Phylogeographic relationships among Asian eggplants and new perspectives on eggplant domestication

The domestication history of eggplant (Solanum melongena L.) has long been debated, with studies unable to narrow down where domestication occurred within a broad region of tropical Asia. The most commonly hypothesized region is India, however China has an equally old written record of eggplant use dating ca. 2000 years before present. Both regions have a high diversity of landraces and populations of putatively wild eggplant: Solanum incanum L. in India and Solanum undatum Lam. in SE Asia. An additional complication is that there is taxonomic confusion regarding the two candidate progenitors. Here, we synthesize historic, morphologic, and molecular data (nrITS sequence and AFLP) to interpret the phylogeographic relationships among candidate progenitors and Asian eggplant landraces in order to test theories of domestication. A minimum of two domestication events is supported: one in India and one in southern China/SE Asia. Results also support separate domestication of S. melongena subsp. ovigerum, a group of morphologically distinct eggplants found in SE Asia, and suggest Asian S. incanum and S. undatum may not be genetically distinct. Routes of the spread of eggplant cultivation throughout Asia are proposed, and evolutionary relationships among allied species are discussed.     

Improved genome assembly and pan-genome provide key insights into eggplant domestication and breeding

Eggplant (Solanum melongena L.) is an important horticultural crop and one of the most widely grown vegetables from the Solanaceae family. It was domesticated from a wild, prickly progenitor carrying small, round, non-anthocyanic fruits. We obtained a novel, highly contiguous genome assembly of the eggplant ‘67/3’ reference line, by Hi-C retrofitting of a previously released short read- and optical mapping-based assembly. The sizes of the 12 chromosomes and the fraction of anchored genes in the improved assembly were comparable to those of a chromosome-level assembly. We resequenced 23 accessions of S. melongena representative of the worldwide phenotypic, geographic, and genetic diversity of the species, and one each from the closely related species Solanum insanum and Solanum incanum. The eggplant pan-genome contained approximately 51.5 additional megabases and 816 additional genes compared with the reference genome, while the pan-plastome showed little genetic variation. We identified 53 selective sweeps related to fruit color, prickliness, and fruit shape in the nuclear genome, highlighting selection leading to the emergence of present-day S. melongena cultivars from its wild ancestors. Candidate genes underlying the selective sweeps included a MYBL1 repressor and CHALCONE ISOMERASE (for fruit color), homologs of Arabidopsis GLABRA1 and GLABROUS INFLORESCENCE STEMS2 (for prickliness), and orthologs of tomato FW2.2, OVATE, LOCULE NUMBER/WUSCHEL, SUPPRESSOR OF OVATE, and CELL SIZE REGULATOR (for fruit size/shape), further suggesting that selection for the latter trait relied on a common set of orthologous genes in tomato and eggplant.     

Conservation of gene function in the solanaceae as revealed by comparative mapping of domestication traits in eggplant

Quantitative trait loci (QTL) for domestication-related traits were identified in an interspecific F(2) population of eggplant (Solanum linnaeanum x S. melongena). Although 62 quantitative trait loci (QTL) were identified in two locations, most of the dramatic phenotypic differences in fruit weight, shape, color, and plant prickliness that distinguish cultivated eggplant from its wild relative could be attributed to six loci with major effects. Comparison of the genomic locations of the eggplant fruit weight, fruit shape, and color QTL with the positions of similar loci in tomato, potato, and pepper revealed that 40% of the different loci have putative orthologous counterparts in at least one of these other crop species. Overall, the results suggest that domestication of the Solanaceae has been driven by mutations in a very limited number of target loci with major phenotypic effects, that selection pressures were exerted on the same loci despite the crops' independent domestications on different continents, and that the morphological diversity of these four crops can be explained by divergent mutations at these loci.     

Comparative genomics reveals novel biochemical pathways

How well do we understand which enzymes are involved in the primary metabolism of the cell? A recent study using comparative genomics and postgenomics approaches revealed a novel pathway in the most studied organism, Escherichia coli. The analysis of a new operon consisting of seven previously uncharacterized genes thought to be involved in the degradation of nucleic acid precursors shows the impact of comparative genomics on the discovery of novel pathways and enzymes.     

