presence of strong positive signals on this chromosome pair. The signals, corresponding to GC-rich DNA regions, were similar in size on the chromosomes of the female individuals, whereas they were different in size in the male individuals. This size difference in the GC-rich heterochromatin regions was also apparent in the interphase nuclei as one small and one large fluorescent spot. The results of the in situ hybridization experiment showed that these chromomycin positive signals on the sex chromosomes also corresponded to the 35S rDNA cluster. The mean 2C DNA value (genome size) obtained for D. dumentorum was 0.71 pg (±0.012), which represents a small genome size. We found no difference in the genome size between the male and female individuals. The results of this study contribute to increasing our knowledge of sex determination in D. dumetorum (standard sex-determining XX/XY system) and may have some agronomic applications.

A Review of Viruses Infecting Yam (Dioscorea spp.)

Yam is an important food staple for millions of people globally, particularly those in the developing countries of West Africa and the Pacific Islands. To sustain the growing population, yam production must be increased amidst the many biotic and abiotic stresses. Plant viruses are among the most detrimental of plant pathogens and have caused great losses of crop yield and quality, including those of yam. Knowledge and understanding of virus biology and ecology are important for the development of diagnostic tools and disease management strategies to combat the spread of yam-infecting viruses. This review aims to highlight current knowledge on key yam-infecting viruses by examining their characteristics, genetic diversity, disease symptoms, diagnostics, and elimination to provide a synopsis for consideration in developing diagnostic strategy and disease management for yam.

Lactobacillus acidophilus and Non-Digestible Carbohydrates: A Review

In the recent years, lactic acid bacteria species such as Lactobacillus are considering one of the important species of probiotics used in the food processing sector to produce fermented products and play a significant role for the transformation and preservation of food products. Besides, there is a huge exploration of new molecules that promote health and exhibit potential for technological applications such as non-digestible carbohydrates. The non-digestible carbohydrates provide various health benefits such as balancing and sustaining the microbiota in the intestine and increasing the production of short chain fatty acids (SCFA). The aim of this review is to review some types of non-digestible carbohydrates as an enhancer for the growth of probiotics. These compounds can help in improving many characteristics of food such as sensory and textural properties.

Potential of Neglected and Underutilized Yams (Dioscorea spp.) for Improving Nutritional Security and Health Benefits

Food and nutritional security are the major concerns in many countries of the world and may have potential to contribute to sustainable food systems under climate change. To address the food and nutritional insecurity, it has become extremely important to diversify the present-day agricultural system as well as to search for alternative food and feed ingredients. Some wild root and tuber crops occupy a remarkable position toward food security of the developing world due to their high calorific value and superior carbohydrate content. Yam (Dioscorea spp.) provides food and medicines to millions of people in the world especially in the tropics and sub tropics. It is recognized as the fourth most important tuber crop after potatoes, cassava, and sweet potatoes. It contributes about 10% of the total root and tubers production around the world. Yams also considered as famine food and plays a prime role in the food habit of small and marginal rural families and forest-dwelling communities during the food scarcity periods. These species are unique for their food, medicinal and economic values but their wider utilization is limited due to the presence of anti-nutritional compositions. This paper describes the ethnobotany of yam species in relation to their nutritional, anti-nutritional and pharmacological properties and highlights the potentiality for food and nutritional security for combating the “hidden hunger” caused by micronutrient deficiencies.

The First Glimpse of Streptocarpus ionanthus (Gesneriaceae) Phylogenomics: Analysis of Five Subspecies' Chloroplast Genomes

Streptocarpus ionanthus (Gesneriaceae) comprise nine herbaceous subspecies, endemic to Kenya and Tanzania. The evolution of Str. ionanthus is perceived as complex due to morphological heterogeneity and unresolved phylogenetic relationships. Our study seeks to understand the molecular variation within Str. ionanthus using a phylogenomic approach. We sequence the chloroplast genomes of five subspecies of Str. ionanthus, compare their structural features and identify divergent regions. The five genomes are identical, with a conserved structure, a narrow size range (170 base pairs (bp)) and 115 unique genes (80 protein-coding, 31 tRNAs and 4 rRNAs). Genome alignment exhibits high synteny while the number of Simple Sequence Repeats (SSRs) are observed to be low (varying from 37 to 41), indicating high similarity. We identify ten divergent regions, including five variable regions (psbM, rps3, atpF-atpH, psbC-psbZ and psaA-ycf3) and five genes with a high number of polymorphic sites (rps16, rpoC2, rpoB, ycf1 and ndhA) which could be investigated further for phylogenetic utility in Str. ionanthus. Phylogenomic analyses here exhibit low polymorphism within Str. ionanthus and poor phylogenetic separation, which might be attributed to recent divergence. The complete chloroplast genome sequence data concerning the five subspecies provides genomic resources which can be expanded for future elucidation of Str. ionanthus phylogenetic relationships.

