Phylogeny and disparate selection signatures suggest two genetically independent domestication events in pea (Pisum L.).

Domestication is considered a model of adaptation that can be used to draw conclusions about the modus operandi of selection in natural systems. Investigating domestication may give insights into how plants react to different intensities of human manipulation, which has direct implication for the continuing efforts of crop improvement. Therefore, scientists of various disciplines study domestication-related questions to understand the biological and cultural bases of the domestication process. We employed restriction site-associated DNA sequencing (RAD-seq) of 494 Pisum sativum (pea) samples from all wild and domesticated groups to analyze the genetic structure of the collection. Patterns of ancient admixture were investigated by analysis of admixture graphs. We used two complementary approaches, one diversity based and one based on differentiation, to detect the selection signatures putatively associated with domestication. An analysis of the subpopulation structure of wild P. sativum revealed five distinct groups with a notable geographic pattern. Pisum abyssinicum clustered unequivocally within the P. sativum complex, without any indication of hybrid origin. We detected 32 genomic regions putatively subjected to selection: 29 in P. sativum ssp. sativum and three in P. abyssinicum. The two domesticated groups did not share regions under selection and did not display similar haplotype patterns within those regions. Wild P. sativum is structured into well-diverged subgroups. Although Pisum sativum ssp. humile is not supported as a taxonomic entity, the so-called 'southern humile' is a genuine wild group. Introgression did not shape the variation observed within the sampled germplasm. The two domesticated pea groups display distinct genetic bases of domestication, suggesting two genetically independent domestication events.

Prospects for the natural distribution of crop wild-relatives with limited adaptability: The case of the wild pea Pisum fulvum.

Plant breeders and conservationist depend on knowledge about the genetic variation of their species of interest. Pisum fulvum, a wild relative of domesticated pea, has attracted attention as a genetic resource for crop improvement, yet little information about its diversity in the wild has been published hitherto. We sampled 15 populations of P. fulvum from Israeli natural habitats and conducted genotyping by sequencing to analyse their genetic diversity and adaptive state. We also attempted to evaluate the species past demography and the prospects of its future reaction to environmental changes. The results suggest that genetic diversity of P. fulvum is low to medium and is distributed between well diverged populations. Surprisingly, with 56 % in the total population the selfing rate was found to be significantly lower than expected from a species that is commonly assumed to be a predominant selfer. We found a strong genetic bottleneck during the last glacial period and only limited patterns of isolation by distance and environment, which explained 13 %-18 % of the genetic variation. Despite the weak signatures of genome-wide IBE, 1,354 markers were significantly correlated with environmental factors, 1,233 of which were located within known genes with a nonsynonymous to synonymous ratio of 0.382. Species distribution modelling depicted an ongoing fragmentation and decreased habitable area over the next 80 years under two different socio-economic pathways. Our results suggest that complex interactions of substantial drift and selection shaped the genome of P. fulvum. Climate changeis likely to cause further erosion of genetic diversity in P. fulvum. Systematic ex-situ conservation may be advisable to safeguard genetic variability for future utilization of this species.

Limited divergent adaptation despite a substantial environmental cline in wild pea.

Isolation by environment (IBE) is a widespread phenomenon in nature. It is commonly expected that the degree of difference among environments is proportional to the level of divergence between populations in their respective environments. It is therefore assumed that a species' genetic diversity displays a pattern of IBE in the presence of a strong environmental cline if gene flow does not mitigate isolation. We tested this common assumption by analysing the genetic diversity and demographic history of Pisum fulvum, which inhabits contrasting habitats in the southern Levant and is expected to display only minor migration rates between populations, making it an ideal test case. Ecogeographical and subpopulation structure were analysed and compared. The correlation of genetic with environmental distances was calculated to test the effect of isolation by distance and IBE and detect the main drivers of these effects. Historical effective population size was estimated using stairway plot. Limited overlap of ecogeographical and genetic clustering was observed, and correlation between genetic and environmental distances was statistically significant but small. We detected a sharp decline of effective population size during the last glacial period. The low degree of IBE may be the result of genetic drift due to a past bottleneck. Our findings contradict the expectation that strong environmental clines cause IBE in the absence of extensive gene flow.

Environmental and genetic determinants of amphicarpy in Pisum fulvum, a wild relative of domesticated pea.

Pisum fulvum is an annual legume native to Syria, Lebanon, Israel and Jordan. In certain locations, P. fulvum individuals were documented to display a reproductive dimorphism - amphicarpy, with both above and below ground flowers and pods. Herein we aimed to study the possible role of soil texture on amphicarpy in P. fulvum, to investigate the possible bio-climatic associations of P. fulvum amphicarpy and to identify genetic markers associated with this phenotype. A set of 127 germplasm accessions sampled across the Israeli distribution range of the species was phenotyped in two common garden nurseries. Land use and bioclimatic data were used to delineate the eco-geographic clustering of accession's sampling sites. Single nucleotide polymorphism (SNP) markers were employed in genome-wide association study to identify associated loci. Amphicarpy was subject to strong experimental site x genotype interaction with higher phenotypic expression in fine textured soil relative to sandy loam. Amphicarpy was more prevalent among accessions sampled in eastern Judea and Samaria and was weakly associated with early phenology and relatively modest above ground biomass production. Twelve SNP markers were significantly associated with amphicarpy, each explaining between 8 and 12 % of the phenotypic variation. In P. fulvum amphicarpy seems to be a polygenetic trait controlled by an array of genes that is likely to be affected by environmental stimuli. The probable selective advantage of the association between amphicarpy and early flowering is in line with its relative prevalence in drought prone territories subject to heavy grazing.