Activation of seminal root primordia during wheat domestication reveals underlying mechanisms of plant resilience

Seminal roots constitute the initial wheat root system and provide the main route for water absorption during early stages of development. Seminal root number (SRN) varies among species. However, the mechanisms through which SRN is controlled and in turn contribute to environmental adaptation are poorly understood. Here, we show that SRN increased upon wheat domestication from 3 to 5 due to the activation of 2 root primordia that are suppressed in wild wheat, a trait controlled by loci expressed in the germinating embryo. Suppression of root primordia did not limit water uptake, indicating that 3 seminal roots is adequate to maintain growth during seedling development. The persistence of roots at their primordial state promoted seedling recovery from water stress through reactivation of suppressed primordia upon rehydration. Our findings suggest that under well‐watered conditions, SRN is not a limiting factor, and excessive number of roots may be costly and maladaptive. Following water stress, lack of substantial root system suppresses growth and rapid recovery of the root system is essential for seedling recovery. This study underscores SRN as key adaptive trait that was reshaped upon domestication. The maintenance of roots at their primordial state during seedling development may be regarded as seedling protective mechanism against water stress.     

Ancient DNA analysis reveals woolly rhino evolutionary relationships

With ancient DNA technology, DNA sequences have been added to the list of characters available to infer the phyletic position of extinct species in evolutionary trees. We have sequenced the entire 12S rRNA and partial cytochrome b (cyt b) genes of one 60-70,000-year-old sample, and partial 12S rRNA and cyt b sequences of two 40-45,000-year-old samples of the extinct woolly rhinoceros (Coelodonta antiquitatis). Based on these two mitochondrial markers, phylogenetic analyses show that C. antiquitatis is most closely related to one of the three extant Asian rhinoceros species, Dicerorhinus sumatrensis. Calculations based on a molecular clock suggest that the lineage leading to C. antiquitatis and D. sumatrensis diverged in the Oligocene, 21-26 MYA. Both results agree with morphological models deduced from palaeontological data. Nuclear inserts of mitochondrial DNA were identified in the ancient specimens. These data should encourage the use of nuclear DNA in future ancient DNA studies. It also further establishes that the degraded nature of ancient DNA does not completely protect ancient DNA studies based on mitochondrial data from the problems associated with nuclear inserts.     

Laboratory evolution of glutathione biosynthesis reveals natural compensatory pathways

The first and highly conserved step in glutathione (GSH) biosynthesis is formation of γ-glutamyl cysteine by the enzyme glutamate-cysteine ligase (GshA). However, bioinformatic analysis revealed that many prokaryotic species that encode GSH-dependent proteins lack the gene for this enzyme. To understand how bacteria cope without gshA, we isolated Escherichia coli ΔgshA multigenic suppressors that accumulated physiological levels of GSH. Mutations in both proB and proA, the first two genes in L-proline biosynthesis, provided a new pathway for γ-glutamyl cysteine formation via the selective interception of ProB-bound γ-glutamyl phosphate by amino acid thiols, likely through an S-to-N acyl shift mechanism. Bioinformatic analysis suggested that the L-proline biosynthetic pathway may have a second role in γ-glutamyl cysteine formation in prokaryotes. Also, we showed that this mechanism could be exploited to generate cytoplasmic redox buffers bioorthogonal to GSH.     

Ancient and modern steps during the domestication of guinea pigs (Cavia porcellus L.)