High Contiguity De Novo Genome Sequence Assembly of Trifoliate Yam (Dioscorea dumetorum) Using Long Read Sequencing

Trifoliate yam (Dioscorea dumetorum) is one example of an orphan crop, not traded internationally. Post-harvest hardening of the tubers of this species starts within 24 h after harvesting and renders the tubers inedible. Genomic resources are required for D. dumetorum to improve breeding for non-hardening varieties as well as for other traits. We sequenced the D. dumetorum genome and generated the corresponding annotation. The two haplophases of this highly heterozygous genome were separated to a large extent. The assembly represents 485 Mbp of the genome with an N50 of over 3.2 Mbp. A total of 35,269 protein-encoding gene models as well as 9941 non-coding RNA genes were predicted, and functional annotations were assigned.

Distribution and diversity of viruses infecting yams (Dioscorea spp.) in Cameroon

Yam (Dioscorea spp.) is an important food crop cultivated for its edible tubers in Cameroon. Surveys were conducted in Cameroon to determine the incidence and severity of yam mosaic disease and associated viruses in 124 yam farms in four agro-ecological zones in 2014 and 2016. Dioscorea rotundata, D. cayenensis, D. alata, D. Dumetorum and D. bulbifera were most frequently detected yam species in the fields. Symptoms of virus disease were observed on 81.5% of the farms surveyed and the disease incidence ranged from 0 to 96.7%, with an overall mean of 26.5%. Mean symptom severity estimated using a numerical rating scale of 1-5, ranged from 2 to 4.1, with an overall mean of 2.6. Representative set of leaf samples collected from farmers' fields were tested for three viruses known to cause yam mosaic disease in West Africa, viz., Yam mosaic virus (YMV), Yam mild mosaic virus (YMMV) and Cucumber mosaic virus (CMV), using multiplex RT-PCR. YMV and YMMV were detected in 220 (37.2%) of the 591 samples tested and 75% of the farms surveyed. None of the samples tested positive to CMV. Phylogenetic analysis based on the coat protein sequencing of 27 YMV isolates clustered these isolates into three phylogenetic groups. This study demonstrated high prevalence of mosaic disease in yam fields and YMV as main causal agent. Knowledge generated in this study will be useful to augment diagnostic tools and yam mosaic disease control with a view to improve on yam production in Cameroon.

Molecular characterization of PgUFGT gene and R2R3-PgMYB transcription factor involved in flavonoid biosynthesis in four tissues of wild pomegranate (Punica granatum L.)

The diversity on fruit colouration in plants directly depends on the flavonoids that explain the development of different pigmentation patterns. Anthocyanins are the major class of flavonoid pigments that are synthesized through flavonoid biosynthetic pathway. In the present study, two genes: PgUFGT gene and R2R3-PgMYB gene, involved in anthocyanin biosynthesis were analysed in four tissues of wild pomegranate. The structural genes, UDP-glucose: flavonoid-3-O-glucosyl transferase (PgUFGT; GenBank accession number: MK058491) and its myeloblastosis transcription factor (R2R3-PgMYB; GenBank accession number: MK092063) were isolated and their expression pattern were studied. Molecular modelling indicated that the main secondary structures of PgUFGT and R2R3-PgMYB genes are α-helix and random coil. In addition, expression profiling of PgUFGT and R2R3-PgMYB by quantitative-real time PCR indicated a positive correlation between anthocyanin content and their expression in leaves, flowers, green and red fruits of wild pomegranate. Among all the tissues, the red fruit exhibited high transcripts levels of PgUFGT as well as R2R3-PgMYB transcription factor. An extensive homology with UFGTs from other plants was revealed on comparative and bioinformatic analyses. Present study reveals that PgUFGT plays a predominant role in anthocyanin content in wild pomegranate fruits. Further, it is strongly suggested that R2R3-PgMYB transcription factor regulates the anthocyanin biosynthesis in wild pomegranate via expression of PgUFGT gene. This is the first study which provides an insight on expression profile of PgUFGT and R2R3-PgMYB that are involved in colour development and fruit ripening in wild pomegranate.