To test whether there are differences between living lineages of domestic guinea pigs Cavia porcellus , we studied 118 specimens from six breeds collected along six Andean countries as well as 15 from the wild cavy species ( Cavia tschudii ). The mean weight and body length of 15 adult wild cavies (295±31 g, 242±8.3 mm) were significantly smaller than 25 creole guinea pigs from Bolivia and Chile (639±157 g, 287±23.7 mm, respectively). Eighteen laboratory/pet guinea pigs (including the English Pirbright breed) were also smaller (900±173 g, 308±21 mm) than 25 improved ones from Peru (Tamborada breed, 1241±75.4 g, 317±12 mm) and Ecuador (Auqui breed, 1138±65.5 g, 307±8 mm). Similar size increases appeared in the first axis of a principal component analysis of six skeletal measurements, recovering 84% of total variation. Phylogenetic and haplotype analyses of complete cytochrome b gene sequences consistently joined all 22 domestic individuals (13 shared unambiguous substitutions, 100% bootstrap in 1000 replicates), probably from a single first ancient domestication in the western Andes. Six laboratory/pet sequences were also joined within a common branch (six shared substitutions, 96% bootstrap), probably from a documented European second phase. By contrast, those from improved Auqui joined a northern creole subgroup (one shared substitution, 84% bootstrap), and those from Nativa and improved Tamborada clustered together and with a southern creole subgroup (four shared substitutions, 86% bootstrap); this suggests at least two independent modern events during a more complex third phase, producing two improved guinea pigs selected for size and meat. Cavia tschudii sequences showed some unexpected geographic variation.     

Ancient DNA from Nubian and Somali wild ass provides insights into donkey ancestry and domestication

Genetic data from extant donkeys (Equus asinus) have revealed two distinct mitochondrial DNA haplogroups, suggestive of two separate domestication events in northeast Africa about 5000 years ago. Without distinct phylogeographic structure in domestic donkey haplogroups and with little information on the genetic makeup of the ancestral African wild ass, however, it has been difficult to identify wild ancestors and geographical origins for the domestic mitochondrial clades. Our analysis of ancient archaeological and historic museum samples provides the first genetic information on the historic Nubian wild ass (Equus africanus africanus), Somali wild ass (Equus africanus somaliensis) and ancient donkey. The results demonstrate that the Nubian wild ass was an ancestor of the first donkey haplogroup. In contrast, the Somali wild ass has considerable mitochondrial divergence from the Nubian wild ass and domestic donkeys. These findings resolve the long-standing issue of the role of the Nubian wild ass in the domestication of the donkey, but raise new questions regarding the second ancestor for the donkey. Our results illustrate the complexity of animal domestication, and have conservation implications for critically endangered Nubian and Somali wild ass.     

Genetic analysis of wheat domestication and evolution under domestication

Wheat is undoubtedly one of the world's major food sources since the dawn of Near Eastern agriculture and up to the present day. Morphological, physiological, and genetic modifications involved in domestication and subsequent evolution under domestication were investigated in a tetraploid recombinant inbred line population, derived from a cross between durum wheat and its immediate progenitor wild emmer wheat. Experimental data were used to test previous assumptions regarding a protracted domestication process. The brittle rachis (Br) spike, thought to be a primary characteristic of domestication, was mapped to chromosome 2A as a single gene, suggesting, in light of previously reported Br loci (homoeologous group 3), a complex genetic model involved in spike brittleness. Twenty-seven quantitative trait loci (QTLs) conferring threshability and yield components (kernel size and number of kernels per spike) were mapped. The large number of QTLs detected in this and other studies suggests that following domestication, wheat evolutionary processes involved many genomic changes. The Br gene did not show either genetic (co-localization with QTLs) or phenotypic association with threshability or yield components, suggesting independence of the respective loci. It is argued here that changes in spike threshability and agronomic traits (e.g. yield and its components) are the outcome of plant evolution under domestication, rather than the result of a protracted domestication process. Revealing the genomic basis of wheat domestication and evolution under domestication, and clarifying their inter-relationships, will improve our understanding of wheat biology and contribute to further crop improvement.     

Molecular dating of human-to-bovid host jumps by Staphylococcus aureus reveals an association with the spread of domestication

Host species switches by bacterial pathogens leading to new endemic infections are important evolutionary events that are difficult to reconstruct over the long term. We investigated the host switching of Staphylococcus aureus over a long evolutionary timeframe by developing Bayesian phylogenetic methods to account for uncertainty about past host associations and using estimates of evolutionary rates from serially sampled whole-genome data. Results suggest multiple jumps back and forth between human and bovids with the first switch from humans to bovids taking place around 5500 BP, coinciding with the expansion of cattle domestication throughout the Old World. The first switch to poultry is estimated at around 275 BP, long after domestication but still preceding large-scale commercial farming. These results are consistent with a central role for anthropogenic change in the emergence of new endemic diseases.