Genetic variability and inter species relationship between wild and cultivated yams (Dioscorea spp.) from Koraput, India based on molecular and morphological markers

Wild yams (Dioscorea spp.) are important tuber crops used both as vegetable and medicine by the tribal people of Koraput, India. There is deficiency of documented information on genetic structure and diversity of wild yams and its genetic assessment is necessary for crop improvement program. The present study assessed the level of genetic diversity of eight wild and one cultivated yam species of Koraput by using different morphological and molecular markers. Significant variation in different yield and morphological traits was observed among the studied yam species. The major morphological traits such as branch number, stem thickness, tuber depth, tuber length, number of tubers per plant and yield showed high genetic heritability accompanied with high genetic advance and major determinants of phenotypic diversity. Molecular profiling was carried out by taking five simple sequence repeat markers. A total of 10 polymorphic bands with an average of two were detected at the loci of the five markers across the nine yam species. Genetic similarity analysis revealed that some wild yam species such as D. oppositifolia, D. hamiltonii and D. pubera showed higher genetic similarity with cultivated (D. alata) species. The knowledge of the extent of genetic variations of wild yam species is important for planning of the genetic conservation and the utilization of this resource especially for genetic improvement.

Development of high-resolution DNA barcodes for Dioscorea species discrimination and phylogenetic analysis

The genus Dioscorea is widely distributed in tropical and subtropical regions, and is economically important in terms of food supply and pharmaceutical applications. However, DNA barcodes are relatively unsuccessful in discriminating between Dioscorea species, with the highest discrimination rate (23.26%) derived from matK sequences. In this study, we compared genic and intergenic regions of three Dioscorea chloroplast genomes and found that the density of SNPs and indels in intergenic sites was about twice and seven times higher than that of SNPs and indels in the genic regions, respectively. A total of 52 primer pairs covering highly variable regions were designed and seven pairs of primers had 80%-100% PCR success rate. PCR amplicons of 73 Dioscorea individuals and assembled sequences of 47 Dioscorea SRAs were used for estimating intraspecific and interspecific divergence for the seven loci: The rpoB-trnC locus had the highest interspecific divergence. Automatic barcoding gap discovery (ABGD), Poisson tree processes (PTP), and generalized mixed Yule coalescence (GMYC) analysis were applied for species delimitation based on the seven loci and successfully identified the majority of species, except for species in the Enantiophyllum section. Phylogenetic analysis of 51 Dioscorea individuals (28 species) showed that most individuals belonging to the same species tended to cluster in the same group. Our results suggest that the variable loci derived from comparative analysis of plastid genome sequences could be good DNA barcode candidates for taxonomic analysis and species delimitation.

Evaluation of mineral bioavailability and heavy metal content in indigenous food plant wild yams (Dioscorea spp.) from Koraput, India

Wild yam (Dioscorea spp.) tubers, an indigenous food makes a significant contribution to the diets of tribal people of Koraput, India. However, there is dearth of documented information of their mineral bioavailability and heavy metal content. To know their quality and safety concerns about their use, minerals and heavy metal concentrations were evaluated in eight wild and one cultivated yam species from Koraput. The samples were further investigated for their antinutrients to determine bioavailability of minerals. The majority of the wild yam tubers were rich in some of the essential minerals like calcium (18.08-74.79 mg/100 g), iron (11.15-28.61 mg/100 g), zinc (2.11-6.21 mg/100 g) and phosphorous (179-248 mg/100 g). The heavy metals concentration (mg/100 g) ranged from: cobalt (1.06-1.98), nickel (0.30-0.89), chromium (2.10-4.53) and lead (0.11-0.93) among the studied yam species. These values were lower than the recommended tolerable levels proposed by WHO Expert Committee on Food Additives. Based on these results of molar ratio between phytate and Fe, Zn and Ca were below the suggested critical values indicating the bioavailability of Fe, Zn and Ca to be high. The study also suggests these wild yam species as the safe food sources for mass consumption and can be beneficial for health.